Provided by: gcc-9_9.4.0-1ubuntu1~20.04.2_amd64 
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same
options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall
options" allow you to stop this process at an intermediate stage. For example, the -c option says not to
run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options control the preprocessor
and others the compiler itself. Yet other options control the assembler and linker; most of these are
not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is
only useful with another language (usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use that option with all supported
languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or
machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke
GCC as g++ instead.
The gcc program accepts options and file names as operands. Many options have multi-letter names;
therefore multiple single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order
does matter when you use several options of the same kind; for example, if you specify -L more than once,
the directories are searched in the order specified. Also, the placement of the -l option is
significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat
and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo.
This manual documents only one of these two forms, whichever one is not the default.
Some options take one or more arguments typically separated either by a space or by the equals sign (=)
from the option name. Unless documented otherwise, an argument can be either numeric or a string.
Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal
arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort
may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a
multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for
megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are
designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and
international standards for the full listing and explanation of the binary and decimal byte size
prefixes.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -x language -v -### --help[=class[,...]] --target-help --version
-pass-exit-codes -pipe -specs=file -wrapper @file -ffile-prefix-map=old=new -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline -fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin -fno-builtin-function -fgimple
-fhosted -ffreestanding -fopenacc -fopenacc-dim=geom -fopenmp -fopenmp-simd -fms-extensions
-fplan9-extensions -fsso-struct=endianness -fallow-single-precision -fcond-mismatch
-flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n -fchar8_t -fcheck-new
-fconstexpr-depth=n -fconstexpr-loop-limit=n -fconstexpr-ops-limit=n -fno-elide-constructors
-fno-enforce-eh-specs -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fpermissive
-fno-pretty-templates -frepo -fno-rtti -fsized-deallocation -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -Wabi=n -Wabi-tag
-Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wdeprecated-copy
-Wdeprecated-copy-dtor -Wliteral-suffix -Wmultiple-inheritance -Wno-init-list-lifetime -Wnamespaces
-Wnarrowing -Wpessimizing-move -Wredundant-move -Wnoexcept -Wnoexcept-type -Wclass-memaccess
-Wnon-virtual-dtor -Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel -Wtemplates
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wno-class-conversion -Wno-terminate -Wsign-promo -Wvirtual-inheritance
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers
-fobjc-abi-version=n -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc
-fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link -gen-decls
-Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always] -fdiagnostics-format=[text|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fdiagnostics-minimum-margin-width=width -fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type -fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress
-Waddress-of-packed-member -Waggregate-return -Waligned-new -Walloc-zero
-Walloc-size-larger-than=byte-size -Walloca -Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=n -Wno-attributes
-Wattribute-alias=n -Wbool-compare -Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat -Wc11-c2x-compat -Wc++-compat
-Wc++11-compat -Wc++14-compat -Wc++17-compat -Wcast-align -Wcast-align=strict
-Wcast-function-type -Wcast-qual -Wchar-subscripts -Wcatch-value -Wcatch-value=n -Wclobbered
-Wcomment -Wconditionally-supported -Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else
-Wdate-time -Wdelete-incomplete -Wno-attribute-warning -Wno-deprecated -Wno-deprecated-declarations
-Wno-designated-init -Wdisabled-optimization -Wno-discarded-qualifiers
-Wno-discarded-array-qualifiers -Wno-div-by-zero -Wdouble-promotion -Wduplicated-branches
-Wduplicated-cond -Wempty-body -Wenum-compare -Wno-endif-labels -Wexpansion-to-defined -Werror
-Werror=* -Wextra-semi -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n -Wformat-security
-Wformat-signedness -Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=byte-
size -Wno-free-nonheap-object -Wjump-misses-init -Whsa -Wif-not-aligned -Wignored-qualifiers
-Wignored-attributes -Wincompatible-pointer-types -Wimplicit -Wimplicit-fallthrough
-Wimplicit-fallthrough=n -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline
-Wno-int-conversion -Wint-in-bool-context -Wno-int-to-pointer-cast -Winvalid-memory-model
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=byte-size -Wlogical-op -Wlogical-not-parentheses
-Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation -Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wmissing-profile
-Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd -Woverride-init-side-effects
-Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses
-Wno-pedantic-ms-format -Wplacement-new -Wplacement-new=n -Wpointer-arith -Wpointer-compare
-Wno-pointer-to-int-cast -Wno-pragmas -Wno-prio-ctor-dtor -Wredundant-decls -Wrestrict
-Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar -Wshadow=global,
-Wshadow=local, -Wshadow=compatible-local -Wshift-overflow -Wshift-overflow=n
-Wshift-count-negative -Wshift-count-overflow -Wshift-negative-value -Wsign-compare
-Wsign-conversion -Wfloat-conversion -Wno-scalar-storage-order -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wsizeof-array-argument -Wstack-protector -Wstack-usage=byte-size
-Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wstringop-overflow=n
-Wstringop-truncation -Wsubobject-linkage -Wsuggest-attribute=[pure|const|noreturn|format|malloc]
-Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override -Wswitch -Wswitch-bool
-Wswitch-default -Wswitch-enum -Wswitch-unreachable -Wsync-nand -Wsystem-headers
-Wtautological-compare -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized
-Wunknown-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wunused-const-variable -Wunused-const-variable=n -Wunused-but-set-parameter
-Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla
-Wvla-larger-than=byte-size -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes
-Wnested-externs -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches -gno-record-gcc-switches -gstabs
-gstabs+ -gstrict-dwarf -gno-strict-dwarf -gas-loc-support -gno-as-loc-support
-gas-locview-support -gno-as-locview-support -gcolumn-info -gno-column-info -gstatement-frontiers
-gno-statement-frontiers -gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points
-gno-inline-points -gvms -gxcoff -gxcoff+ -gz[=type] -gsplit-dwarf -gdescribe-dies
-gno-describe-dies -fdebug-prefix-map=old=new -fdebug-types-section
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list] -feliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]] -falign-jumps[=n[:m:[n2[:m2]]]]
-falign-labels[=n[:m:[n2[:m2]]]] -falign-loops[=n[:m:[n2[:m2]]]] -fassociative-math -fauto-profile
-fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize
-fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments
-fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans
-fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects -ffast-math
-ffinite-math-only -ffloat-store -fexcess-precision=style -fforward-propagate -ffp-contract=style
-ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm
-fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions
-finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fipa-stack-alignment -fipa-icf -fira-algorithm=algorithm
-flive-patching=level -fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage -floop-block
-floop-interchange -floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto -flto-compression-level -flto-partition=alg
-fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact
-fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole
-fno-peephole2 -fno-printf-return-value -fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning
-fprefetch-loop-arrays -fprofile-correction -fprofile-use -fprofile-use=path -fprofile-values
-fprofile-reorder-functions -freciprocal-math -free -frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks -fsched-pressure -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic
-fsched-rank-heuristic -fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-fusion
-fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling
-fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types
-fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing -fthread-jumps
-ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars
-ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-fcode-hoisting -ftree-loop-if-convert -ftree-loop-im -ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta
-ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion
-ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp -funconstrained-commons -funit-at-a-time
-funroll-all-loops -funroll-loops -funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa
-fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path -fprofile-dir=path
-fprofile-generate -fprofile-generate=path -fprofile-update=method -fprofile-filter-files=regex
-fprofile-exclude-files=regex -fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error
-fbounds-check -fcf-protection=[full|branch|return|none|check] -fstack-protector
-fstack-protector-all -fstack-protector-strong -fstack-protector-explicit -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers -fexec-charset=charset -fextended-identifiers
-finput-charset=charset -fmacro-prefix-map=old=new -fno-canonical-system-headers -fpch-deps
-fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion -fwide-exec-charset=charset
-fworking-directory -H -imacros file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-no-integrated-cpp -P -pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef
-Wp,option -Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nolibc -nostdlib -e
entry --entry=entry -pie -pthread -r -rdynamic -s -static -static-pie -static-libgcc
-static-libstdc++ -static-libasan -static-libtsan -static-liblsan -static-libubsan -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir -iprefix file
-iquote dir -isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir
-no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions
-fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-plt -fno-jump-tables -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-wchar
-fverbose-asm -fpack-struct[=n] -fleading-underscore -ftls-model=model -fstack-reuse=reuse_level
-ftrampolines -ftrapv -fwrapv -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fchecking -fchecking=n
-fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name
-fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-debug -fdump-earlydebug -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all
-fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename -fdump-passes
-fdump-rtl-pass -fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch
-fdump-tree-switch-options -fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-list -fira-verbose=n -flto-report
-flto-report-wpa -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report
-fopt-info -fopt-info-options[=file] -fprofile-report -frandom-seed=string -fsched-verbose=n
-fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats -fstack-usage
-ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large -mstrict-align -mno-strict-align -momit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769
-mfix-cortex-a53-843419 -mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div
-mpc-relative-literal-loads -msign-return-address=scope -mbranch-protection=none|standard|pac-
ret[+leaf]|bti -mharden-sls=opts -march=name -mcpu=name -mtune=name -moverride=string
-mverbose-cost-dump -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num -mcmove
-mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg
AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes
ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp
-mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm
-mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock
-mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs
-mlpc-width=width -G num -mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg
-mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact mlra-priority-
noncompact -mmillicode -mmixed-code -mq-class -mRcq -mRcw -msize-level=level -mtune=cpu
-mmultcost=num -mcode-density-frame -munalign-prob-threshold=probability -mmpy-option=multo
-mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check
-mapcs-reentrant -mno-apcs-reentrant -mgeneral-regs-only -msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mbe8 -mbe32 -mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name -mprint-tune-info
-mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm
-mflip-thumb -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -munaligned-access
-mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it -mverbose-cost-dump
-mpure-code -mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost -mcall-prologues
-mgas-isr-prologues -mint8 -mn_flash=size -mno-interrupts -mmain-is-OS_task -mrelax -mrmw
-mstrict-X -mtiny-stack -mfract-convert-truncate -mshort-calls -nodevicelib -nodevicespecs
-Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id=n -mleaf-id-shared-library -mno-leaf-id-shared-library
-msep-data -mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt -mmulticore
-mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4
-metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux
-sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops -mdata-model=model
C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB -mlittle-endian -EL -mhard-float
-msoft-float -mfpu=fpu -mdouble-float -mfdivdu -melrw -mistack -mmp -mcp -mcache -msecurity
-mtrust -mdsp -medsp -mvdsp -mdiv -msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld
-mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc -msched-prolog
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load
-bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip
-dependency-file -dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names
-iframework -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols
-sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table
-seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels
-mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch
-mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options -march=architecture-type -mcaller-copies -mdisable-fpregs -mdisable-indexing
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0
-mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld
-static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop
-mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency -minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64
-msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc
-msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled
-muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030
-m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float
-m68881 -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot -mno-xgot
-mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210
-m340 -mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip -mconfig=name -mcop
-mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult
-mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul
-mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model -mpic-data-is-text-relative
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2
-mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16
-mno-mips16 -mflip-mips16 -minterlink-compressed -mno-interlink-compressed -minterlink-mips16
-mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt
-mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float
-msingle-float -mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp
-mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mcrc
-mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa -mloongson-mmi
-mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2 -mno-loongson-ext2 -mfpu=fpu-
type -msmartmips -mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum
-mlocal-sdata -mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data
-mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks -mload-store-pairs -mno-load-store-pairs
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd
-mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely
-mcompact-branches=policy -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align
-msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls
-mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt -mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware
-mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu -mcode-region=
-mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov -mext-perf
-mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push -m16bit
-mno-16bit -misr-vector-size=num -mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor
-mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mgprel-sec=regexp -mr0rel-sec=regexp
-mel -meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile -mcache-volatile -mno-fast-sw-div
-mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N
-mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name -msys-lib=name -march=arch
-mbmx -mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
OpenRISC Options -mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div -mcmov -mror
-msext -msfimm -mshftimm
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16 -mint16
-mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv
-mno-div -march=ISA-string -mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore -mstrict-align -mno-strict-align
-mcmodel=medlow -mcmodel=medany -mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
-mriscv-attribute -mmo-riscv-attribute
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14
-mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mpowerpc64 -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf
-mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple -mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux -mcall-netbsd
-mcall-openbsd -mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi -mtraceback=traceback_type
-maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -mlongcall
-mno-longcall -mpltseq -mno-pltseq -mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz -mno-friz
-mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector
-mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic
-mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mfloat128 -mno-float128
-mfloat128-hardware -mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data
-mlittle-endian-data -msmall-data -msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax
-mmax-constant-size= -mint-register= -mpid -mallow-string-insns -mno-allow-string-insns -mjsr
-mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp
-mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack
-mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug
-mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard -mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e -m4-nofpu
-m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml
-mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee
-mbitops -misize -minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number
-mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot
-mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64
-mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode
-mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4
-mno-vis4 -mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc
-mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints -msmall-mem
-mlarge-mem -mstdmain -mfixed-range=register-range -mea32 -mea64 -maddress-space-conversion
-mno-address-space-conversion -mcache-size=cache-size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function
-mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt
-mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps
-msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=cpu-type
-mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list -mdump-tune-features
-mno-default -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387
-mhard-float -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip -mrecip=opt
-mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmmx -msse -msse2 -msse3 -mssse3
-msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl
-mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd
-mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mprefetchwt1 -mclflushopt -mclwb -mxsavec
-mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt
-mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni
-mvaes -mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call -mavx512vbmi2 -mvpclmulqdq
-mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni
-mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx -mcldemote -mms-bitfields -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128
-mregparm=num -msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80
-mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model
-mabi=name -maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=num
-msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv -minstrument-return=type
-mfentry-name=name -mfentry-section=name -mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only -mcall-ms2sysv-xlogues
-mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread -municode
-mwin32 -mwindows -fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic
-mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals
-mauto-litpools -mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking,
always in that order. GCC is capable of preprocessing and compiling several files either into several
assembler input files, or into one assembler input file; then each assembler input file produces an
object file, and linking combines all the object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C
program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++
program work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or
C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be
literally x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.d
D source code.
file.di
D interface file.
file.dd
D documentation code (Ddoc).
file.ads
Ada source code file that contains a library unit declaration (a declaration of a package,
subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a
package, generic, or subprogram renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are
also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated
this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler
choose a default based on the file name suffix). This option applies to all following input files
until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that subsequent files are handled according to their
file name suffixes (as they are if -x has not been used at all).
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc
where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some
combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The
ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc.,
with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an
assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc.,
with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of output is being produced, whether it be
an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print
the version number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted unless they contain only
alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated
command lines.
--help
Print (on the standard output) a description of the command-line options understood by gcc. If the
-v option is also specified then --help is also passed on to the various processes invoked by gcc, so
that they can display the command-line options they accept. If the -Wextra option has also been
specified (prior to the --help option), then command-line options that have no documentation
associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific command-line options for each tool.
For some targets extra target-specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line options understood by the compiler
that fit into all specified classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the compiler.
warnings
Display all of the options controlling warning messages produced by the compiler.
target
Display target-specific options. Unlike the --target-help option however, target-specific
options of the linker and assembler are not displayed. This is because those tools do not
currently support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is the name of one of the languages
supported in this version of GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal sign in the same continuous piece
of text, such as: --help=target.
separate
Display options taking an argument that appears as a separate word following the original option,
such as: -o output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler,
use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to
display all binary warning options (i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there
is nothing to display. One case where it does work, however, is when one of the classes is target.
For example, to display all the target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use displays its requested
class of options, skipping those that have already been displayed. If --help is also specified
anywhere on the command line then this takes precedence over any --help= option.
If the -Q option appears on the command line before the --help= option, then the descriptive text
displayed by --help= is changed. Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler
knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for example it is
possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success
return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically
highest error produced by any phase returning an error indication. The C, C++, and Fortran front
ends return 4 if an internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation.
This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults
which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld,
etc. More than one -specs=file can be specified on the command line, and they are processed in
order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters
are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1
....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any references to them in the result of the
compilation as if the files resided in directory new instead. Specifying this option is equivalent
to specifying all the individual -f*-prefix-map options. This can be used to make reproducible
builds that are location independent. See also -fmacro-prefix-map and -fdebug-prefix-map.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler.
The base name of the shared object file is used to identify the plugin for the purposes of argument
parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions
specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This generates Go
"const", "type", "var", and "func" declarations which may be a useful way to start writing a Go
interface to code written in some other language.
@file
Read command-line options from file. The options read are inserted in place of the original @file
option. If file does not exist, or cannot be read, then the option will be treated literally, and
not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by
surrounding the entire option in either single or double quotes. Any character (including a
backslash) may be included by prefixing the character to be included with a backslash. The file may
itself contain additional @file options; any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++
header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use
the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call
the compiler the same way as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically
specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of
C source files unless -x is used. This program is also useful when precompiling a C header file with a
.h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for
compiling programs in any language; or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and
Objective-C++) that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or
of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined
macros such as "unix" and "vax" that identify the type of system you are using. It also enables the
undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++
style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work
despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to
put them in header files that might be included in compilations done with -ansi. Alternate
predefined macros such as "__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic
is required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may
notice this macro and refrain from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any programs that might use these
names for other things.
Functions that are normally built in but do not have semantics defined by ISO C (such as "alloca" and
"ffs") are not built-in functions when -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those
standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all
programs following that standard plus those using GNU extensions that do not contradict it. For
example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the
"asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such
as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard
is specified, all features supported by the compiler are enabled, even when those features change the
meaning of the base standard. As a result, some strict-conforming programs may be rejected. The
particular standard is used by -Wpedantic to identify which features are GNU extensions given that
version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while
-std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled).
Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported, modulo bugs and floating-point
issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See
<http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are
deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely
supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11
features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L
(Analyzability). The name c1x is deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as
C11 except for corrections of defects (all of which are also applied with -std=c11) and a new
value of "__STDC_VERSION__", and so is supported to the same extent as C11.
c2x The next version of the ISO C standard, still under development. The support for this version is
experimental and incomplete.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17. This is the default for C code.
gnu2x
The next version of the ISO C standard, still under development, plus GNU extensions. The
support for this version is experimental and incomplete.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports.
Same as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for C++ code. The name gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. The name gnu++1z is deprecated.
c++2a
The next revision of the ISO C++ standard, tentatively planned for 2020. Support is highly
experimental, and will almost certainly change in incompatible ways in future releases.
gnu++2a
GNU dialect of -std=c++2a. Support is highly experimental, and will almost certainly change in
incompatible ways in future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when
in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline
functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99
or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or
-std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which
semantics are in effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations
and constants with a semantic type that is an interchange or extended format should be evaluated to
the precision and range of that type. These new values are a superset of those permitted under
C99/C11, which does not specify the meaning of other positive values of "FLT_EVAL_METHOD". As such,
code conforming to C11 may not have been written expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of
"FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS
18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is
-fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a
translation unit, including those in header files. This option is silently ignored in any language
other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file
and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and the colon), and whether it came
from a declaration or a definition (C or F, respectively, in the following character). In the case
of function definitions, a K&R-style list of arguments followed by their declarations is also
provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to
read the arguments. This is only supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard
keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for
instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls
to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but
since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor
can you change the behavior of the functions by linking with a different library. In addition, when
a function is recognized as a built-in function, GCC may use information about that function to warn
about problems with calls to that function, or to generate more efficient code, even if the resulting
code still contains calls to that function. For example, warnings are given with -Wformat for bad
calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must
not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this
option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-
in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that
allows unit testing of GIMPLE passes.
-fhosted
Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment
is one in which the entire standard library is available, and in which "main" has a return type of
"int". Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This implies -fno-builtin. A
freestanding environment is one in which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to
-fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and "!$acc" in Fortran. When -fopenacc
is specified, the compiler generates accelerated code according to the OpenACC Application
Programming Interface v2.0 <https://www.openacc.org>. This option implies -pthread, and thus is only
supported on targets that have support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that do not explicitly specify. The
geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can
be omitted, to use a target-specific default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is
specified, the compiler generates parallel code according to the OpenMP Application Program Interface
v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets
that have support for -pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with "#pragma omp" in C/C++ and "!$omp" in Fortran. Other
OpenMP directives are ignored.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's
current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an
experimental feature whose interface may change in future versions of GCC, as the official
specification changes. Please note that not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions
(-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets but x86 targets using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to
functions that expect pointers to elements of the type of the field, and permits referring to
anonymous fields declared using a typedef. This is only supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of
such an expression is void. This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible
element types. This option should not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by
default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been written to use plain "char" and expect it to be
signed, or expect it to be unsigned, depending on the machines they were written for. This option,
and its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though
its behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.
Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use
either "signed" or "unsigned". By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to the specified endianness. The
accepted values are big-endian, little-endian and native for the native endianness of the target (the
default). This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code
generated without it if the specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs. You can also
use most of the GNU compiler options regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other
options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the
ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through
G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on
function pointer types, decltype of a plain decl, and use of a function parameter in the declaration
of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and
the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope
function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the
mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with
function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity,
such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and
operator names. For multiple entities with the same name within a function, that are declared in
different scopes, the mangling now changes starting with the twelfth occurrence. It also implies
-fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the
x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the
calling convention for classes with a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling changes by creating an alias with
the correct mangled name when defining a symbol with an incorrect mangled name. This switch
specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility). If another ABI
version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through
4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If
this option is provided along with -Wabi (without the version), the version from this option is used
for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access
control code.
-faligned-new
Enable support for C++17 "new" of types that require more alignment than "void* ::operator
new(std::size_t)" provides. A numeric argument such as "-faligned-new=32" can be used to specify how
much alignment (in bytes) is provided by that function, but few users will need to override the
default of "alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++2a. This includes the addition of a new "char8_t"
fundamental type, changes to the types of UTF-8 string and character literals, new signatures for
user-defined literals, associated standard library updates, and new "__cpp_char8_t" and
"__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO
C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is
ill-formed when -fchar8_t is specified.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage
allocated. This check is normally unnecessary because the C++ standard specifies that "operator new"
only returns 0 if it is declared "throw()", in which case the compiler always checks the return value
even without this option. In all other cases, when "operator new" has a non-empty exception
specification, memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), which
allows code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to
detect endless recursion during constant expression evaluation. The minimum specified by the
standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed
to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single constexpr evaluation. Even when number of
iterations of a single loop is limited with the above limit, if there are several nested loops and
each of them has many iterations but still smaller than the above limit, or if in a body of some loop
or even outside of a loop too many expressions need to be evaluated, the resulting constexpr
evaluation might take too long. The default is 33554432 (1<<25).
-fdeduce-init-list
Enable deduction of a template type parameter as "std::initializer_list" from a brace-enclosed
initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the originally proposed semantics for the
C++11 standard, but was not part of the final standard, so it is disabled by default. This option is
deprecated, and may be removed in a future version of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that is only used to
initialize another object of the same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial
member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial
member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run time. This option
violates the C++ standard, but may be useful for reducing code size in production builds, much like
defining "NDEBUG". This does not give user code permission to throw exceptions in violation of the
exception specifications; the compiler still optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and "threadprivate" variables to have dynamic
(runtime) initialization. To support this, any use of such a variable goes through a wrapper
function that performs any necessary initialization. When the use and definition of the variable are
in the same translation unit, this overhead can be optimized away, but when the use is in a different
translation unit there is significant overhead even if the variable doesn't actually need dynamic
initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs
to trigger dynamic initialization (either because the variable is statically initialized, or a use of
the variable in the defining TU will be executed before any uses in another TU), they can avoid this
overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not
support symbol aliases, the default is -fno-extern-tls-init.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use
the keyword "__typeof__" instead. This option is implied by the strict ISO C++ dialects: -ansi,
-std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit
code for explicit instantiations. If you use this option, you must take care to structure your code
to include all the necessary explicit instantiations to avoid getting undefined symbols at link time.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The default is to handle
inlines differently so that compiles with and without optimization need the same set of explicit
instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by "#pragma
implementation". This causes linker errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer
to member function via non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17
but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default
unless -fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template parameters and default arguments:
this allows a template with default template arguments as an argument for a template template
parameter with fewer template parameters. This flag is enabled by default for -std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs",
"alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or
eliminate the text size overhead relative to a function with no exception specification. If the
function has local variables of types with non-trivial destructors, the exception specification
actually makes the function smaller because the EH cleanups for those variables can be optimized
away. The semantic effect is that an exception thrown out of a function with such an exception
specification results in a call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as
synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only
such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using
-fpermissive allows some nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler normally prints
the signature of the template followed by the template arguments and any typedefs or typenames in the
signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which
template is involved. When an error message refers to a specialization of a class template, the
compiler omits any template arguments that match the default template arguments for that template.
If either of these behaviors make it harder to understand the error message rather than easier, you
can use -fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ run-
time type identification features ("dynamic_cast" and "typeid"). If you don't use those parts of the
language, you can save some space by using this flag. Note that exception handling uses the same
information, but G++ generates it as needed. The "dynamic_cast" operator can still be used for casts
that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example,
programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled
with -frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for
example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14
and above. The flag -Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one
of the values of the enumeration (as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the enumerators). This assumption
may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated
type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions in left-to-right order, and
evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with
-std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift
expressions, and is the default without -std=c++17.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single warning or error to n. The
default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation
depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The
default value is 900, as the compiler can run out of stack space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe
initialization of local statics. You can use this option to reduce code size slightly in code that
doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather
than the "atexit" function. This option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be
incorrect, but is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or
methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can have a dramatic effect on load and
link times of a DSO as it massively reduces the size of the dynamic export table when the library
makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because
it does not affect static variables local to the function or cause the compiler to deduce that the
function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that
method. For example, if you do want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class with explicit visibility has no
effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise
cross a shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of
Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are
defined in more than one shared object: those declarations are permitted if they are permitted
when this option is not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be
externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the
Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same
name but defined in different shared objects are different, so changing one does not change the
other; and that pointers to function members defined in different shared objects may not compare
equal. When this flag is given, it is a violation of the ODR to define types with the same name
differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak
symbols if they are available. This option exists only for testing, and should not be used by end-
users; it results in inferior code and has no benefits. This option may be removed in a future
release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the
other standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++
programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not compatible with the vendor-neutral C++ ABI.
Since G++ now defaults to updating the ABI with each major release, normally -Wabi will warn only if
there is a check added later in a release series for an ABI issue discovered since the initial
release. -Wabi will warn about more things if an older ABI version is selected (with
-fabi-version=n).
-Wabi can also be used with an explicit version number to warn about compatibility with a particular
-fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version
number from this option is used for compatibility aliases. If no explicit version number is provided
with this option, but -fabi-compat-version is specified, that version number is used for ABI
warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code. There may also be cases where
warnings are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code
generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way
that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and "decltype" of a plain
declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function are promoted like unscoped
enumerators, causing "va_arg" to complain. On most targets this does not actually affect the
parameter passing ABI, as there is no way to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast",
prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the un-qualified function type was
incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1, leading to unaligned accesses. Note that
this did not affect the ABI of a function with a "nullptr_t" parameter, as parameters have a
minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a type, such as ia32 calling conventions
on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name
collisions when function pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI changes at this point include:
* For SysV/x86-64, unions with "long double" members are passed in memory as specified in psABI.
For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++
Attributes for more information about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are
private, and it has neither friends nor public static member functions. Also warn if there are no
non-private methods, and there's at least one private member function that isn't a constructor or
destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that has virtual functions and non-
virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base
class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if
the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This
warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-
provided destructor.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers.
Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object,
it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For
example:
* If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the
array's lifetime ends at the end of the return statement, so the value returned has a dangling
pointer.
* If a new-expression creates an "initializer_list", the array only lives until the end of the
enclosing full-expression, so the "initializer_list" in the heap has a dangling pointer.
* When an "initializer_list" variable is assigned from a brace-enclosed initializer list, the
temporary array created for the right side of the assignment only lives until the end of the
full-expression, so at the next statement the "initializer_list" variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from the "initializer_list" argument, this
doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary
"initializer_list", the pointer is left dangling by the end of the variable declaration
statement.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-suffix which does not begin with an
underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in
order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>".
For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing token.
Additionally, warn when a user-defined literal operator is declared with a literal suffix identifier
that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore
are reserved for future standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type mismatches in global declarations from different
compilation units. Requires -flto to be enabled. Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the
standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from
a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does
not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in
SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { },
e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call to a function that does not have
a non-throwing exception specification (i.e. "throw()" or "noexcept") but is known by the compiler to
never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a
symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls "f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an
object of class type, and when writing into such an object might bypass the class non-trivial or
deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt
virtual table pointers. Modifying the representation of such objects may violate invariants
maintained by member functions of the class. For example, the call to "memset" below is undefined
because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either
initialize or clear the storage of objects of such types is by using the appropriate constructor or
assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object
to "void *" or to a type that can be safely accessed by the raw memory function suppresses the
warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an
accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a
derived class through a pointer to the class itself or base class. This warning is automatically
enabled if -Weffc++ is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit
Register Variables extension. The use of the "register" keyword as storage class specifier has been
deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not match the order in which they
must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the
members, emitting a warning to that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy
elision can occur is when returning in a function with a class return type, when the expression being
returned is the name of a non-volatile automatic object, and is not a function parameter, and has the
same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that is, when a move operation would have
been performed even without the "std::move" call. This happens because the compiler is forced to
treat the object as if it were an rvalue in certain situations such as returning a local variable,
where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the type of the expression being returned and
the function return type differ, yet G++ treats the return value as if it were designated by an
rvalue.
This warning is enabled by -Wextra.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When
this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98,
-std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of
books:
* Define a copy constructor and an assignment operator for classes with dynamically-allocated
memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations.
However, the check is extended to warn about the lack of virtual destructor in accessible non-
polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these
guidelines; use grep -v to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid
sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant rather than a
null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable
across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared within a template. In very old
versions of GCC that predate implementation of the ISO standard, declarations such as friend int
foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular
specialization of a template function; the warning exists to diagnose compatibility problems, and is
enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style
casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to
unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type,
over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some coding rules disallow templates, and
this may be used to enforce that rule. The warning is inactive inside a system header file, such as
the STL, so one can still use the STL. One may also instantiate or specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple
inheritance, and this may be used to enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL. One may also define classes that
indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple
inheritance, and this may be used to enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL. One may also define classes that
indirectly use virtual inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be
used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so
one can still use the STL. One may also use using directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately result in a call to "terminate".
-Wno-class-conversion (C++ and Objective-C++ only)
Disable the warning about the case when a conversion function converts an object to the same type, to
a base class of that type, or to void; such a conversion function will never be called.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and
Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For
example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you
can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use
options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the
syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option,
if also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid
out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for
most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems,
including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit
ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to
be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported
only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support
for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with
support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI.
If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit
target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial
default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which runs
non-trivial default constructors on any such instance variables, in order, and then return "self".
Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so,
synthesize a special "- (void) .cxx_destruct" method which runs all such default destructors, in
reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated only operate on
instance variables declared in the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an
object's inheritance hierarchy. The "- (id) .cxx_construct" methods are invoked by the runtime
immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods are
invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "-
(id) .cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered
by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and
@synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not
available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only
available with the NeXT runtime; the GNU runtime has a different garbage collection implementation
that does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before
doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way no matter what this flag is set
to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime
ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only
affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option flags. When this option is used
with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC
4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled
with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and
allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded
in the course of program execution, without the need to restart the program itself. Currently, Fix-
and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time) with static class references that get
initialized at load time, which improves run-time performance. Specifying the -fzero-link flag
suppresses this behavior and causes calls to "objc_getClass("...")" to be retained. This is useful
in Zero-Link debugging mode, since it allows for individual class implementations to be modified
during program execution. The GNU runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if they were local variables from
within the methods of the class they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a class method or globally with the same
name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing
issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified option so that instance variables
declared outside the scope of any access modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued for every method in the protocol
that is not implemented by the class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implementation is inherited from the
superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are
considered to be implemented, and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are found during compilation. The
check is performed on the list of methods in the final stage of compilation. Additionally, a check
is performed for each selector appearing in a "@selector(...)" expression, and a corresponding
method for that selector has been found during compilation. Because these checks scan the method
table only at the end of compilation, these warnings are not produced if the final stage of
compilation is not reached, for example because an error is found during compilation, or because the
-fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types are found for a given selector
when attempting to send a message using this selector to a receiver of type "id" or "Class". When
this flag is off (which is the default behavior), the compiler omits such warnings if any differences
found are confined to types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is
considered undeclared if no method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol declaration, or implicitly in an
@implementation section. This option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in the final stage of compilation.
This also enforces the coding style convention that methods and selectors must be declared before
being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g.
its width, ...). You can use the options described below to control the formatting algorithm for
diagnostic messages, e.g. how many characters per line, how often source location information should be
reported. Note that some language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. If n is zero, then no
line-wrapping is done; each error message appears on a single line. This is the default for all
front ends.
Note - this option also affects the display of the #error and #warning pre-processor directives, and
the deprecated function/type/variable attribute. It does not however affect the pragma GCC warning
and pragma GCC error pragmas.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source
location information once; that is, in case the message is too long to fit on a single physical line
and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in
subsequent continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same
source location information (as prefix) for physical lines that result from the process of breaking a
message which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler
has been configured, it can be any of the above WHEN options or also never if GCC_COLORS environment
variable isn't present in the environment, and auto otherwise. auto means to use color only when the
standard error is a terminal. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases
for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list
of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator. (See the section in the documentation of your text terminal for
permitted values and their meanings as character attributes.) These substring values are integers in
decimal representation and can be concatenated with semicolons. Common values to concatenate include
1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37
for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color
and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background
colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and
256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is
bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities
are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
"type-diff="
SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly
controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line and a caret ^ indicating the
column. This option suppresses this information. The source line is truncated to n characters, if
the -fmessage-length=n option is given. When the output is done to the terminal, the width is
limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of
source code with pertinent information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long int
This option suppresses the printing of these labels (in the example above, the vertical bars and the
"char *" and "long int" text).
-fno-diagnostics-show-line-numbers
By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed,
showing line numbers. This option suppresses this left margin.
-fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers.
It defaults to 6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it,
a line will be printed after the relevant diagnostic, starting with the string "fix-it:". For
example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for
the initial column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be
replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double
quotes as "\"", non-printable characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to be removed. An empty range (e.g.
"45:3-45:3") indicates that the string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as for diagnostics (see
-fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common
and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double" and "float" in this case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing mismatching template types, common parts
of the types are printed as "[...]" to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of
the -fdiagnostics-show-template-tree flag.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned
by a program that does not understand the column numbers, such as dejagnu.
-fdiagnostics-format=FORMAT
Select a different format for printing diagnostics. FORMAT is text or json. The default is text.
The json format consists of a top-level JSON array containing JSON objects representing the
diagnostics.
The JSON is emitted as one line, without formatting; the examples below have been formatted for
clarity.
Diagnostics can have child diagnostics. For example, this error and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^
might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"message": "...this statement, but the latter is ..."
}
]
},
...
]
where the "note" is a child of the "warning".
A diagnostic has a "kind". If this is "warning", then there is an "option" key describing the
command-line option controlling the warning.
A diagnostic can contain zero or more locations. Each location has up to three positions within it:
a "caret" position and optional "start" and "finish" positions. A location can also have an optional
"label" string. For example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}
has three locations. Its primary location is at the "+" token at column 23. It has two secondary
locations, describing the left and right-hand sides of the expression, which have labels. It might
be printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"message": "invalid operands to binary + ..."
}
If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals,
similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a
replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| color
might be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"message": "\u2018struct s\u2019 has no member named ..."
}
where the fix-it hint suggests replacing the text from "start" up to but not including "next" with
"string"'s value. Deletions are expressed via an empty value for "string", insertions by having
"start" equal "next".
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are
risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of
diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting
to continue processing the source code. If n is 0 (the default), there is no limit on the number of
error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence
over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a warning is appended; for example
-Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes
-Wswitch warnings not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning.
That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the
option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply
anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying
to keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wimplicit to request
warnings on implicit declarations. Each of these specific warning options also has a negative form
beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two
forms, whichever is not the default. For further language-specific options also refer to C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on
further options, such as -Wunused-value. The combined effect of positive and negative forms is that more
specific options have priority over less specific ones, independently of their position in the command-
line. For options of the same specificity, the last one takes effect. Options enabled or disabled via
pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic
stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly
different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being
produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For example
-Wsuggest-final-types is more effective with link-time optimization and -Wmaybe-uninitialized will not
warn at all unless optimization is enabled.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden
extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare
few require -ansi or a -std option specifying the required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are supported as well. With this
option, they are rejected.
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and
end with __. Pedantic warnings are also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes; application programs should avoid
them.
Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that
it does not do quite what they want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would
require considerable additional work and would be quite different from -Wpedantic. We don't have
plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or
gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect
is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does
not make sense for such warnings to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the compiler supports with the given
option, and there would be nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where
there is undefined behavior at compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic,
since there are errors enabled by this option and not enabled by the latter and vice versa.
-Wall
This enables all the warnings about constructions that some users consider questionable, and that are
easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables
some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++
Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare -Wbool-operation -Wc++11-compat
-Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default
in C++) -Wformat -Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and
Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Winit-self (only for C++)
-Wlogical-not-parentheses -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation (only for C/C++)
-Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for
C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move (only for C++)
-Wpointer-sign -Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in C++)
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch
-Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label
-Wunused-value -Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that
users generally do not consider questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in some cases, and there is no
simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many
of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called
-W. The older name is still supported, but the newer name is more descriptive.)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only) -Wempty-body -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wredundant-move (only for
C++) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused or
-Wall)
The option -Wextra also prints warning messages for the following cases:
* A pointer is compared against integer zero with "<", "<=", ">", or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared "register".
* (C++ only) Taking the address of a variable that has been declared "register".
* (C++ only) A base class is not initialized in the copy constructor of a derived class.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often
forget that this type is signed on some machines. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is
changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile
feedback can fail to match the source file and GCC cannot use the profile feedback information. By
default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to
disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the
error for this warning can result in poorly optimized code and is useful only in the case of very
minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not
recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit
"single-precision" floating-point unit implement "float" in hardware, but emulate "double" in
software. On such a machine, doing computations using "double" values is much more expensive because
of the overhead required for software emulation.
It is easy to accidentally do computations with "double" because floating-point literals are
implicitly of type "double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because the floating-point literal is a
"double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This
warning is enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types
appropriate to the format string specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified by format attributes, in the
"printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include
all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and
GNU extensions. Other library implementations may not support all these features; GCC does not
support warning about features that go beyond a particular library's limitations. However, if
-Wpedantic is used with -Wformat, warnings are given about format features not in the selected
standard version (but not for "strfmon" formats, since those are not in any version of the C
standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since
-Wformat also checks for null format arguments for several functions, -Wformat also implies
-Wnonnull. Some aspects of this level of format checking can be disabled by the options:
-Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is
enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format
function. The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number
specifications, normally warnings are still given, since the implementation could not know what
type to pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats,
this option suppresses the warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as "sprintf" and "vsprintf" that might
overflow the destination buffer. When the exact number of bytes written by a format directive
cannot be determined at compile-time it is estimated based on heuristics that depend on the level
argument and on optimization. While enabling optimization will in most cases improve the
accuracy of the warning, it may also result in false positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns
only about calls that most likely overflow the buffer. At this level, numeric arguments to
format directives with unknown values are assumed to have the value of one, and strings of
unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of
their type, or string arguments whose output is bounded either by their directive's precision
or by a finite set of string literals, are assumed to take on the value within the range that
results in the most bytes on output. For example, the call to "sprintf" below is diagnosed
because even with both a and b equal to zero, the terminating NUL character ('\0') appended
by the function to the destination buffer will be written past its end. Increasing the size
of the buffer by a single byte is sufficient to avoid the warning, though it may not be
sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the destination buffer given an argument
of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have
the minimum representable value for signed types with a precision greater than 1, and the
maximum representable value otherwise. Unknown string arguments whose length cannot be
assumed to be bounded either by the directive's precision, or by a finite set of string
literals they may evaluate to, or the character array they may point to, are assumed to be 1
character long.
At level 2, the call in the example above is again diagnosed, but this time because with a
equal to a 32-bit "INT_MIN" the first %i directive will write some of its digits beyond the
end of the destination buffer. To make the call safe regardless of the values of the two
variables, the size of the destination buffer must be increased to at least 34 bytes. GCC
includes the minimum size of the buffer in an informational note following the warning.
An alternative to increasing the size of the destination buffer is to constrain the range of
formatted values. The maximum length of string arguments can be bounded by specifying the
precision in the format directive. When numeric arguments of format directives can be
assumed to be bounded by less than the precision of their type, choosing an appropriate
length modifier to the format specifier will reduce the required buffer size. For example,
if a and b in the example above can be assumed to be within the precision of the "short int"
type then using either the %hi format directive or casting the argument to "short" reduces
the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that
zero-length formats are allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be
checked, unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible
security problems. At present, this warns about calls to "printf" and "scanf" functions where
the format string is not a string literal and there are no format arguments, as in "printf
(foo);". This may be a security hole if the format string came from untrusted input and contains
%n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings
may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string requires an unsigned argument and the
argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as "snprintf" and "vsnprintf" that
might result in output truncation. When the exact number of bytes written by a format directive
cannot be determined at compile-time it is estimated based on heuristics that depend on the level
argument and on optimization. While enabling optimization will in most cases improve the
accuracy of the warning, it may also result in false positives. Except as noted otherwise, the
option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns
only about calls to bounded functions whose return value is unused and that will most likely
result in output truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose return value is used and that might
result in truncation given an argument of sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit
year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull"
function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull" function attribute against null inside the
function.
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing
a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is
enabled by optimizations in most targets. The precision of the warnings depends on the optimization
options used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with themselves. Note this option can only
be used with the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following snippet only when -Winit-self
has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared. In C99 mode (-std=c99 or
-std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors.
This warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the
same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case cannot fall through, e.g. when there is
a return statement or a call to function declared with the noreturn attribute.
-Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns
when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is desirable, GCC provides an attribute,
"__attribute__ ((fallthrough))", that is to be used along with a null statement to suppress this
warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using "[[fallthrough]];"
instead of the GNU attribute. In C++11 or C++14 users can use "[[gnu::fallthrough]];", which is a
GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to
silence the warning. The whole body of the C or C++ style comment should match the given regular
expressions listed below. The option argument n specifies what kind of comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other comments by "case" or "default"
keywords or by a user label that precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wif-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warning triggered by the "warn_if_not_aligned" attribute should be issued. This is
enabled by default. Use -Wno-if-not-aligned to disable it.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type
qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C prohibits qualified "void" return types on
function definitions, so such return types always receive a warning even without this option.
This warning is also enabled by -Wextra.
-Wignored-attributes (C and C++ only)
Warn when an attribute is ignored. This is different from the -Wattributes option in that it warns
whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in
a wrong place, etc. This warning is enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a function with external linkage,
returning int, taking either zero arguments, two, or three arguments of appropriate types. This
warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block structure. Specifically, a warning
is issued for "if", "else", "while", and "for" clauses with a guarded statement that does not use
braces, followed by an unguarded statement with the same indentation.
In the following example, the call to "bar" is misleadingly indented as if it were guarded by the
"if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the
statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic such as the following
example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this typically indicates autogenerated
code, and no assumptions can be made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more attributes that a related function is
declared with and whose absence may adversely affect the correctness or efficiency of generated code.
For example, the warning is issued for declarations of aliases that use attributes to specify less
restrictive requirements than those of their targets. This typically represents a potential
optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when
the alias is more restrictive than the target, which could lead to incorrect code generation.
Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc",
"nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".
In C++, the warning is issued when an explicit specialization of a primary template declared with
attribute "alloc_align", "alloc_size", "assume_aligned", "format", "format_arg", "malloc", or
"nonnull" is declared without it. Attributes "deprecated", "error", and "warning" suppress the
warning..
You can use the "copy" attribute to apply the same set of attributes to a declaration as that on
another declaration without explicitly enumerating the attributes. This attribute can be applied to
declarations of functions, variables, or types.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute
"malloc" and "alloc_size" the declaration of the explicit specialization of the template is diagnosed
because it is missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the
initializer for "a" is not fully bracketed, but that for "b" is fully bracketed. This warning is
enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wmissing-profile
Warn if feedback profiles are missing when using the -fprofile-use option. This option diagnoses
those cases where a new function or a new file is added to the user code between compiling with
-fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the
profile feedback data files do not contain any profile feedback information for the newly added
function or file respectively. Also, in the case when profile count data (.gcda) files are removed,
GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform
the user that a profile generation step is due. -Wno-missing-profile can be used to disable the
warning. Ignoring the warning can result in poorly optimized code. Completely disabling the warning
is not recommended and should be done only when non-existent profile data is justified.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded by a clause such as "if",
"else", "for", "switch", or "while", in which only the first statement is actually guarded after the
macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping
the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context
where a truth value is expected, or when operators are nested whose precedence people often get
confused about.
Also warn if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y ? 1 : 0) <= z", which
is a different interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the
condition in the "?": operator is a boolean expression, the omitted value is always 1. Often
programmers expect it to be a value computed inside the conditional expression instead.
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate
an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in
the C and C++ standards.
The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in
terms of sequence points, which represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those executed after it. These occur after
the evaluation of a full expression (one which is not part of a larger expression), after the
evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function is
called (but after the evaluation of its arguments and the expression denoting the called function),
and in certain other places. Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All these rules describe only a
partial order rather than a total order, since, for example, if two functions are called within one
expression with no sequence point between them, the order in which the functions are called is not
specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect.
Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that
"Between the previous and next sequence point an object shall have its stored value modified at most
once by the evaluation of an expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these rules, the results on any particular
implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some
more complicated cases are not diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting this sort of problem in
programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it
requires that the right-hand side of an assignment be evaluated before the left-hand side, so the
above examples are no longer undefined. But this warning will still warn about them, to help people
avoid writing code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the
sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope
after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults to "int". Also warn about any
"return" statement with no return value in a function whose return type is not "void" (falling off
the end of the function body is considered returning without a value).
For C only, warn about a "return" statement with an expression in a function whose return type is
"void", unless the expression type is also "void". As a GNU extension, the latter case is accepted
without a warning unless -Wpedantic is used. Attempting to use the return value of a non-"void"
function other than "main" that flows off the end by reaching the closing curly brace that terminates
the function is undefined.
Unlike in C, in C++, flowing off the end of a non-"void" function other than "main" results in
undefined behavior even when the value of the function is not used.
This warning is enabled by default in C++ and by -Wall otherwise.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled by default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 and C++11 modes
(and newer).
-Wshift-overflow
-Wshift-overflow=n
Warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and
newer).
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes
(and newer). This warning level does not warn about left-shifting 1 into the sign bit.
(However, in C, such an overflow is still rejected in contexts where an integer constant
expression is required.) No warning is emitted in C++2A mode (and newer), as signed left shifts
always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or
newer) is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more
of the named codes of that enumeration. (The presence of a "default" label prevents this warning.)
"case" labels outside the enumeration range also provoke warnings when this option is used (even if
there is a "default" label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more
of the named codes of that enumeration. "case" labels outside the enumeration range also provoke
warnings when this option is used. The only difference between -Wswitch and this option is that this
option gives a warning about an omitted enumeration code even if there is a "default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of boolean type and the case values are outside the
range of a boolean type. It is possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wswitch-unreachable
Warn whenever a "switch" statement contains statements between the controlling expression and the
first case label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the controlling expression and the first
case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These
functions changed semantics in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration).
This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is
unused. This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its
return value. The default is -Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its declaration. This option implies
-Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its declaration.
-Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares
variable storage, but in C++ this is not an error since const variables take the place of "#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by -Wunused-variable for C. It warns only about unused
static const variables defined in the main compilation unit, but not about static const variables
declared in any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static variables in headers (excluding system
headers). This is the warning level of -Wunused-const-variable and must be explicitly requested
since in C++ this isn't an error and in C it might be harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning
cast the unused expression to "void". This includes an expression-statement or the left-hand side of
a comma expression that contains no side effects. For example, an expression such as "x[i,j]" causes
a warning, while "x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra
-Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be
clobbered by a "setjmp" call. In C++, warn if a non-static reference or non-static "const" member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the variable in its own
initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array
variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur
for variables or elements declared "volatile". Because these warnings depend on optimization, the
exact variables or elements for which there are warnings depends on the precise optimization options
and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself
is never used, because such computations may be deleted by data flow analysis before the warnings are
printed.
-Winvalid-memory-model
Warn for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with
a memory consistency argument that is either invalid for the operation or outside the range of values
of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n"
built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the
following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic (i.e. local) variable, if there exists a path from the function entry to a use of
the variable that is initialized, but there exist some other paths for which the variable is not
initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed
at run time.
These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track
of the state of variables.
These warnings are made optional because GCC may not be able to determine when the code is correct in
spite of appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this.
To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to
"longjmp". The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be
called; in fact, a signal handler could call it at any point in the code. As a result, you may get a
warning even when there is in fact no problem because "longjmp" cannot in fact be called at the place
that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as
"noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line
option is used, warnings are even issued for unknown pragmas in system header files. This is not the
case if the warnings are only enabled by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts
between pragmas. See also -Wunknown-pragmas.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of
constructor and destructor attributes allow you to assign a priority to the constructor/destructor to
control its order of execution before "main" is called or after it returns. The priority values must
be greater than 100 as the compiler reserves priority values between 0--100 for the implementation.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code that might break
the strict aliasing rules that the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is
equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code that might break
the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to
higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to
the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many
false positives. Warns for all pointer conversions between possibly incompatible types, even if
never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as
level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only
warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the
common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled,
it also runs in the back end, where it deals with multiple statement cases using flow-sensitive
points-to information. Only warns when the converted pointer is dereferenced. Does not warn about
incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined. It warns about cases where the
compiler optimizes based on the assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only warns about cases where the compiler
implements some optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of
the variables involved are such that overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not actually a problem. To help focus on
important issues, several warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop requires, in particular when
determining whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid. For example the compiler
simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels
are not, and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: "abs
(x) >= 0". This can only be simplified when signed integer overflow is undefined, because "abs
(INT_MIN)" overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is
the same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is
simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: "(x * 10) /
5" is simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a
comparison. For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at
the highest warning level because this simplification applies to many comparisons, so this
warning level gives a very large number of false positives.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as "memcpy" and "strcpy" that are determined to
overflow the destination buffer. The optional argument is one greater than the type of Object Size
Checking to perform to determine the size of the destination. The argument is meaningful only for
functions that operate on character arrays but not for raw memory functions like "memcpy" which
always make use of Object Size type-0. The option also warns for calls that specify a size in excess
of the largest possible object or at most "SIZE_MAX / 2" bytes. The option produces the best results
with optimization enabled but can detect a small subset of simple buffer overflows even without
optimization in calls to the GCC built-in functions like "__builtin_memcpy" that correspond to the
standard functions. In any case, the option warns about just a subset of buffer overflows detected
by the corresponding overflow checking built-ins. For example, the option will issue a warning for
the "strcpy" call below because it copies at least 5 characters (the string "blue" including the
terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of
destination objects. This is the default setting of the option. At this setting the option will
not warn for writes past the end of subobjects of larger objects accessed by pointers unless the
size of the largest surrounding object is known. When the destination may be one of several
objects it is assumed to be the largest one of them. On Linux systems, when optimization is
enabled at this setting the option warns for the same code as when the "_FORTIFY_SOURCE" macro is
defined to a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of
destination objects. At this setting the option will warn about overflows when writing to
members of the largest complete objects whose exact size is known. It will, however, not warn
for excessive writes to the same members of unknown objects referenced by pointers since they may
point to arrays containing unknown numbers of elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of
destination objects. At this setting the option warns about overflowing the smallest object or
data member. This is the most restrictive setting of the option that may result in warnings for
safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of
destination objects. At this setting the option will warn about overflowing any data members,
and when the destination is one of several objects it uses the size of the largest of them to
decide whether to issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option
may result in warnings for benign code.
-Wstringop-truncation
Warn for calls to bounded string manipulation functions such as "strncat", "strncpy", and "stpncpy"
that may either truncate the copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less than the length of the
source string. As a result, the copy of the source will be truncated and so the call is diagnosed.
To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results in copying to "d" just the characters
preceding the terminating NUL, without appending the NUL to the end. Assuming the result of
"strncpy" is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed.
To avoid the warning when the result is not expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the size of the destination buffer as the
bound. If the length of the source string is equal to or greater than this size the result of the
copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning,
specify "sizeof buf - 1" as the bound and set the last element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a sequence of bytes with no terminating
"NUL" such an array may be annotated with attribute "nonstring" to avoid this warning. Such arrays,
however, are not suitable arguments to functions that expect "NUL"-terminated strings. To help
detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is
safe.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
Warn for cases where adding an attribute may be beneficial. The attributes currently supported are
listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for attributes "pure", "const" or "noreturn" or
"malloc". The compiler only warns for functions visible in other compilation units or (in the
case of "pure" and "const") if it cannot prove that the function returns normally. A function
returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling
"abort" or trapping. This analysis requires option -fipa-pure-const, which is enabled by default
at -O and higher. Higher optimization levels improve the accuracy of the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format" attributes. Note these are
only possible candidates, not absolute ones. GCC guesses that function pointers with "format"
attributes that are used in assignment, initialization, parameter passing or return statements
should have a corresponding "format" attribute in the resulting type. I.e. the left-hand side of
the assignment or initialization, the type of the parameter variable, or the return type of the
containing function respectively should also have a "format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for "format" attributes.
Again, these are only possible candidates. GCC guesses that "format" attributes might be
appropriate for any function that calls a function like "vprintf" or "vscanf", but this might not
always be the case, and some functions for which "format" attributes are appropriate may not be
detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold" attribute. This is based on static
detection and generally will only warn about functions which always leads to a call to another
"cold" function such as wrappers of C++ "throw" or fatal error reporting functions leading to
"abort".
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be improved if the type were declared
with the C++11 "final" specifier, or, if possible, declared in an anonymous namespace. This allows
GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link
time optimization, where the information about the class hierarchy graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if the method were declared with the
C++11 "final" specifier, or, if possible, its type were declared in an anonymous namespace or with
the "final" specifier. This warning is more effective with link-time optimization, where the
information about the class hierarchy graph is more complete. It is recommended to first consider
suggestions of -Wsuggest-final-types and then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with the override keyword.
-Walloc-zero
Warn about calls to allocation functions decorated with attribute "alloc_size" that specify zero
bytes, including those to the built-in forms of the functions "aligned_alloc", "alloca", "calloc",
"malloc", and "realloc". Because the behavior of these functions when called with a zero size
differs among implementations (and in the case of "realloc" has been deprecated) relying on it may
result in subtle portability bugs and should be avoided.
-Walloc-size-larger-than=byte-size
Warn about calls to functions decorated with attribute "alloc_size" that attempt to allocate objects
larger than the specified number of bytes, or where the result of the size computation in an integer
type with infinite precision would exceed the value of PTRDIFF_MAX on the target.
-Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be
disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disable -Walloc-size-larger-than= warnings. The option is equivalent to
-Walloc-size-larger-than=SIZE_MAX or larger.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=byte-size
This option warns on calls to "alloca" with an integer argument whose value is either zero, or that
is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns
for calls to "alloca" where the bound value is unknown. Arguments of non-integer types are
considered unbounded even if they appear to be constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would not issue a warning because the call
to "alloca" is known to be at most 1000 bytes. However, if "-Walloca-larger-than=500" were passed,
the compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with no controlling predicate constraining
its integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would trigger a warning, but this time because
of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than expected argument to be implicitly
cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp
is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than=SIZE_MAX or
larger.
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about
subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and
must be explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds access for arrays at the end of a struct and
for arrays accessed through pointers. This warning level may give a larger number of false
positives and is deactivated by default.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar attributes whose target is incompatible with
the type of the alias.
-Wattribute-alias=1
The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the
type of the alias declaration and that of its target. Such incompatibilities are typically
indicative of bugs.
-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases where the attributes of the alias
declaration are more restrictive than the attributes applied to its target. These mismatches can
potentially result in incorrect code generation. In other cases they may be benign and could be
resolved simply by adding the missing attribute to the target. For comparison, see the
-Wmissing-attributes option, which controls diagnostics when the alias declaration is less
restrictive than the target, rather than more restrictive.
Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf",
"malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these
warnings with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbool-compare
Warn about boolean expression compared with an integer value different from "true"/"false". For
instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of
a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or
decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement. This warning also warn for
conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater
than 0. Such calls may return indeterminate values or crash the program. The warning is included in
-Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a
"const char *" variable is passed to a function that takes a "char *" parameter. This option can be
used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically,
the compiler warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]"
parameter. This option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have incompatible types. This warning
is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment
with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning
is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero is not
warned about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are
normally suppressed, on the assumption that they usually do not indicate real problems and would only
make the compiler output harder to read. Using this command-line option tells GCC to emit warnings
from system headers as if they occurred in user code. However, note that using -Wall in conjunction
with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas
must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes
such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always evaluate to true or false, for
instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of
data or code that is created at run time on the stack when the address of a nested function is taken,
and is used to call the nested function indirectly. For some targets, it is made up of data only and
thus requires no special treatment. But, for most targets, it is made up of code and thus requires
the stack to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-
point values as approximations to infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error
that the computation introduces, and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of testing for equality, you should
check to see whether the two values have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO
C constructs that have no traditional C equivalent, and/or problematic constructs that should be
avoided.
* Macro parameters that appear within string literals in the macro body. In traditional C macro
replacement takes place within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist. Traditional preprocessors only
considered a line to be a directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C understands but ignores because the #
does not appear as the first character on the line. It also suggests you hide directives like
"#pragma" not understood by traditional C by indenting them. Some traditional implementations do
not recognize "#elif", so this option suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C
does support the L suffix on integer constants.) Note, these suffixes appear in macros defined
in the system headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of
these macros in user code might normally lead to spurious warnings, however GCC's integrated
preprocessor has enough context to avoid warning in these cases.
* A function declared external in one block and then used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct is not accepted by
some traditional C compilers.
* The ISO type of an integer constant has a different width or signedness from its traditional
type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal
values, which typically represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
* Initialization of unions. If the initializer is zero, the warning is omitted. This is done
under the assumption that the zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in
the traditional C case.
* Conversions by prototypes between fixed/floating-point values and vice versa. The absence of
these prototypes when compiling with traditional C causes serious problems. This is a subset of
the possible conversion warnings; for the full set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This warning intentionally is not issued for prototype
declarations or variadic functions because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also
bypassed for nested functions because that feature is already a GCC extension and thus not
relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to the same
argument in the absence of a prototype. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a fixed-point argument except when
the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was
introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter, type, class
member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed.
Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it
shadows a struct/class/enum. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in an Objective-C method.
-Wshadow=global
The default for -Wshadow. Warns for any (global) shadowing.
-Wshadow=local
Warn when a local variable shadows another local variable or parameter. This warning is enabled by
-Wshadow=global.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter whose type is compatible with
that of the shadowing variable. In C++, type compatibility here means the type of the shadowing
variable can be converted to that of the shadowed variable. The creation of this flag (in addition to
-Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible
type, it is most likely intentional, not a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible types, enabling only
-Wshadow=compatible-local will not emit a warning. Because their types are incompatible, if a
programmer accidentally uses one in place of the other, type checking will catch that and emit an
error or warning. So not warning (about shadowing) in this case will not lead to undetected bugs. Use
of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by
intentional shadowing.
This warning is enabled by -Wshadow=local.
-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is
enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size
of SIZE_MAX or more or by -Wno-larger-than.
-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.
-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack
frame size is approximate and not conservative. The actual requirements may be somewhat greater than
byte-size even if you do not get a warning. In addition, any space allocated via "alloca", variable-
length arrays, or related constructs is not included by the compiler when determining whether or not
to issue a warning. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by
the option can be disabled either by specifying byte-size of SIZE_MAX or more or by
-Wno-frame-larger-than.
-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or
larger.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not allocated on the heap.
-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size. The computation done to determine the
stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related
constructs is included by the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled
either by specifying byte-size of SIZE_MAX or more or by -Wno-stack-usage.
-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the
loop indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the
warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on
Windows targets, which depend on the MS runtime.
-Waligned-new
Warn about a new-expression of a type that requires greater alignment than the
"alignof(std::max_align_t)" but uses an allocation function without an explicit alignment parameter.
This option is enabled by -Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about
class member allocation functions.
-Wplacement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, such as constructing an object in a
buffer that is smaller than the type of the object. For example, the placement new expression below
is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes
large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At this level the warning is not issued
for some strictly undefined constructs that GCC allows as extensions for compatibility with
legacy code. For example, the following "new" expression is not diagnosed at this level even
though it has undefined behavior according to the C++ standard because it writes past the end of
the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is
also issued for placement new expressions that construct an object in the last member of
structure whose type is an array of a single element and whose size is less than the size of the
object being constructed. While the previous example would be diagnosed, the following construct
makes use of the flexible member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these
types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions.
In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by
-Wpedantic.
-Wpointer-compare
Warn if a pointer is compared with a zero character constant. This usually means that the pointer
was meant to be dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do
not warn for constant expressions. For example, warn if an unsigned variable is compared against
zero with "<" or ">=". This warning is also enabled by -Wextra.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute value of an argument when a more
appropriate standard function is available. For example, calling "abs(3.14)" triggers the warning
because the appropriate function to call to compute the absolute value of a double argument is
"fabs". The option also triggers warnings when the argument in a call to such a function has an
unsigned type. This warning can be suppressed with an explicit type cast and it is also enabled by
-Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline
appears in a // comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within
comments are not warned about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless
trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive. Such identifiers are replaced
with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro (including the case where the macro
is expanded by an #if directive). Such usage is not portable. This warning is also enabled by
-Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or
tested for existence at least once. The preprocessor also warns if the macro has not been used at
the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not
warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor
reports it as unused. To avoid the warning in such a case, you might improve the scope of the
macro's definition by, for example, moving it into the first skipped block. Alternatively, you could
provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by text. This sometimes happens in older
programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For example, warn if a call to a function
returning an integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of
variable length arrays, "long long" type, "bool" type, compound literals, designated initializers,
and so on. This option is independent of the standards mode. Warnings are disabled in the
expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of
anonymous structures and unions, "_Atomic" type qualifier, "_Thread_local" storage-class specifier,
"_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and so on. This option is independent
of the standards mode. Warnings are disabled in the expression that follows "__extension__".
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C2X. For instance, warn about
omitting the string in "_Static_assert". This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request
for implicit conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g.,
identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and
is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning
is enabled by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning
is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example,
warn if a "const char *" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting
"char **" to "const char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For
example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the target is increased. For
example, warn if a "char *" is cast to an "int *" regardless of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function pointer. In a cast involving
function types with a variable argument list only the types of initial arguments that are provided
are considered. Any parameter of pointer-type matches any other pointer-type. Any benign
differences in integral types are ignored, like "int" vs. "long" on ILP32 targets. Likewise type
qualifiers are ignored. The function type "void (*) (void)" is special and matches everything, which
can be used to suppress this warning. In a cast involving pointer to member types this warning warns
whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of
one into a non-"const" "char *" pointer produces a warning. These warnings help you find at compile
time code that can try to write into a string constant, but only if you have been very careful about
using "const" in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did
not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This
warning is enabled by default for C++ programs.
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value
for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn
about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are
not caught by reference. -Wcatch-value is enabled by -Wall.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork". This warning is also enabled by
-Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and
integer, like "abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned
ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like
"abs ((int) x)" and "ui = (unsigned) -1", or if the value is not changed by the conversion like in
"abs (2.0)". Warnings about conversions between signed and unsigned integers can be disabled by
using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions
that never use a type conversion operator: conversions to "void", the same type, a base class or a
reference to them. Warnings about conversions between signed and unsigned integers are disabled by
default in C++ unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by
default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This can be useful to facilitate the
conversion to "nullptr" in C++11.
-Wsubobject-linkage (C++ and Objective-C++ only)
Warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a
type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in
multiple translation units would be an ODR violation because the meaning of B is different in each
translation unit. If A only appears in a single translation unit, the best way to silence the
warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler
doesn't give this warning for types defined in the main .C file, as those are unlikely to have
multiple definitions. -Wsubobject-linkage is enabled by default.
-Wdangling-else
Warn about constructions where there may be confusion to which "if" statement an "else" branch
belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in this example
is "if (b)". This is often not what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this confusion, GCC issues a
warning when this flag is specified. To eliminate the warning, add explicit braces around the
innermost "if" statement so there is no way the "else" can belong to the enclosing "if". The
resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might prevent bit-
wise-identical reproducible compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This
warning is enabled by default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while" statement. This warning is also
enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ enumerated type
mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C
this warning is enabled by -Wall.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolon after in-class function definition.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a
variable, or jumps backward to a label after the variable has been initialized. This only warns
about variables that are initialized when they are declared. This warning is only supported for C
and Objective-C; in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init
option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the
signed value is converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it is
also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like assigning a signed
integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this
option is enabled also by -Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real value. This includes conversions
from real to integer, and from higher precision real to lower precision real values. This option is
also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element
size, which is the usual way to compute the array size but won't work out correctly with pointers.
This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])" if "ptr" is not an array, but a
pointer. This warning is enabled by -Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions if the argument
uses "sizeof". This warning triggers for example for "memset (ptr, 0, sizeof (ptr));" if "ptr" is
not an array, but a pointer, and suggests a possible fix, or about "memcpy (&foo, ptr, sizeof
(&foo));". -Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like
"strncat" or "strncpy" that specify as the bound a "sizeof" expression of the source array. For
example, in the following function the call to "strncat" specifies the size of the source string as
the bound. That is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a parameter that is declared as an array in a function
definition. This warning is enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the first argument references an
array, and the third argument is a number equal to the number of elements, but not equal to the size
of the array in memory. This indicates that the user has omitted a multiplication by the element
size. This warning is enabled by -Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function where the second argument is not zero and
the third argument is zero. For example, the call "memset (buf, sizeof buf, 0)" is diagnosed because
"memset (buf, 0, sizeof buf)" was meant instead. The diagnostic is only emitted if the third
argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of
zero to some type, it is far less likely that the arguments have been mistakenly transposed and no
warning is emitted. This warning is enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include using the address of a function in a
conditional expression, such as "void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer
error: the address of a function always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function call; and comparisons against
string literals result in unspecified behavior and are not portable in C, so they usually indicate
that the programmer intended to use "strcmp". This warning is enabled by -Wall.
-Waddress-of-packed-member
Warn when the address of packed member of struct or union is taken, which usually results in an
unaligned pointer value. This is enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical
operators in contexts where a bit-wise operator is likely to be expected. Also warns when the
operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison. This option does not warn
if the right operand is considered to be a boolean expression. Its purpose is to detect suspicious
code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where
you can return an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some
statement during one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function
attributes applied to variables, etc. This does not stop errors for incorrect use of supported
attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible signature or as a non-function, or when
a built-in function declared with a type that does not include a prototype is called with arguments
whose promoted types do not match those expected by the function. When -Wextra is specified, also
warn when a built-in function that takes arguments is declared without a prototype. The
-Wno-builtin-declaration-mismatch warning is enabled by default. To avoid the warning include the
appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to "memset" below is diagnosed by the warning because the function expects a
value of type "size_t" as its argument but the type of 32 is "int". With -Wextra, the declaration of
the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of
"__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style
function definition is permitted without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if
storage-class specifiers like "static" are not the first things in a declaration. This warning is
also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous
prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration. This warning is
issued even if the definition itself provides a prototype. Use this option to detect global
functions that do not have a matching prototype declaration in a header file. This option is not
valid for C++ because all function declarations provide prototypes and a non-matching declaration
declares an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to
detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the definition
itself provides a prototype. Use this option to detect global functions that are not declared in
header files. In C, no warnings are issued for functions with previous non-prototype declarations;
use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function
templates, or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following code causes
such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification does not
trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the
user's code, as they have implementation-defined values, and should not be used in portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters.
However, sometimes when characters outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets
out some normalization rules which when applied ensure that two sequences that look the same are
turned into the same sequence. GCC can warn you if you are using identifiers that have not been
normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns
about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when
turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning
for these characters. It is hoped that future versions of the standards involved will correct this,
which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized.
You should only do this if you are using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display
methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", displays just like a regular "n" that has been placed in a superscript. ISO
10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and
GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to
warning about every identifier that contains the letter O because it might be confused with the digit
0, and so is not the default, but may be useful as a local coding convention if the programming
environment cannot be fixed to display these characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions declared with "warning" attribute. By default, this warning is
enabled. -Wno-attribute-warning can be used to disable the warning or -Wno-error=attribute-warning
can be used to disable the error when compiled with -Werror flag.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the
"deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time optimization. Requires
-flto-odr-type-merging to be enabled. Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-override-init.
-Woverride-init-side-effects (C and Objective-C only)
Warn if an initialized field with side effects is overridden when using designated initializers.
This warning is enabled by default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the
layout or size of the structure. Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar"
does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This
has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a
4-bit padding between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a packed struct or union is
misaligned. For example, a warning will be issued on "struct S", like, "warning: alignment 1 of
'struct S' is less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align
the whole structure. Sometimes when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple
declaration is valid and changes nothing.
-Wno-restrict
Warn when an object referenced by a "restrict"-qualified parameter (or, in C++, a
"__restrict"-qualified parameter) is aliased by another argument, or when copies between such objects
overlap. For example, the call to the "strcpy" function below attempts to truncate the string by
replacing its initial characters with the last four. However, because the call writes the
terminating NUL into "a[4]", the copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple overlap even without optimization but works
best at -O2 and above. It is included in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a
C variadic constructor; the warning is on by default because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler
does not warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For
example, the compiler takes into account the size of the function being inlined and the amount of
inlining that has already been done in the current function. Therefore, seemingly insignificant
changes in the source program can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD type. According to the 2014 ISO
C++ standard, applying "offsetof" to a non-standard-layout type is undefined. In existing C++
implementations, however, "offsetof" typically gives meaningful results. This flag is for users who
are aware that they are writing nonportable code and who have deliberately chosen to ignore the
warning about it.
The restrictions on "offsetof" may be relaxed in a future version of the C++ standard.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are expected, such as conditional
expressions (?:) using non-boolean integer constants in boolean context, like "if (a <= b ? 2 : 3)".
Or left shifting of signed integers in boolean context, like "for (a = 0; 1 << a; a++);". Likewise
for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a
pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90
and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99
mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use
-Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle variable arguments like "va_start". This
is default. To inhibit the warning messages, use -Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful
for the performance tuning. Vector operation can be implemented "piecewise", which means that the
scalar operation is performed on every vector element; "in parallel", which means that the vector
operation is implemented using scalars of wider type, which normally is more performance efficient;
and "as a single scalar", which means that vector fits into a scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment
operator. This is dangerous because if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the moved-from state. If the move
assignment operator is written to avoid moving from a moved-from object, this warning can be
disabled.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the
variable-length array.
-Wvla-larger-than=byte-size
If this option is used, the compiler will warn for declarations of variable-length arrays whose size
is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes.
This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this
warning may not get triggered for such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp
is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or
larger.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all
optimizations that may eliminate reads and/or writes to register variables. This warning is enabled
by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that
there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to
handle the code effectively. Often, the problem is that your code is too big or too complex; GCC
refuses to optimize programs when the optimization itself is likely to take inordinate amounts of
time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness. This option is only
supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled
with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that are not
protected against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified in the C
standard. Modern compilers generally allow string constants that are much longer than the standard's
minimum limit, but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90,
the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When used together with
-Wsystem-headers it warns about such constants in system header files. This can be useful when
preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point
extension to C99.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize a structure that has been
marked with the "designated_init" attribute.
-Whsa
Issue a warning when HSAIL cannot be emitted for the compiled function or OpenMP construct.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g
to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising:
some variables you declared may not exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they compute constant results or their values are
already at hand; some statements may execute in different places because they have been moved out of
loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og with -g. With no -O option at
all, some compiler passes that collect information useful for debugging do not run at all, so that -Og
may result in a better debugging experience.
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF).
GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB
can use; this extra information makes debugging work better in GDB but probably makes other debuggers
crash or refuse to read the program. If you want to control for certain whether to generate the
extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if
at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The value of version may be
either 2, 3, 4 or 5; the default version for most targets is 4. DWARF Version 5 is only
experimental.
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3
extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later.
For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a
reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This
is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this
option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems
this option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions understood
only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the
DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood
only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash
or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail
with an error.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format
used by DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level
is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you
don't plan to debug. This includes descriptions of functions and external variables, and line number
tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some
debuggers support macro expansion when you use -g3.
If you use multiple -g options, with or without level numbers, the last such option is the one that
is effective.
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead
use an additional -glevel option to change the debug level for DWARF.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only symbols that are
actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object file, emit it in all
object files using the class. This option should be used only with debuggers that are unable to
handle the way GCC normally emits debugging information for classes because using this option
increases the size of debugging information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are identical in
different object files. Merging is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of increasing link processing time.
Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging information describing them as if
the files resided in directory new instead. This can be used to replace a build-time path with an
install-time path in the debug info. It can also be used to change an absolute path to a relative
path by using . for new. This can give more reproducible builds, which are location independent, but
may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better
debugging information is then generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information
(-g) and the debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations
over throughout the compilation all the way to the end, in an attempt to improve debug information
while optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and
maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking,
except when selective scheduling is enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible into a separate output file with the
extension .dwo. This option allows the build system to avoid linking files with debug information.
To be useful, this option requires a debugger capable of reading .dwo files.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name attribute, such as artificial
variables, external references and call site parameter DIEs.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a
GDB index. This option is only useful with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section
instead of making them part of the ".debug_info" section. It is more efficient to put them in a
separate comdat section since the linker can then remove duplicates. But not all DWARF consumers
support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of
smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code
generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each other and from the compiler version.
It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options
into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On
most targets using non-conflicting DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives. It may then use them for the
assembler to generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact
than those the compiler can generate itself.
This option will be enabled by default if, at GCC configure time, the assembler was found to support
such directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be
generated.
gas-locview-support
Inform the compiler that the assembler supports "view" assignment and reset assertion checking in
".loc" directives.
This option will be enabled by default if, at GCC configure time, the assembler was found to support
them.
gno-as-locview-support
Force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information, rather than just file and line.
This option is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal representation at the beginning of
statements, and to keep them roughly in place throughout compilation, using them to guide the output
of "is_stmt" markers in the line number table. This is enabled by default when compiling with
optimization (-Os, -O, -O2, ...), and outputting DWARF 2 debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied from the line number table.
This enables debug information consumers to inspect state at certain points of the program, even if
no instructions associated with the corresponding source locations are present at that point. If the
assembler lacks support for view numbers in line number tables, this will cause the compiler to emit
the line number table, which generally makes them somewhat less compact. The augmented line number
tables and location lists are fully backward-compatible, so they can be consumed by debug information
consumers that are not aware of these augmentations, but they won't derive any benefit from them
either.
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as
there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When
assembler support is not available, this may still be enabled, but it will force GCC to output
internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most
certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not backward compatible with the location
list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5.
This option may be removed in the future, is only provided as a reference implementation of the
proposed representation. Debug information consumers are not expected to support this extended
format, and they would be rendered unable to decode location lists using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location view lists. This requires the
compiler to have very accurate insn length estimates, which isn't always the case, and it may cause
incorrect view lists to be generated silently when using an assembler that does not support location
view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only
enabled on ports that define a reliable estimation function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions. Location view tracking markers are
inserted at inlined entry points, so that address and view numbers can be computed and output in
debug information. This can be enabled independently of location views, in which case the view
numbers won't be output, but it can only be enabled along with statement frontiers, and it is only
enabled by default if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is supported. If type is not given, the
default type depends on the capabilities of the assembler and linker used. type may be one of none
(don't compress debug sections), zlib (use zlib compression in ELF gABI format), or zlib-gnu (use
zlib compression in traditional GNU format). If the linker doesn't support writing compressed debug
sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently
ignored when producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the struct is defined.
This option substantially reduces the size of debugging information, but at significant potential
loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive
option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the type is defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging information, with some potential loss in type
information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug information. The intent is to
reduce duplicate struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly,
which serves for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used directly (dir:) or used
indirectly (ind:). A struct type is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid,
the use is indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs
(gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes within the above. Other programming
languages have generics, but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug
information. The values none and any have the normal meaning. The value base means that the base of
name of the file in which the type declaration appears must match the base of the name of the main
compilation file. In practice, this means that when compiling foo.c, debug information is generated
for types declared in that file and foo.h, but not other header files. The value sys means those
types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*"
directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are
nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation unit, regardless of whether or not they are
actually used in that compilation unit, for example if, in the debugger, you want to cast a value to
a type that is not actually used in your program (but is declared). More often, however, this
results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make
debugging produce the expected results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any variable or change the program
counter to any other statement in the function and get exactly the results you expect from the source
code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at
the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple
files at once to a single output file mode allows the compiler to use information gained from all of the
files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed
in this section.
Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even
if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be
enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find
out the exact set of optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large
function.
With -O, the compiler tries to reduce code size and execution time, without performing any
optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce
-fdefer-pop -fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fmerge-constants -fmove-loop-invariants -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt
-ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-pta
-ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-
speed tradeoff. As compared to -O, this option increases both compilation time and the performance
of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization
flags:
-falign-functions -falign-jumps -falign-labels -falign-loops -fcaller-saves -fcode-hoisting
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra
-fipa-vrp -fisolate-erroneous-paths-dereference -flra-remat -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing -fthread-jumps
-ftree-builtin-call-dce -ftree-pre -ftree-switch-conversion -ftree-tail-merge -ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2 is set by default, and is activated when
-O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc
functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following
optimization flags:
-fgcse-after-reload -finline-functions -fipa-cp-clone -floop-interchange -floop-unroll-and-jam
-fpeel-loops -fpredictive-commoning -fsplit-paths -ftree-loop-distribute-patterns
-ftree-loop-distribution -ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model -fversion-loops-for-strides
-O0 Reduce compilation time and make debugging produce the expected results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops -fprefetch-loop-arrays
-freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to tune for code size rather than execution
speed, and performs further optimizations designed to reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables
optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and
the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and
-fno-protect-parens.
-Og Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-
compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation
and a good debugging experience. It is a better choice than -O0 for producing debuggable code
because some compiler passes that collect debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization passes so that individual options
controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those
that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fif-conversion -fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp -ftree-pta -ftree-sra
If you use multiple -O options, with or without level numbers, the last such option is the one that is
effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative
forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the
one you typically use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are
related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
-fno-defer-pop
For machines that must pop arguments after a function call, always pop the arguments as soon as each
function returns. At levels -O1 and higher, -fdefer-pop is the default; this allows the compiler to
let arguments accumulate on the stack for several function calls and pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if
the result can be simplified. If loop unrolling is active, two passes are performed and the second
is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables
floating-point expression contraction such as forming of fused multiply-add operations if the target
has native support for them. -ffp-contract=on enables floating-point expression contraction if
allowed by the language standard. This is currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set
up and restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame
pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions.
Several targets always omit the frame pointer in leaf functions.
Enabled by default at -O and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen", "strchr" or "strcpy") and their
"_FORTIFY_SOURCE" counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those marked with the "always_inline" attribute. This
is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function call code
(so overall size of program gets smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining applies to all functions, even
those not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous
inlining. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler
heuristically decides which functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the
function is normally not output as assembler code in its own right.
Enabled at levels -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not
marked "inline". If a call to a given function is integrated, then the function is not output as
assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function
call overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs having large chains of
nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and
replacement of parameters passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of
this limit. n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by
using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults
of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of
function's size. In no way does it represent a count of assembly instructions and as such its exact
meaning might change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that
are declared using the "dllexport" attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function
has been inlined into all of its callers. This switch does not affect functions using the "extern
inline" extension in GNU C90. In C++, emit any and all inline functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the function is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables
aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is
referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts
option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across
compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even
constant initialized arrays or initialized constant variables with integral or floating-point types.
Languages like C or C++ require each variable, including multiple instances of the same variable in
recursive calls, to have distinct locations, so using this option results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at
innermost loops and reorders their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this
flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on
the life-range analysis. This option is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use "decrement and branch" instructions
on a count register instead of instruction sequences that decrement a register, compare it against
zero, and then branch based upon the result. This option is only meaningful on architectures that
support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the
-fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated
instruction stream introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function
contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output
may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into
BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the
data section---e.g., so that the resulting executable can find the beginning of that section and/or
make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where another comparison
subsumed by the first is found. If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on whether the condition is known to
be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the
registers apart and allocate them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump
is not reached by any other path. For example, when CSE encounters an "if" statement with an "else"
clause, CSE follows the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over
blocks. When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes
CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and
copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time
performance if you disable the global common subexpression elimination pass by adding -fno-gcse to
the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are
only killed by stores into themselves. This allows a loop containing a load/store sequence to be
changed to a load outside the loop, and a copy/store within the loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination.
This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops
containing a load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant
loads that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload.
The purpose of this pass is to clean up redundant spilling.
Enabled by -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive bounds for the number of
iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The bounds for the number of
iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations.
This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be
overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing
the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and saves code
size. The resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on
architectures that do not have instructions to support this. Enabled by default at -O and higher on
architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This includes use of
conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it is available is controlled by
-fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less
equivalents.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and destructors: one for a base
subobject, one for a complete object, and one for a virtual destructor that calls operator delete
afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which
means two copies of the function. With this option, the base and complete variants are changed to be
thunks that call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data element
resides at address zero. This option enables simple constant folding optimizations at all
optimization levels. In addition, other optimization passes in GCC use this flag to control global
dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access
to address zero always results in a trap, so that if a pointer is checked after it has already been
dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use
-fno-delete-null-pointer-checks to disable this optimization for programs that depend on that
behavior.
This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR,
CR16, and MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure
and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant
propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of
the type inheritance graph, determine for a given call the set of likely targets. If the set is
small, preferably of size 1, change the call into a conditional deciding between direct and indirect
calls. The speculative calls enable more optimizations, such as inlining. When they seem useless
after further optimization, they are converted back into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running the link-time optimizer
in local transformation mode. This option enables more devirtualization but significantly increases
the size of streamed data. For this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64
architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit
half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within its lifetime: when the constructor
begins, the object has an indeterminate value, and any changes during the lifetime of the object are
dead when the object is destroyed. Normally dead store elimination will take advantage of this; if
your code relies on the value of the object storage persisting beyond the lifetime of the object, you
can use this flag to disable this optimization. To preserve stores before the constructor starts
(e.g. because your operator new clears the object storage) but still treat the object as dead after
the destructor you, can use -flifetime-dse=1. The default behavior can be explicitly selected with
-flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range shrinkage. This is helpful for
fast processors with small or moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument
can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs
coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets
that do support it, it is the default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The region argument should be one of
the following:
all Use all loops as register allocation regions. This can give the best results for machines with a
small and/or irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions. This value usually
gives the best results in most cases and for most architectures, and is enabled by default when
compiling with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the smallest code size, and is
enabled by default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions.
This option usually results in smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option
usually results in generation of faster and smaller code on machines with large register files (>= 32
registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each
hard register gets a separate stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not
get a hard register gets a separate stack slot, and as a result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA
tries to rematerialize (recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots
available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os, but not at -Og.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls
due to required data being unavailable. This helps machines that have slow floating point or memory
load instructions by allowing other instructions to be issued until the result of the load or
floating-point instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register
allocation has been done. This is especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is normally enabled when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is normally enabled when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation. This only makes sense
when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or
higher. Usage of this option can improve the generated code and decrease its size by preventing
register pressure increase above the number of available hard registers and subsequent spills in
register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the
ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved
prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved
prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a
candidate for premature removal from the queue of stalled insns. This has an effect only during the
second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is
equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent
to -fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This allows motion across
basic block boundaries, resulting in faster schedules. This option is experimental, as not all
machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the
algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at
-O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a
schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the
critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative
instructions with greater dependency weakness. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a
basic block with greater size or frequency. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that
is less dependent on the last instruction scheduled. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that
has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later
scheduling passes may change its schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the
first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the
second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling. This option has no effect
unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops. This option has no
effect unless -fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that
for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining
and other optimizations in anticipation that the function or variable in question may change. While
this feature is useful, for example, to rewrite memory allocation functions by a debugging
implementation, it is expensive in the terms of code quality. With -fno-semantic-interposition the
compiler assumes that if interposition happens for functions the overwriting function will have
precisely the same semantics (and side effects). Similarly if interposition happens for variables,
the constructor of the variable will be the same. The flag has no effect for functions explicitly
declared inline (where it is never allowed for interposition to change semantics) and for symbols
explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the
function. This flag is enabled by default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts are only
executed when needed. This option is on by default, but has no effect unless -fshrink-wrap is also
turned on and the target supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls, by emitting extra
instructions to save and restore the registers around such calls. Such allocation is done only when
it seems to result in better code.
This option is always enabled by default on certain machines, usually those which have no call-
preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to
combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are not used by any called function. In
that case it is not necessary to save and restore them around calls. This is only possible if called
functions are part of same compilation unit as current function and they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be
instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this
happens on targets that do not expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes
the program slower. This option implies setting the large-stack-frame parameter to 100 and the
large-stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all
paths to the function exit as early as possible. This is especially useful as a code size
optimization, but it often helps for code speed as well. This flag is enabled by default at -O2 and
higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and
-O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE
only considers expressions that are computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default
at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O
and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent
locations in the same structure and the target architecture has a conditional move instruction. This
flag is enabled by default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is
enabled by default at -O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O and higher.
-fipa-reference
Discover which static variables do not escape the compilation unit. Enabled by default at -O and
higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables. Enabled by default at -O and
higher.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference analysis.
This option can cause excessive memory and compile-time usage on large compilation units. It is not
enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are
marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or
destructors) are identified. Cold functions and loop less parts of functions executed once are then
optimized for size. Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to determine
when values passed to functions are constants and then optimizes accordingly. This optimization can
substantially increase performance if the application has constants passed to functions. This flag
is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When enabled,
interprocedural constant propagation performs function cloning when externally visible function can
be called with constant arguments. Because this optimization can create multiple copies of
functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag
is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default
at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at
-O2. It requires that -fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only variables. The optimization reduces code
size and may disturb unwind stacks by replacing a function by equivalent one with a different name.
The optimization works more effectively with link-time optimization enabled.
Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different
levels and thus the optimizations are not same - there are equivalences that are found only by GCC
and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for live-patching.
If the compiler's optimization uses a function's body or information extracted from its body to
optimize/change another function, the latter is called an impacted function of the former. If a
function is patched, its impacted functions should be patched too.
The impacted functions are determined by the compiler's interprocedural optimizations. For example,
a caller is impacted when inlining a function into its caller, cloning a function and changing its
caller to call this new clone, or extracting a function's pureness/constness information to optimize
its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each
function. In order to control the number of impacted functions and more easily compute the list of
impacted function, IPA optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural
scalar replacement of aggregates and partial inlining. As a result, when patching a function,
all its callers and its clones' callers are impacted, therefore need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program
-fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables
-fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment
inline-only-static
Only enable inlining of static functions. As a result, when patching a static function, all its
callers are impacted and so need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone disables,
-flive-patching=inline-only-static disables the following additional optimization flags:
-fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer.
Isolate those paths from the main control flow and turn the statement with erroneous or undefined
behavior into a trap. This flag is enabled by default at -O2 and higher and depends on
-fdelete-null-pointer-checks also being enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way
forbidden by a "returns_nonnull" or "nonnull" attribute. Isolate those paths from the main control
flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently
enabled, but may be enabled by -O2 in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment
information. This pass only operates on local scalar variables and is enabled by default at -O1 and
higher, except for -Og. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local
scalar variables and is enabled by default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain in order to simplify the
definitions. For example, this pass strips sign operations if the sign of a value never matters.
The flag is enabled by default at -O and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by
default at -O1 and higher, except for -Og.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array.
This flag is enabled by default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other. This
optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and
higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter
and max-tail-merge-iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions that may set "errno"
but are otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is
not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range
propagation and expression simplification) based on a dominator tree traversal. This also performs
jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that
is later overwritten by another store without any intervening loads. In this case the earlier store
can be deleted. This flag is enabled by default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code
motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It
is not enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC
has to be configured with --with-isl to enable the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral
representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also
performed by the code generator isl, like index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto
optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism.
This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize
all the loops that can be analyzed to not contain loop carried dependences without checking that it
is profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to reduce copying by coalescing
versions of different user-defined variables, instead of just compiler temporaries. This may
severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments.
In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by
default if optimization is enabled, and it does very little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent
is to remove control-flow from the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow
further loop optimizations, like parallelization or vectorization, to take place. For example, the
loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library. This flag
is enabled by default at -O3, and by -fprofile-use and -fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero. For example, the
loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero. This flag is enabled by
default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-interchange
Perform loop interchange outside of graphite. This flag can improve cache performance on loop nest
and allow further loop optimizations, like vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop
by some factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3.
It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at
RTL level (function calls, operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we
can use just trivial invariantness analysis in loop unswitching. The pass also includes store
motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of
iterations requires complicated analysis. Later optimizations then may determine the number easily.
Useful especially in connection with unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a loop in such a way that its value
when exiting the loop can be determined using only its initial value and the number of loop
iterations, replace uses of the final value by such a computation, provided it is sufficiently cheap.
This reduces data dependencies and may allow further simplifications. Enabled by default at -O and
higher.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and
induction variable elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for
loops whose iterations are independent and can be arbitrarily reordered. The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g.
by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have
support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at -O1 and
higher, except for -Og.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to
prevent committing structures to memory too early. This flag is enabled by default at -O1 and
higher, except for -Og.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores
of immediate values narrower than a word into fewer wider stores to reduce the number of
instructions. This is enabled by default at -O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def
temporaries are replaced at their use location with their defining expression. This results in non-
GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL
generation. This is enabled by default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related expressions involving
multiplications and replaces them by less expensive calculations when possible. This is enabled by
default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if
not explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O3 and by -ftree-vectorize,
-fprofile-use, and -fauto-profile.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O3 and by
-ftree-vectorize, -fprofile-use, and -fauto-profile.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument should be one of unlimited, dynamic
or cheap. With the unlimited model the vectorized code-path is assumed to be profitable while with
the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration
counts that will likely execute faster than when executing the original scalar loop. The cheap model
disables vectorization of loops where doing so would be cost prohibitive for example due to required
runtime checks for data dependence or alignment but otherwise is equal to the dynamic model. The
default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with the OpenMP simd directive. The
model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning
as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but
instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and
higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination and common
subexpression elimination. This is enabled by default at -O3 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the unrolled loop using
the value in the first iteration. This breaks long dependency chains, thus improving efficiency of
the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not
reliable in cases where the loop body is more complicated than a single basic block. It also does
not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when unrolling a loop,
which can result in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and
stores) performed in previous iterations of loops.
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the
performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops
within the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted output functions such as "sprintf",
"snprintf", "vsprintf", and "vsnprintf" (but not "printf" of "fprintf"). This transformation allows
GCC to optimize or even eliminate branches based on the known return value of these functions called
with arguments that are either constant, or whose values are known to be in a range that makes
determining the exact return value possible. For example, when -fprintf-return-value is in effect,
both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized
away when "i" is a 32-bit or smaller integer because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and
above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The
-fprintf-return-value option is enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and
-fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch
probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch
probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into
account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some
cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are
easier to understand.
It is also possible to specify expected probability of the expression with
"__builtin_expect_with_probability" built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve
code locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The algorithm argument can be simple, which
does not increase code size (except sometimes due to secondary effects like alignment), or stc, the
"software trace cache" algorithm, which tries to put all often executed code together, minimizing the
number of branches executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken
branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files,
to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling or unwind tables
(on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions
with a user-defined section attribute and on any architecture that does not support named sections.
When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be
enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using
special subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for
unlikely executed functions. Reordering is done by the linker so object file format must support
named sections and linker must place them in a reasonable way.
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for
details) or manually annotate functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled.
For C (and C++), this activates optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an object of a different type,
unless the types are almost the same. For example, an "unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called
"type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above works as expected. However, this code
might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result
has undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than n, skipping up to m-1 bytes. This
ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an
n-byte alignment boundary.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24
aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less,
-falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6
bytes or less.
The second pair of n2:m2 values allows you to specify a secondary alignment:
-falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6
bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes
or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option
value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It
attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to
skip more bytes than the size of the function.
-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and
-falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values
are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1,
meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any
execution of the dummy padding instructions.
Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and
-falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option
value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be
reached by jumping. In this case, no dummy operations need be executed.
Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and
-falign-jumps=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option
value is 65536.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect, while
-fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order
that they appear in the input file. When this option is used, unreferenced static variables are not
removed. This option is intended to support existing code that relies on a particular ordering. For
new code, it is better to use attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is
explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual
pseudo register. This allows the register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code
remover. It can, however, make debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public
functions and variables with the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are optimized more aggressively by
interprocedural optimizers.
This option should not be used in combination with -flto. Instead relying on a linker plugin should
provide safer and more precise information.
-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it generates
GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object
file. When the object files are linked together, all the function bodies are read from these ELF
sections and instantiated as if they had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options should be specified at compile time
and during the final link. It is recommended that you compile all the files participating in the
same link with the same options and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections
inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges
the two files into a single internal image, and compiles the result as usual. Since both foo.o and
bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations
in GCC to work across the two files as if they were a single one. This means, for example, that the
inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE
representation and optimizes them as usual to produce myprog.
The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC
driver to perform the link step. GCC automatically performs link-time optimization if any of the
objects involved were compiled with the -flto command-line option. You can always override the
automatic decision to do link-time optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain whole program
assumptions. The compiler needs to know what functions and variables can be accessed by libraries
and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin
(see -fuse-linker-plugin) passes information to the compiler about used and externally visible
symbols. When the linker plugin is not available, -fwhole-program should be used to allow the
compiler to make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger
than a regular object file because it contains GIMPLE bytecodes and the usual final code (see
-ffat-lto-objects. This means that object files with LTO information can be linked as normal object
files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that
when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular,
non-LTO link on them.
When producing the final binary, GCC only applies link-time optimizations to those files that contain
bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and
final object code. GCC automatically selects which files to optimize in LTO mode and which files to
link without further processing.
Generally, options specified at link time override those specified at compile time, although in some
cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time, then GCC uses the highest
optimization level used when compiling the object files. Note that it is generally ineffective to
specify an optimization level option only at link time and not at compile time, for two reasons.
First, compiling without optimization suppresses compiler passes that gather information needed for
effective optimization at link time. Second, some early optimization passes can be performed only at
compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be
used during the final link. Currently, the following options and their settings are taken from the
first object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions,
-fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all compilation units, and trying to override
this at link time with a conflicting value is ignored. This includes options such as
-freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or
-fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting
translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and
for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at
link time.
When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile
such translation units with -fno-lto or consistently use the same assembler options on all
translation units. You can alternatively also specify assembler options at LTO link time.
To enable debug info generation you need to supply -g at compile-time. If any of the input files at
link time were built with debug info generation enabled the link will enable debug info generation as
well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitely
repeated at the linker command line and mixing different settings in different translation units is
discouraged.
If LTO encounters objects with C linkage declared with incompatible types in separate translation
units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic
may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for
other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written
in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added
to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use
the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is
possible to extract and use them in an LTO link if you are using a linker with plugin support. To
create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show
the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib
and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to
ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes
them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the
objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO
optimization process. In order to make a static library suitable for both LTO optimization and usual
linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program
does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to
allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved
optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see
-fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between
different types of hosts. The bytecode files are versioned and there is a strict version check, so
bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information on systems other
than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in
parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE
may be used to override the program used.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of
parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You
must prepend a + to the command recipe in the parent Makefile for this to work. This option likely
only works if MAKE is GNU make.
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of
the number of CPU threads present in your system.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to
specify a partitioning mirroring the original source files or balanced to specify partitioning into
equally sized chunks (whenever possible) or max to create new partition for every symbol where
possible. Specifying none as an algorithm disables partitioning and streaming completely. The
default value is balanced. While 1to1 can be used as an workaround for various code ordering issues,
the max partitioning is intended for internal testing only. The value one specifies that exactly one
partition should be used while the value none bypasses partitioning and executes the link-time
optimization step directly from the WPA phase.
-flto-odr-type-merging
Enable streaming of mangled types names of C++ types and their unification at link time. This
increases size of LTO object files, but enables diagnostics about One Definition Rule violations.
-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to LTO object
files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no
compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If
the option is not given, a default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin
support in the linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This
improves the quality of optimization by exposing more code to the link-time optimizer. This
information specifies what symbols can be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries (and shared libraries that use hidden
visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag
and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use
with a linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code.
This makes them usable for both LTO linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to
be aware of LTO. It requires a linker with linker plugin support for basic functionality.
Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build
environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib
wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to
use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into
$libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-
ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify arithmetic
instructions that compute processor flags similar to a comparison operation based on that arithmetic.
If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation
before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, perform a copy-
propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due
to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth
out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is
detected.
This option is enabled by -fauto-profile.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following optimizations, many of which are
generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt
-finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-reorder-functions
Before you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match the source code. This
error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in
poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback
profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See
-fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which
are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt
-finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-correction
path is the name of a file containing AutoFDO profile information. If omitted, it defaults to
fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a
supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC.
You must also supply the unstripped binary for your program to this tool. See
<https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic. These options trade
off between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might change
whether a floating-point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating
registers (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only good, but a few programs rely on
the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision on machines where floating-point operations
occur in a format with more precision or range than the IEEE standard and interchange floating-point
types. By default, -fexcess-precision=fast is in effect; this means that operations may be carried
out in a wider precision than the types specified in the source if that would result in faster code,
and it is unpredictable when rounding to the types specified in the source code takes place. When
compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules
specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their
semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default
for C if a strict conformance option such as -std=c99 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages other than C. On the x86, it has no
effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply
without excess precision, and in the latter, rounding is unpredictable.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output
for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do not require the guarantees of
these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed with a single instruction, e.g.,
"sqrt". A program that relies on IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the
compiler to consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are
valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries
or startup files that change the default FPU control word or other similar optimizations.
This option is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and
C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign
of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used
on code that relies on rounding behavior like "(x + 2**52) - 2**52". May also reorder floating-point
comparisons and thus may not be used when ordered comparisons are required. This option requires
that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much
sense with -frounding-math. For Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this enables
optimizations. For example "x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is
subject to common subexpression elimination. Note that this loses precision and increases the number
of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs
or +-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE
arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits
simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option
implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps
include division by zero, overflow, underflow, inexact result and invalid operation. This option
requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one
relies on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for
programs that depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior. This
is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified for programs that change the FP rounding
mode dynamically, or that may be executed with a non-default rounding mode. This option disables
constant folding of floating-point expressions at compile time (which may be affected by rounding
mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding
modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that
are affected by rounding mode. Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will be used to specify the default state
for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point
operations. Setting this option disables optimizations that may change the number of exceptions
visible with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that
affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and "trunc", and their "float" and "long
double" variants, to generate code that raises the "inexact" floating-point exception for noninteger
arguments. ISO C99 and C11 allow these functions to raise the "inexact" exception, but ISO/IEC TS
18661-1:2014, the C bindings to IEEE 754-2008, does not allow these functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised. This option does
nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function
then the "inexact" exception may be raised if the library implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly converting them to double-
precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex
division. Also, there is no checking whether the result of a complex multiplication or division is
"NaN + I*NaN", with an attempt to rescue the situation in that case. The default is
-fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the
option applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex
division, but there is no checking whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O
options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken.
When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called
sourcename.gcda for each source file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code and the same optimization
options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can
be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of
guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly
determine which path is taken more often.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the
program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for
usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first time of execution of a function
and orders these functions in ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information
about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations
based on them. Currently the optimizations include specialization of division operations using the
knowledge about the value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after
register allocation. This optimization most benefits processors with lots of registers. Depending
on the debug information format adopted by the target, however, it can make debugging impossible,
since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule instructions of same type
together because target machine can execute them more efficiently if they are adjacent to each other
in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow
of the function allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on
complete loop peeling (i.e. complete removal of loops with a small constant number of iterations).
This option makes code larger, and may or may not make it run faster.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from profile feedback
or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with
small constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher,
except for -Og.
-fsplit-loops
Split a loop into two if it contains a condition that's always true for one side of the iteration
space and false for the other.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both
branches (modified according to result of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create another version of the loop that
assumes the stride is always one. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better
vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also
enabled by -fprofile-use and -fauto-profile.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports
arbitrary sections. The name of the function or the name of the data item determines the section's
name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of
reference in the instruction space. Most systems using the ELF object format have linkers with such
optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The
performance impact varies.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to
smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There
could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these
options, the assembler and linker create larger object and executable files and are also slower.
These options affect code generation. They prevent optimizations by the compiler and assembler using
relative locations inside a translation unit since the locations are unknown until link time. An
example of such an optimization is relaxing calls to short call instructions.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of
target registers can typically be exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse branch target registers within
any basic block.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of those arguments.
NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++,
ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address
nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on
some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it with
-fsection-anchors, it accesses the variables from a common anchor point instead. The effect is
similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For
example, GCC does not inline functions that contain more than a certain number of instructions. You
can control some of these constants on the command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the
compiler, and are subject to change without notice in future releases.
In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q
options.
In each case, the value is an integer. The allowable choices for name are:
predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in percent),
then it is considered well predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and replace them with
conditionally executed instructions. This parameter gives the maximum number of instructions in
a block which should be considered for if-conversion. The compiler will also use other
heuristics to decide whether if-conversion is likely to be profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches around a block and replace them with
conditionally executed instructions. These parameters give the maximum permissible cost for the
sequence that would be generated by if-conversion depending on whether the branch is statically
determined to be predictable or not. The units for this parameter are the same as those for the
GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this
parameter using the BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping. The algorithm used by
-fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean
more aggressive optimization, making the compilation time increase with probably small
improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks before cross-
jumping is performed on them. This value is ignored in the case where all instructions in the
block being cross-jumped from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The
expansion is relative to a jump instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To
avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation
process, and unfactors them as late as possible. Only computed jumps at the end of a basic
blocks with no more than max-goto-duplication-insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay
slot. If more than this arbitrary number of instructions are searched, the time savings from
filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive
optimization, making the compilation time increase with probably small improvement in execution
time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching
for a block with valid live register information. Increasing this arbitrarily chosen value means
more aggressive optimization, increasing the compilation time. This parameter should be removed
when the delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that can be allocated in order to perform the global
common subexpression elimination optimization. If more memory than specified is required, the
optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any expression,
then RTL PRE inserts or removes the expression and thus leaves partially redundant computations
in the instruction stream.
max-pending-list-length
The maximum number of pending dependencies scheduling allows before flushing the current state
and starting over. Large functions with few branches or calls can create excessively large lists
which needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop.
Larger values can exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets the maximum number of
instructions (counted in GCC's internal representation) in a single function that the tree
inliner considers for inlining. This only affects functions declared inline and methods
implemented in a class declaration (C++).
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be
considered for inlining by the compiler are investigated. To those functions, a different (more
restrictive) limit compared to functions declared inline can be applied.
max-inline-insns-small
This is bound applied to calls which are considered relevant with -finline-small-functions.
max-inline-insns-size
This is bound applied to calls which are optimized for size. Small growth may be desirable to
anticipate optimization oppurtunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function overhead such as function prologue and
epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead such as time needed to execute function
prologue and epilogue
uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param uninlined-function-time but applied to
function thunks
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this threshold (in
percent), the function can be inlined regardless of the limit on --param max-inline-insns-single
and --param max-inline-insns-auto.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after
inlining, inlining is constrained by --param large-function-growth. This parameter is useful
primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. For example,
parameter value 100 limits large function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this
limit is limited by --param inline-unit-growth. For small units this might be too tight. For
example, consider a unit consisting of function A that is inline and B that just calls A three
times. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very
sane. For very large units consisting of small inlineable functions, however, the overall unit
growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the
size is increased to --param large-unit-insns before applying --param inline-unit-growth.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. For example,
parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either
marked cold via an attribute or by profile feedback) are not accounted into the unit size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural constant
propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size.
large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is trying to not grow past
this limit too much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. For example,
parameter value 1000 limits large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline
function can grow into by performing recursive inlining.
--param max-inline-insns-recursive applies to functions declared inline. For functions not
declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-insns-recursive-auto applies instead.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
--param max-inline-recursive-depth applies to functions declared inline. For functions not
declared inline, recursive inlining happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-recursive-depth-auto applies instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt
for function having little recursion depth by increasing the prologue size or complexity of
function body to other optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be
guessed from the probability that function recurses via a given call expression. This parameter
limits inlining only to call expressions whose probability exceeds the given threshold (in
percents).
early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the amount of inlining
for code having a large abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number of nested indirect
calls the early inliner can resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility are shared across
multiple compilation units.
profile-func-internal-id
A parameter to control whether to use function internal id in profile database lookup. If the
value is 0, the compiler uses an id that is based on function assembler name and filename, which
makes old profile data more tolerant to source changes such as function reordering etc.
min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is
used. The number of iterations after vectorization needs to be greater than the value specified
by this option to allow vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE
optimizations. This is currently supported only in the code hoisting pass. The bigger the
ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that
have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple
expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE
optimizations do not constrain the distance an expression can travel. This is currently
supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting
is. Specifying 0 allows all expressions to travel unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions to hoist. This is used to avoid
quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may
slow down compilation of huge functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior
in tree tail merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is used to limit
compilation time in tree tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the store merging pass.
max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled,
this parameter also determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop may
have to be unrolled. If a loop is unrolled, this parameter also determines how many times the
loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this
parameter also determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all candidates are considered
for each use in induction variable optimizations. If there are more candidates than this, only
the most relevant ones are considered to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value, always try to remove
unnecessary ivs from the set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may
result in larger compilation times.
dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store. Larger values result in larger
compilation times and may result in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the
analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex
expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked
with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop versioning for
alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alias
in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no
limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for analysis of the number
of iterations of the loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it contributes to the given permillage (i.e.
0...1000) of the entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of basic block in function given basic
block needs to have to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where a
function contains a single loop with known bound and another loop with unknown bound. The known
number of iterations is predicted correctly, while the unknown number of iterations average to
roughly 10. This means that the loop without bounds appears artificially cold relative to the
other one.
builtin-expect-probability
Control the probability of the expression having the specified value. This parameter takes a
percentage (i.e. 0 ... 100) as input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of a basic block in a function to align
the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed
instructions is covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available.
The real profiles (as opposed to statically estimated ones) are much less balanced allowing the
threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is a rather artificial
limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much
higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in
percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this threshold.
Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-
feedback is used for compilation with profile feedback and tracer-min-branch-probability
compilation without. The value for compilation with profile feedback needs to be more
conservative (higher) in order to make tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher
values may reduce the number of explicit probes, but a value larger than the operating system
provided guard will leave code vulnerable to stack clash style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated. This param controls the
maximum distance between probes into the stack as 2 raised to num bytes. Higher values may
reduce the number of explicit probes, but a value larger than the operating system provided guard
will leave code vulnerable to stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers.
max-cse-insns
The maximum number of instructions CSE processes before flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the
minimum percentage by which the garbage collector's heap should be allowed to expand between
collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit"
is available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS".
If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used.
Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every
opportunity. This is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The
first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize.
Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA
or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound
of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the
lower bound is used. Setting this parameter very large effectively disables garbage collection.
Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every
opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register.
Increasing values mean more aggressive optimization, making the compilation time increase with
probably slightly better performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing values mean
more aggressive optimization, making the compilation time increase with probably slightly better
performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given
time during the first scheduling pass. Increasing values mean more thorough searches, making the
compilation time increase with probably little benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective
scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective
scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative
scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0 disables region
extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insns are
scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of search for
available instructions.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective scheduling. This
is the limit on the number of iterations through which the instruction may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered for renaming in the
selective scheduler.
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for
a pseudo register as last known value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.
integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler's memory usage and
increasing its speed. This sets the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when
-fstack-protection is used.
This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of
functions protected by the stack protector.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner during pointer
analysis.
prefetch-latency
Estimate on average number of instructions that are executed before prefetch finishes. The
distance prefetched ahead is proportional to this constant. Increasing this number may also lead
to less streams being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software prefetch hints for strides that are
non-constant. In some cases this may be beneficial, though the fact the stride is non-constant
may make it hard to predict when there is clear benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch
hints should be issued only for strides that are known to be constant and below prefetch-minimum-
stride.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than
this threshold, prefetch hints will not be issued.
This setting is useful for processors that have hardware prefetchers, in which case there may be
conflicts between the hardware prefetchers and the software prefetchers. If the hardware
prefetchers have a maximum stride they can handle, it should be used here to improve the use of
software prefetchers.
A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any
constant stride.
This setting is only useful for strides that are known and constant.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to enable
prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references to
enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system. Should always be 1, which uses a
more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs
in the canonical type system are causing compilation failures, set this value to 0 to disable
canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-
max-branch-ratio times the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination
optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the
enhanced partial redundancy elimination optimization can run away, consuming all of the memory
available on the host machine. This parameter sets a limit on the length of the sets that are
computed, which prevents the runaway behavior. Setting a value of 0 for this parameter allows an
unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-
vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically
and the remaining ones not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for redundancies for loads and
stores. If this limit is hit the search is aborted and the load or store is not considered
redundant. The number of queries is algorithmically limited to the number of stores on all paths
from the load to the function entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the
number given by this parameter, only at most the given number of the most frequently-executed
loops form regions for regional register allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still
require excessive amounts of memory for huge functions. If the conflict table for a function
could be more than the size in MB given by this parameter, the register allocator instead uses a
faster, simpler, and lower-quality algorithm that does not require building a pseudo-register
conflict table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop
invariants (see -O3). The number of available registers reserved for some other purposes is
given by this parameter. Default of the parameter is the best found from numerous experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called
inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal
fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The
default value was chosen from numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount of needed
compile-time memory, with very large loops. Loops with more basic blocks than this parameter
won't have loop invariant motion optimization performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This parameter limits the number
of data references in loops that are considered for data dependence analysis. These large loops
are no handled by the optimizations using loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of
any function. If this limit is exceeded with variable tracking at assignments enabled, analysis
for that function is retried without it, after removing all debug insns from the function. If
the limit is exceeded even without debug insns, var tracking analysis is completely disabled for
the function. Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug
temporaries to value expressions. This trades compilation time for more complete debug
information. If this is set too low, value expressions that are available and could be
represented in debug information may end up not being used; setting this higher may enable the
compiler to find more complex debug expressions, but compile time and memory use may grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity
explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set
limit, such stmts will be dropped from the inlined copy of a function, and from its RTL
expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the parameter is
reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may
get (non-overlapping) uids above it if the reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their
cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original
pointer parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of
aggregates with uses of independent scalar variables. These parameters control the maximum size,
in storage units, of aggregate which is considered for replacement when compiling for speed (sra-
max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track,
per one local variable, in order to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter specifies the size
in bytes after which variables are saved with the logging functions as opposed to save/restore
code sequence pairs. This option only applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of parameters in a
Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound. A
variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of
the SCoP.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip
mine each loop in the loop nest by a given number of iterations. The strip length can be changed
using the loop-block-tile-size parameter.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's parameter in order
to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of
values and types it stores per one formal parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning
opportunities with scores that exceed ipa-cp-eval-threshold.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function will receive when they
are evaluated for cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-
agg-items controls the maximum number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of iterations of a loop
known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate.
ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate would make the index of an array access known, it
adds a bonus of ipa-cp-array-index-hint-bonus to the profitability score of the candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in order to track values
pointed to by function parameters. In order not spend too much time analyzing huge functions, it
gives up and consider all memory clobbered after examining ipa-max-aa-steps statements modifying
memory.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number of partitions
should exceed the number of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of
splitting very small programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to provide an upper bound for
individual size of partition. Meant to be used only with balanced partitioning.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an
identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a
statement's original block to allow statement sinking of a statement. Larger numbers result in
more aggressive statement sinking. A small positive adjustment is applied for statements with
memory operands as those are even more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0.
case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a
tree of conditional branches. If the value is 0, use the default for the machine.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated tree. This parameter
overrides target dependent heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the
original implementation and is the more likely to prevent instructions from being reordered.
Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by
algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more
heavily on having a regular register file and accurate register pressure classes. See
haifa-sched.c in the GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when seeking a basis for a new
straight-line strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects. This kind of protection is enabled by
default if you are using -fsanitize=address option. To disable global objects protection use
--param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This kind of protection is enabled by
default when using -fsanitize=address. To disable stack protection use --param asan-stack=0
option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of protection is enabled by default
when using -fsanitize=address. To disable memory reads protection use --param
asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind of protection is enabled by
default when using -fsanitize=address. To disable memory writes protection use --param
asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of protection is enabled by default when
using -fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of protection is enabled by default when using
the -fsanitize=address option. To disable it use --param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable it, add
"detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is greater or equal to this number,
use callbacks instead of inline checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this number, directly poison (or
unpoison) shadow memory instead of using run-time callbacks.
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump
thread path.
max-fsm-thread-length
Maximum number of basic blocks on a finite state automaton jump thread path.
max-fsm-thread-paths
Maximum number of new jump thread paths to create for a finite state automaton.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto,
runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an innermost parallelized loop for which the
parallelized variant is preferred over the single threaded one. Note that for a parallelized
loop nest the minimum number of iterations of the outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things like fold routines.
One level of recursion corresponds to following a use-def chain.
hsa-gen-debug-stores
Enable emission of special debug stores within HSA kernels which are then read and reported by
libgomp plugin. Generation of these stores is disabled by default, use --param
hsa-gen-debug-stores=1 to enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type
of an object that invokes a virtual call we may be able to devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default edge of a switch statement during VRP.
unroll-jam-min-percent
The minimum percentage of memory references that must be optimized away for the unroll-and-jam
transformation to be considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by the unroll-and-jam
transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass
for a branch that is considered unpredictable.
max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum number of times that an individual
variable will be expanded during loop unrolling.
tracer-min-branch-probability-feedback
Stop forward growth if the probability of best edge is less than this threshold (in percent).
Used when profile feedback is available.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent relative to entry BB of the
function) to make partial inlining happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization pass will track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy elimination after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that permit performing redundancy
elimination after reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by the copy loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.
slp-max-insns-in-bb
Maximum number of instructions in basic block to be considered for SLP vectorization.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers when checking conflicts using DFA.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic block
needs to have to be considered hot (used in non-LTO mode)
max-inline-insns-recursive-auto
The maximum number of instructions non-inline function can grow to via recursive inlining.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number of iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.
unlikely-bb-count-fraction
The minimum fraction of profile runs a given basic block execution count must be not to be
considered unlikely.
tracer-dynamic-coverage-feedback
The percentage of function, weighted by execution frequency, that must be covered by trace
formation. Used when profile feedback is available.
max-inline-recursive-depth-auto
The maximum depth of recursive inlining for non-inline functions.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a threading path when comparing to the
number of (scaled) blocks.
fsm-maximum-phi-arguments
Maximum number of arguments a PHI may have before the FSM threader will not try to thread through
its block.
uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies during uninitialized variable
analysis.
indir-call-topn-profile
Track top N target addresses in indirect-call profile.
max-once-peeled-insns
The maximum number of insns of a peeled loop that rolls only once.
sra-max-scalarization-size-Osize
Maximum size, in storage units, of an aggregate which should be considered for scalarization when
compiling for size.
fsm-scale-path-blocks
Scale factor to apply to the number of blocks in a threading path when comparing to the number of
(scaled) statements.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks
into; at ' ' only enable instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have before the loop versioning pass
considers it too big to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have before the loop versioning pass
considers it too big to copy, discounting any instructions in inner loops that directly benefit
from versioning.
ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in determining a property of a variable such
as its value. This limits the number of iterations or recursive calls GCC performs when
optimizing certain statements or when determining their validity prior to issuing diagnostics.
Program Instrumentation Options
GCC supports a number of command-line options that control adding run-time instrumentation to the code it
normally generates. For example, one purpose of instrumentation is collect profiling statistics for use
in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of
program instrumentation is adding run-time checking to detect programming errors like invalid pointer
dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack
smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other
forms of tracing or function-level instrumentation for debug or program analysis purposes.
-p
-pg Generate extra code to write profile information suitable for the analysis program prof (for -p) or
gprof (for -pg). You must use this option when compiling the source files you want data about, and
you must also use it when linking.
You can use the function attribute "no_instrument_function" to suppress profiling of individual
functions when compiling with these options.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many
times each branch and call is executed and how many times it is taken or returns. On targets that
support constructors with priority support, profiling properly handles constructors, destructors and
C++ constructors (and destructors) of classes which are used as a type of a global variable.
When the compiled program exits it saves this data to a file called auxname.gcda for each source
file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test
coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the
output file, if explicitly specified and it is not the final executable, otherwise it is the basename
of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or
dir/foo.gcda for output file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a
synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the
documentation for those options for more details.
* Compile the source files with -fprofile-arcs plus optimization and code generation options. For
test coverage analysis, use the additional -ftest-coverage option. You do not need to profile
every source file in a program.
* Compile the source files additionally with -fprofile-abs-path to create absolute path names in
the .gcno files. This allows gcov to find the correct sources in projects where compilations
occur with different working directories.
* Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
* Run the program on a representative workload to generate the arc profile information. This may
be repeated any number of times. You can run concurrent instances of your program, and provided
that the file system supports locking, the data files will be correctly updated. Unless a strict
ISO C dialect option is in effect, "fork" calls are detected and correctly handled without double
counting.
* For profile-directed optimizations, compile the source files again with the same optimization and
code generation options plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human readable information from the .gcno and
.gcda files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a
spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented:
the compiler adds code to count the number of times that these arcs are executed. When an arc is the
only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise,
a new basic block must be created to hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each
source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test coverage data. Coverage data matches
the source files more closely if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path names in the .gcno files. This
allows gcov to find the correct sources in projects where compilations occur with different working
directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the
profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths
can be used. By default, GCC uses the current directory as path, thus the profile data file appears
in the same directory as the object file. In order to prevent the file name clashing, if the object
file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it
as the file name of a .gcda file.
When an executable is run in a massive parallel environment, it is recommended to save profile to
different folders. That can be done with variables in path that are exported during run-time:
%p process ID.
%q{VAR}
value of environment variable VAR
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for later
recompilation with profile feedback based optimization. You must use -fprofile-generate both when
compiling and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and
-fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See
-fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
-fprofile-update=method
Alter the update method for an application instrumented for profile feedback based optimization. The
method argument should be one of single, atomic or prefer-atomic. The first one is useful for
single-threaded applications, while the second one prevents profile corruption by emitting thread-
safe code.
Warning: When an application does not properly join all threads (or creates an detached thread), a
profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single
otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the
command line.
-fprofile-filter-files=regex
Instrument only functions from files where names match any regular expression (separated by a semi-
colon).
For example, -fprofile-filter-files=main.c;module.*.c will instrument only main.c and all C files
starting with 'module'.
-fprofile-exclude-files=regex
Instrument only functions from files where names do not match all the regular expressions (separated
by a semi-colon).
For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are
located in /usr/ folder.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented
to detect out-of-bounds and use-after-free bugs. The option enables
-fsanitize-address-use-after-scope. See <https://github.com/google/sanitizers/wiki/AddressSanitizer>
for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment
variable. When set to "help=1", the available options are shown at startup of the instrumented
program. See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a
list of supported options. The option cannot be combined with -fsanitize=thread.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See <https://github.com/google/kasan/wiki> for more
details.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be combined
with either -fsanitize=kernel-address or -fsanitize=address The option cannot be combined with
-fsanitize=thread. Note: By default the check is disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be combined with either
-fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread.
Note: By default the check is disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to
detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more
details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options.
The option cannot be combined with -fsanitize=address, -fsanitize=leak.
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory
addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables
and the executable is linked against a library that overrides "malloc" and other allocator functions.
See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The
run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot
be combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are
instrumented to detect undefined behavior at runtime. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift operation is not undefined. Note that
what exactly is considered undefined differs slightly between C and C++, as well as between ISO
C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and
-fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation is not negative and is
smaller than the precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range, check that the result of a shift
operation is not undefined. Note that what exactly is considered undefined differs slightly
between C and C++, as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1" division.
-fsanitize=unreachable
With this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message
call instead. When reaching the "__builtin_unreachable" call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the application built with this option
turned on will issue an error message when it tries to dereference a NULL pointer, or if a
reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on
an object pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs built with this option turned on will
issue an error message when the end of a non-void function is reached without actually returning
a value. This option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check that the result of "+", "*", and
both unary and binary "-" does not overflow in the signed arithmetics. Note, integer promotion
rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various out of bounds accesses are
detected. Flexible array members, flexible array member-like arrays, and initializers of
variables with static storage are not instrumented.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most out of bounds accesses are
detected, including flexible array members and flexible array member-like arrays. Initializers
of variables with static storage are not instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they are dereferenced, or when a
reference is bound to insufficiently aligned target, or when a method or constructor is invoked
on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using the "__builtin_object_size"
function. Various out of bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar options,
-fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point
division by zero can be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking. We check that the result
of the conversion does not overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work
well with "FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether null values are not passed to
arguments marked as requiring a non-null value by the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in functions marked with
"returns_nonnull" function attribute, to detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a
run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type. If a value outside the range of
values for the enum type is loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function calls, member accesses and some
conversions between pointers to base and derived classes, to verify the referenced object has the
correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If the pointer arithmetics
overflows, a run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected builtin functions. If an invalid
value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to
"__builtin_ctz" or "__builtin_clz" invokes undefined behavior and is diagnosed by this option.
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a
diagnostic message. This currently works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some
sanitizers cannot be used together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for
experimenting with different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-separated list of
opts. Enabling this option for a sanitizer component causes it to attempt to continue running the
program as if no error happened. This means multiple runtime errors can be reported in a single
program run, and the exit code of the program may indicate success even when errors have been
reported. The -fno-sanitize-recover= option can be used to alter this behavior: only the first
detected error is reported and program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for
-fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and
-fsanitize=address. For these sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and
-fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime
library side, otherwise the failures are still fatal. The runtime library defaults to
"halt_on_error=0" for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for
AddressSanitizer is "halt_on_error=1". This can be overridden through setting the "halt_on_error"
flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list
of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse
to none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the compiler to report undefined behavior
using "__builtin_trap" rather than a "libubsan" library routine. The advantage of this is that the
"libubsan" library is not needed and is not linked in, so this is usable even in freestanding
environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_pc"
into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_cmp1",
"__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for
integral comparison with both operands variable or "__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2", "__sanitizer_cov_trace_const_cmp4" or
"__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand constant,
"__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for float or double comparisons and
"__sanitizer_cov_trace_switch" for switch statements.
-fcf-protection=[full|branch|return|none|check]
Enable code instrumentation of control-flow transfers to increase program security by checking that
target addresses of control-flow transfer instructions (such as indirect function call, function
return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected
target. This is intended to protect against such threats as Return-oriented Programming (ROP), and
similarly call/jmp-oriented programming (COP/JOP).
The value "branch" tells the compiler to implement checking of validity of control-flow transfer at
the point of indirect branch instructions, i.e. call/jmp instructions. The value "return" implements
checking of validity at the point of returning from a function. The value "full" is an alias for
specifying both "branch" and "return". The value "none" turns off instrumentation.
The value "check" is used for the final link with link-time optimization (LTO). An error is issued
if LTO object files are compiled with different -fcf-protection values. The value "check" is ignored
at the compile time.
The macro "__CET__" is defined when -fcf-protection is used. The first bit of "__CET__" is set to 1
for the value "branch" and the second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which functions and calls should be skipped
from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement
Technology (CET).
NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC,
ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by
adding a guard variable to functions with vulnerable objects. This includes functions that call
"alloca", and functions with buffers larger than 8 bytes. The guards are initialized when a function
is entered and then checked when the function exits. If a guard check fails, an error message is
printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be protected --- those that have local
array definitions, or have references to local frame addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this
flag if you are running in an environment with multiple threads, but you only rarely need to specify
it in a single-threaded environment since stack overflow is automatically detected on nearly all
systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the
language runtime must do that. The switch causes generation of code to ensure that they see the
stack being extended.
You can additionally specify a string parameter: no means no checking, generic means force the use of
old-style checking, specific means use the best checking method and is equivalent to bare
-fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler
but comes with the following drawbacks:
1. Modified allocation strategy for large objects: they are always allocated dynamically if their
size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular
function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation,
code performance is hampered.
Note that old-style stack checking is also the fallback method for specific if no target support has
been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows.
specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect
against stack-clash attacks. To protect against those you want -fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will
only allocate one page of stack space at a time and each page is accessed immediately after
allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the
operating system.
Most targets do not fully support stack clash protection. However, on those targets
-fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also
provide limited protection for static stack allocations if the target supports
-fstack-check=specific.
NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++,
ObjC, ObjC++, unless -fno-stack-clash-protection is found.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a
register or the address of a symbol. If a larger stack is required, a signal is raised at run time.
For most targets, the signal is raised before the stack overruns the boundary, so it is possible to
catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the
flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack
limit of 128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the "no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a
discontiguous stack which can only overflow if the program is unable to allocate any more memory.
This is most useful when running threaded programs, as it is no longer necessary to calculate a good
stack size to use for each thread. This is currently only implemented for the x86 targets running
GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be
much stack space available for the latter code to run. If compiling all code, including library
code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code
compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold
linker in GNU binutils release 2.21 and later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on (or off, if using
-fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that
the vtable pointer through which the call is made is valid for the type of the object, and has not
been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is
reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at program startup, which are used for
verifying the vtable pointers. The options std and preinit control the timing of when these data
structures are built. In both cases the data structures are built before execution reaches "main".
Using -fvtable-verify=std causes the data structures to be built after shared libraries have been
loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have
been loaded and initialized.
If this option appears multiple times in the command line with different values specified, none takes
highest priority over both std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions
of the runtime functions for the vtable verification feature to be called. This flag also causes the
compiler to log information about which vtable pointers it finds for each class. This information is
written to a file named vtv_set_ptr_data.log in the directory named by the environment variable
VTV_LOGS_DIR if that is defined or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any
existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls
it encounters and the number of verifications it inserts. It also counts the number of calls to
certain run-time library functions that it inserts and logs this information for each compilation
unit. The compiler writes this information to a file named vtv_count_data.log in the directory named
by the environment variable VTV_LOGS_DIR if that is defined or the current working directory
otherwise. It also counts the size of the vtable pointer sets for each class, and writes this
information to vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any
existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just
before function exit, the following profiling functions are called with the address of the current
function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the
current function, so the call site information may not be available to the profiling functions
otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up
exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling
calls indicate where, conceptually, the inline function is entered and exited. This means that
addressable versions of such functions must be available. If all your uses of a function are
expanded inline, this may mean an additional expansion of code size. If you use "extern inline" in
your C code, an addressable version of such functions must be provided. (This is normally the case
anyway, but if you get lucky and the optimizer always expands the functions inline, you might have
gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation is
not done. This can be used, for example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions cannot safely be called
(perhaps signal handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of
-finstrument-functions). If the file that contains a function definition matches with one of file,
then that function is not instrumented. The match is done on substrings: if the file parameter is a
substring of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of
function names to be excluded from instrumentation. The function name to be matched is its user-
visible name, such as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the
function name must be given in UTF-8, not using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the function entry point before the Mth
NOP. If M is omitted, it defaults to 0 so the function entry points to the address just at the first
NOP. The NOP instructions reserve extra space which can be used to patch in any desired
instrumentation at run time, provided that the code segment is writable. The amount of space is
controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the
instruction emitted by the internal GCC back-end interface "gen_nop". This behavior is target-
specific and may also depend on the architecture variant and/or other compilation options.
For run-time identification, the starting addresses of these areas, which correspond to their
respective function entries minus M, are additionally collected in the "__patchable_function_entries"
section of the resulting binary.
Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over
command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to
remove it completely on a single function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on M---the function entry address,
even before the prologue.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only
together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to control search paths for include
files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning
Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase
three in a #define directive. In particular, the definition is truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the
shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with
surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells,
so you should quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file
and -include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However,
the first directory searched for file is the preprocessor's working directory instead of the
directory containing the main source file. If not found there, it is searched for in the remainder
of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the
command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it
defines remain defined. This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain
defined.
-pthread
Define additional macros required for using the POSIX threads library. You should use this option
consistently for both compilation and linking. This option is supported on GNU/Linux targets, most
other Unix derivatives, and also on x86 Cygwin and MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the
dependencies of the main source file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the included files, including those
coming from -include or -imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the
source file with any suffix replaced with object file suffix and with any leading directory parts
removed. If there are many included files then the rule is split into several lines using \-newline.
The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such
debug output with the dependency rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the
regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files
that are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in
itself determine whether that header appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header
files are generated files and adds them to the dependency list without raising an error. The
dependency filename is taken directly from the "#include" directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing
each to depend on nothing. These dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the
main input file, deletes any directory components and any file suffix such as .c, and appends the
platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify. If you want multiple targets,
you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d,
otherwise it takes the name of the input file, removes any directory components and suffix, and
applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output
file, but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the
compilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses
things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes comments, so that you can pass a file
preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is
little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and
"#error". Other preprocessor operations, such as macro expansion and trigraph conversion are not
performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such
as "__LINE__", which are contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full
preprocessing of files previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is enabled by default for C99 (and
later C standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in
warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about
the current macro expansion stack when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory. The level parameter can be used
to choose the level of precision of token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option. Value 1 tracks tokens locations in a degraded
mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of
an argument of a function-like macro have the same location. Value 2 tracks tokens locations
completely. This value is the most memory hungry. When this option is given no argument, the default
parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the "__FILE__" and "__BASE_FILE__" macros
as if the files resided in directory new instead. This can be used to change an absolute path to a
relative path by using . for new which can result in more reproducible builds that are location
independent. This option also affects "__builtin_FILE()" during compilation. See also
-ffile-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8.
charset can be any encoding supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is
UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset
can be any encoding supported by the system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the
source character set used by GCC. If the locale does not specify, or GCC cannot get this information
from the locale, the default is UTF-8. This can be overridden by either the locale or this command-
line option. Currently the command-line option takes precedence if there's a conflict. charset can
be any encoding supported by the system's "iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-output flags to also list the files
from the precompiled header's dependencies. If not specified, only the precompiled header are listed
and not the files that were used to create it, because those files are not consulted when a
precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma",
"#pragma GCC pch_preprocess "filename"" in the output to mark the place where the precompiled header
was found, and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads
the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as
input to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH
file is available in a different location. The filename may be absolute or it may be relative to
GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that let the compiler know the current
working directory at the time of preprocessing. When this option is enabled, the preprocessor emits,
after the initial linemarker, a second linemarker with the current working directory followed by two
slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory
emitted as the current working directory in some debugging information formats. This option is
implicitly enabled if debugging information is enabled, but this can be inhibited with the negated
form -fno-working-directory. If the -P flag is present in the command line, this option has no
effect, since no "#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the
older form -A predicate(answer), which is still supported, because it does not use shell special
characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-C Do not discard comments. All comments are passed through to the output file, except for comments in
processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments
as tokens in their own right. For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary source line, since the first
token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments
contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside
a macro to be converted to C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when
running the preprocessor on something that is not C code, and will be sent to a program which might
be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See
the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are
only supported with the -E switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined
by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character
constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std
and -ansi options.
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-
DOS.
-H Print the name of each header file used, in addition to other normal activities. Each name is
indented to show how deep in the #include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a
valid one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by letters. The flags documented here
are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or
reserved for future versions of GCC, and so are silently ignored. If you specify letters whose
behavior conflicts, the result is undefined.
-dM Instead of the normal output, generate a list of #define directives for all the macros defined
during the execution of the preprocessor, including predefined macros. This gives you a way of
finding out what is predefined in your version of the preprocessor. Assuming you have no file
foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include the predefined macros, and it outputs both
the #define directives and the result of preprocessing. Both kinds of output go to the standard
output file.
-dN Like -dD, but emit only the macro names, not their expansions.
-dI Output #include directives in addition to the result of preprocessing.
-dU Like -dD except that only macros that are expanded, or whose definedness is tested in
preprocessor directives, are output; the output is delayed until the use or test of the macro;
and #undef directives are also output for macros tested but undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging
information about location maps. Every token in the output is preceded by the dump of the map its
location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to the
preprocessor. If option contains commas, it is split into multiple options at the commas. However,
many options are modified, translated or interpreted by the compiler driver before being passed to
the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is
undocumented and subject to change, so whenever possible you should avoid using -Wp and let the
driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific
preprocessor options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the
option and once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing
as an integrated part of input tokenization and parsing. If this option is provided, the appropriate
language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead
invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input.
This option may be useful in conjunction with the -B or -wrapper options to specify an alternate
preprocessor or perform additional processing of the program source between normal preprocessing and
compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple
options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler
options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the
option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They
are meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used
as arguments.
-flinker-output=type
This option controls the code generation of the link time optimizer. By default the linker output is
determined by the linker plugin automatically. For debugging the compiler and in the case of
incremental linking to non-lto object file is desired, it may be useful to control the type manually.
If type is exec the code generation is configured to produce static binary. In this case -fpic and
-fpie are both disabled.
If type is dyn the code generation is configured to produce shared library. In this case -fpic or
-fPIC is preserved, but not enabled automatically. This makes it possible to build shared libraries
without position independent code on architectures this is possible, i.e. on x86.
If type is pie the code generation is configured to produce -fpie executable. This result in similar
optimizations as exec except that -fpie is not disabled if specified at compilation time.
If type is rel the compiler assumes that incremental linking is done. The sections containing
intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting
object file. In addition, if -ffat-lto-objects is specified the binary code is produced for future
non-lto linking. The object file produced by incremental linking will be smaller than a static
library produced from the same object files. At link-time the result of incremental linking will
also load faster to compiler than a static library assuming that majority of objects in the library
are used.
Finally nolto-rel configure compiler to for incremental linking where code generation is forced,
final binary is produced and the intermediate code for later link-time optimization is stripped. When
multiple object files are linked together the resulting code will be optimized better than with link
time optimizations disabled (for example, the cross-module inlining will happen), most of benefits of
whole program optimizations are however lost.
During the incremental link (by -r) the linker plugin will default to rel. With current interfaces to
GNU Binutils it is however not possible to link incrementally LTO objects and non-LTO objects into a
single mixed object file. In the case any of object files in incremental link cannot be used for
link-time optimization the linker plugin will output warning and use nolto-rel. To maintain the whole
program optimization it is recommended to link such objects into static library instead.
Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a
separate argument is only for POSIX compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact
details. The general description below applies to the GNU linker.
The linker searches a standard list of directories for the library. The directories searched include
several standard system directories plus any that you specify with -L.
Static libraries are archives of object files, and have file names like liblibrary.a. Some targets
also support shared libraries, which typically have names like liblibrary.so. If both static and
shared libraries are found, the linker gives preference to linking with the shared library unless the
-static option is used.
It makes a difference where in the command you write this option; the linker searches and processes
libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be
loaded.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used
normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify are passed to
the linker, and options specifying linkage of the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is
used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are
usually resolved by entries in libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nolibc
Do not use the C library or system libraries tightly coupled with it when linking. Still link with
the startup files, libgcc or toolchain provided language support libraries such as libgnat,
libgfortran or libstdc++ unless options preventing their inclusion are used as well. This typically
removes -lc from the link command line, as well as system libraries that normally go with it and
become meaningless when absence of a C library is assumed, for example -lpthread or -lm in some
configurations. This is intended for bare-board targets when there is indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only
the libraries you specify are passed to the linker, and options specifying linkage of the system
libraries, such as -static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are
usually resolved by entries in libc. These entry points should be supplied through some other
mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of
internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs
for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other
words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC library subroutines. (An example of
such an internal subroutine is "__main", used to ensure C++ constructors are called.)
-e entry
--entry=entry
Specify that the program entry point is entry. The argument is interpreted by the linker; the GNU
linker accepts either a symbol name or an address.
-pie
Produce a dynamically linked position independent executable on targets that support it. For
predictable results, you must also specify the same set of options used for compilation (-fpie,
-fPIE, or model suboptions) when you specify this linker option.
-no-pie
Don't produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that support it. A static position
independent executable is similar to a static executable, but can be loaded at any address without a
dynamic linker. For predictable results, you must also specify the same set of options used for
compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.
-pthread
Link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix
derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags
for the preprocessor, so it should be used consistently for both compilation and linking.
-r Produce a relocatable object as output. This is also known as partial linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the
linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this overrides -pie and prevents linking with the shared
libraries. On other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all
systems support this option. For predictable results, you must also specify the same set of options
used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared
or static version, respectively. If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the
static version. The most common of these is when the application wishes to throw and catch
exceptions across different shared libraries. In that case, each of the libraries as well as the
application itself should use the shared libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a
main executable, because C++ programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always
linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU
linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of
libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes
away the linking with the shared version of libgcc, linking with the static version of libgcc by
default. This allows exceptions to propagate through such shared libraries, without incurring
relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it
using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared
libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the GCC driver automatically links
against libasan. If libasan is available as a shared library, and the -static option is not used,
then this links against the shared version of libasan. The -static-libasan option directs the GCC
driver to link libasan statically, without necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC driver automatically links
against libtsan. If libtsan is available as a shared library, and the -static option is not used,
then this links against the shared version of libtsan. The -static-libtsan option directs the GCC
driver to link libtsan statically, without necessarily linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC driver automatically links against
liblsan. If liblsan is available as a shared library, and the -static option is not used, then this
links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to
link liblsan statically, without necessarily linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program, the GCC driver automatically links
against libubsan. If libubsan is available as a shared library, and the -static option is not used,
then this links against the shared version of libubsan. The -static-libubsan option directs the GCC
driver to link libubsan statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally automatically links against
libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then
this links against the shared version of libstdc++. That is normally fine. However, it is sometimes
useful to freeze the version of libstdc++ used by the program without going all the way to a fully
static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically,
without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved
references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few
systems support this option.
-T script
Use script as the linker script. This option is supported by most systems using the GNU linker. On
some targets, such as bare-board targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options
that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for
the option and once for the argument. For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions",
because this passes the entire string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using
the option=value syntax than as separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this
syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options
at the commas. You can use this syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also
get the same effect with -Wl,-Map=output.map.
NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable,
use -Wl,-z,norelro.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can
use -u multiple times with different symbols to force loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword keyword. See the section in the
documentation of your linker for permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the
compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for header files during
preprocessing. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot
prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of the directive, "#include "file"".
Directories specified with -I, -isystem, or -idirafter apply to lookup for both the "#include "file""
and "#include <file>" directives.
You can specify any number or combination of these options on the command line to search for header
files in several directories. The lookup order is as follows:
1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are
searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these
directories are searched before the standard system header file directories. However, you should not
use this option to add directories that contain vendor-supplied system header files; use -isystem for
that.
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets
the same special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified
with -I, the -I option is ignored. The directory is still searched but as a system directory at its
normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy
system headers and the ordering for the "#include_next" directive are not inadvertently changed. If
you really need to change the search order for system directories, use the -nostdinc and/or -isystem
options.
-I- Split the include path. This option has been deprecated. Please use -iquote instead for -I
directories before the -I- and remove the -I- option.
Any directories specified with -I options before -I- are searched only for headers requested with
"#include "file""; they are not searched for "#include <file>". If additional directories are
specified with -I options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search
directory for "#include "file"". There is no way to override this effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a
directory, you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the
include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it
where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin
targets, where it applies to both header files and libraries). See the --sysroot option for more
information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
-nostdinc
Do not search the standard system directories for header files. Only the directories explicitly
specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current
file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the
other standard directories. (This option is used when building the C++ library.)
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of
-fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the
driver.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the
compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix
as a prefix for each program it tries to run, both with and without machine/version/ for the
corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name
is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified
program name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds
a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because
the compiler translates these options into -L options for the linker. They also apply to include
files in the preprocessor, because the compiler translates these options into -isystem options for
the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is
not found there, the two standard prefixes above are tried, and that is all. The file is left out of
the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable
GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0
to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when
generating a relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler
normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries,
but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your
linker does not support this option, the header file aspect of --sysroot still works, but the library
aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with --sysroot, depending on the
other options used, so that headers may for example be found in dir/suffix/usr/include instead of
dir/usr/include. This option disables the addition of such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table
below, only one of the forms is listed---the one that is not the default. You can figure out the other
form by either removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto variables and compiler generated
temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local
variables and temporaries, named_vars enables the reuse only for user defined local variables with
names, and none disables stack reuse completely. The default value is all. The option is needed when
the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond
the end point defined by the language. When a lifetime of a variable ends, and if the variable lives
in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or
scoped local variables whose live range does not overlap with it. Legacy code extending local
lifetime is likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime
of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to
reuse its stack space with other temporaries or scoped local variables whose live range does not
overlap with it. However some of the legacy code relies on the behavior of older compilers in which
temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This
option is used to control the temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line
results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction
and multiplication wraps around using twos-complement representation. This flag enables some
optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using
-ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active
options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being
effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic overflow on addition and
subtraction wraps around using twos-complement representation. This flag disables some optimizations
which assume pointer overflow is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets,
this implies GCC generates frame unwind information for all functions, which can produce significant
data size overhead, although it does not affect execution. If you do not specify this option, GCC
enables it by default for languages like C++ that normally require exception handling, and disables
it for languages like C that do not normally require it. However, you may need to enable this option
when compiling C code that needs to interoperate properly with exception handlers written in C++.
You may also wish to disable this option if you are compiling older C++ programs that don't use
exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires
platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not
allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't otherwise contribute to the execution
of the program can be optimized away. This option is enabled by default for the Ada front end, as
permitted by the Ada language specification. Optimization passes that cause dead exceptions to be
removed are enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed static data, but does not affect
the generated code in any other way. You normally do not need to enable this option; instead, a
language processor that needs this handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target machine. The table is exact at each
instruction boundary, so it can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE"
binding to make sure that definitions of template static data members and static local variables in
inline functions are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems
with a library used by two different "RTLD_LOCAL" plugins depending on a definition in one of them
and therefore disagreeing with the other one about the binding of the symbol. But this causes
"dlclose" to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via
"dlclose" and "dlopen", you can use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers.
This convention is less efficient, but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code
compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever
convention is standard for the target. If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can
choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code
compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type is equivalent to the smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned int" instead of the default for the
target. This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface.
-fno-common
In C code, this option controls the placement of global variables defined without an initializer,
known as tentative definitions in the C standard. Tentative definitions are distinct from
declarations of a variable with the "extern" keyword, which do not allocate storage.
Unix C compilers have traditionally allocated storage for uninitialized global variables in a common
block. This allows the linker to resolve all tentative definitions of the same variable in different
compilation units to the same object, or to a non-tentative definition. This is the behavior
specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior
is not required by ISO C, and on some targets may carry a speed or code size penalty on variable
references.
The -fno-common option specifies that the compiler should instead place uninitialized global
variables in the BSS section of the object file. This inhibits the merging of tentative definitions
by the linker so you get a multiple-definition error if the same variable is defined in more than one
compilation unit. Compiling with -fno-common is useful on targets for which it provides better
performance, or if you wish to verify that the program will work on other systems that always treat
uninitialized variable definitions this way.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the function
is split in the middle, and the two halves are placed at locations far apart in memory. This option
is used when compiling crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This
option is generally only of use to those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when
comparing two assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence the precise format of the
comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded into the object file
that is being created. This switch is only implemented on some targets and the exact format of the
recording is target and binary file format dependent, but it usually takes the form of a section
containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only
records information in the assembler output file as comments, so it never reaches the object file.
See also -grecord-gcc-switches for another way of storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the
target machine. Such code accesses all constant addresses through a global offset table (GOT). The
dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of
GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a
machine-specific maximum size, you get an error message from the linker indicating that -fpic does
not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on
AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines.
For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and
avoiding any limit on the size of the global offset table. This option makes a difference on
AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the generated position-independent code can be only
linked into executables. Usually these options are used to compile code that will be linked using
the -pie GCC option.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for
-fpie and 2 for -fPIE.
-fno-plt
Do not use the PLT for external function calls in position-independent code. Instead, load the
callee address at call sites from the GOT and branch to it. This leads to more efficient code by
eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86
where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation
freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols
are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific
external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use
the PLT to use the GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code
generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code
that forms part of a dynamic linker and cannot reference the address of a jump table. On some
targets, jump tables do not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except
perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined
in the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be
allocated for temporaries or variables that do not live across a call. Functions compiled this way
do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model produces disastrous
results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even
for temporaries or variables that live across a call. Functions compiled this way save and restore
the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model produces disastrous
results.
A different sort of disaster results from the use of this flag for a register in which function
values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is
specified (which must be a small power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default alignment requirements larger than
this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible
with code generated without that switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of global-dynamic,
local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the
compiler may use a more efficient model for symbols not visible outside of the translation unit, or
if -fpic is not given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions, always generate them instead of
using descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do
nothing.
A trampoline is a small piece of code that is created at run time on the stack when the address of a
nested function is taken, and is used to call the nested function indirectly. Therefore, it requires
the stack to be made executable in order for the program to work properly.
-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid
generating them, if it computes that this is safe, and replace them with descriptors. Descriptors
are made up of data only, but the generated code must be prepared to deal with them. As of this
writing, -fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary
compatible if nested functions are present. This option must therefore be used on a program-wide
basis and be manipulated with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified option---all symbols are marked with
this unless overridden within the code. Using this feature can very substantially improve linking
and load times of shared object libraries, produce more optimized code, provide near-perfect API
export and prevent symbol clashes. It is strongly recommended that you use this in any shared
objects you distribute.
Despite the nomenclature, default always means public; i.e., available to be linked against from
outside the shared object. protected and internal are pretty useless in real-world usage so the only
other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e.,
make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is
given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at
<https://www.akkadia.org/drepper/>)---however a superior solution made possible by this option to
marking things hidden when the default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with -fvisibility=hidden and "__attribute__
((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with
identical syntax. This is a great boon to those working with cross-platform projects.
For those adding visibility support to existing code, you may find "#pragma GCC visibility" of use.
This works by you enclosing the declarations you wish to set visibility for with (for example)
"#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". Bear in mind that symbol
visibility should be viewed as part of the API interface contract and thus all new code should always
specify visibility when it is not the default; i.e., declarations only for use within the local DSO
should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this
abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++
specification requirements, "operator new" and "operator delete" must always be of default
visibility.
Be aware that headers from outside your project, in particular system headers and headers from any
other library you use, may not be expecting to be compiled with visibility other than the default.
You may need to explicitly say "#pragma GCC visibility push(default)" before including any such
headers.
"extern" declarations are not affected by -fvisibility, so a lot of code can be recompiled with
-fvisibility=hidden with no modifications. However, this means that calls to "extern" functions with
no explicit visibility use the PLT, so it is more effective to use "__attribute ((visibility))"
and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as
hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an
exception class that is be thrown between DSOs must be explicitly marked with default visibility so
that the type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
<http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although
the compiler usually honors those types anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped
peripheral registers might require all such accesses to be 16 bits wide; with this flag you can
declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these targets) to
force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous
example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain
any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied to a structure field, it may not be
possible to access the field with a single read or write that is correctly aligned for the target
machine. In this case GCC falls back to generating multiple accesses rather than code that will
fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non
bit-field members. It is therefore recommended to define all bits of the field's type as bit-field
members.
The default value of this option is determined by the application binary interface for the target
processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync" family of functions may be used
to implement the C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only useful form of the option is
-fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
GCC Developer Options
This section describes command-line options that are primarily of interest to GCC developers, including
options to support compiler testing and investigation of compiler bugs and compile-time performance
problems. This includes options that produce debug dumps at various points in the compilation; that
print statistics such as memory use and execution time; and that print information about GCC's
configuration, such as where it searches for libraries. You should rarely need to use any of these
options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can
specify stdout or - to dump to standard output, and stderr for standard error.
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file
name, a pass number, phase letter, and pass name. The base dump file name is the name of output file
produced by the compiler if explicitly specified and not an executable; otherwise it is the source file
name. The pass number is determined by the order passes are registered with the compiler's pass manager.
This is generally the same as the order of execution, but passes registered by plugins, target-specific
passes, or passes that are otherwise registered late are numbered higher than the pass named final, even
if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-
specific), r (RTL), or t (tree). The files are created in the directory of the output file.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by letters. This is used for
debugging the RTL-based passes of the compiler.
Some -dletters switches have different meaning when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible
letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or
auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization
passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion
passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination
passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation
passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression
elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the x87's stack-like
registers. This pass is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging information.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which pattern and alternative is used.
The length and cost of each instruction are also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp
annotation.
-dx Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
-fdump-debug
Dump debugging information generated during the debug generation phase.
-fdump-earlydebug
Dump debugging information generated during the early debug generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different compiler binaries and/or different text / bss
/ data / heap / stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes it more
feasible to use diff on debugging dumps for compiler invocations with different options, in
particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the
previous and next instructions in a sequence.
-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file
name is generated by appending a switch specific suffix to the source file name, and the file is
created in the same directory as the output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
inline
Dump after function inlining.
Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning
as for -fopt-info, defaulting to -optimized.
For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined,
along with callsites that were not inlined.
By default, the dump will contain messages about successful optimizations (equivalent to -optimized)
together with low-level details about the analysis.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options and filename portions behave as
described in the -fdump-tree option. The following switch values are accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table information is emitted unless 'slim' is
specified. This option is applicable to C++ only.
raw Dump the raw internal tree data. This option is applicable to C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are turned on and off by the current command-
line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is generated by
appending a suffix ending in .statistics to the source file name, and the file is created in the same
directory as the output file. If the -option form is used, -stats causes counters to be summed over
the whole compilation unit while -details dumps every event as the passes generate them. The default
with no option is to sum counters for each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate language tree to a file. If the
-options form is used, options is a list of - separated options which control the details of the
dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The
following options are available
address
Print the address of each node. Usually this is not meaningful as it changes according to the
environment and source file. Its primary use is for tying up a dump file with a debug
environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of
"DECL_NAME". Its primary use is ease of use working backward from mangled names in the assembly
file.
slim
When dumping front-end intermediate representations, inhibit dumping of members of a scope or
body of a function merely because that scope has been reached. Only dump such items when they
are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like
representation.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like
representation.
details
Enable more detailed dumps (not honored by every dump option). Also include information from the
optimization passes.
stats
Enable dumping various statistics about the pass (not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the
control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in
the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in certain passes).
missed
Enable showing missed optimization information (only available in certain passes).
note
Enable other detailed optimization information (only available in certain passes).
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and note.
To determine what tree dumps are available or find the dump for a pass of interest follow the steps
below.
1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the
pass you are interested in. For example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2"
correspond to the three Value Range Propagation passes. The number at the end distinguishes
distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and
invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass,
invoke GCC with the -fdump-tree-evrp option. Optionally, you may specify the name of the dump
file. If you don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period:
the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass
number followed by the letter t for tree passes (and the letter r for RTL passes), and finally
the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp
in the current working directory. Note that the numeric codes are not stable and may change from
one version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options form is used, options
is a list of - separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-overlapping. However, in case of any
conflicts, the later options override the earlier options on the command line.
The following options control which kinds of messages should be emitted:
optimized
Print information when an optimization is successfully applied. It is up to a pass to decide
which information is relevant. For example, the vectorizer passes print the source location of
loops which are successfully vectorized.
missed
Print information about missed optimizations. Individual passes control which information to
include in the output.
note
Print verbose information about optimizations, such as certain transformations, more detailed
messages about decisions etc.
all Print detailed optimization information. This includes optimized, missed, and note.
The following option controls the dump verbosity:
internals
By default, only "high-level" messages are emitted. This option enables additional, more
detailed, messages, which are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to describe a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful
optimizations from all the passes, omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable optimizations are concatenated
into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are
accepted, only one of them can include a filename. If other filenames are provided then all but the
first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined
output from multiple translation units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization passes on stderr. Note
that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization
group names and message types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations from all the inlining
passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is
allowed. In this case, only the first option takes effect and the subsequent options are ignored.
Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those
optimizations that support -fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to
change.
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages
with source file, line number and column number, with the following additional data for each message:
* the execution count of the code being optimized, along with metadata about whether this was from
actual profile data, or just an estimate, allowing consumers to prioritize messages by code
hotness,
* the function name of the code being optimized, where applicable,
* the "inlining chain" for the code being optimized, so that when a function is inlined into
several different places (which might themselves be inlined), the reader can distinguish between
the copies,
* objects identifying those parts of the message that refer to expressions, statements or symbol-
table nodes, which of these categories they are, and, when available, their source code location,
* the GCC pass that emitted the message, and
* the location in GCC's own code from which the message was emitted
Additionally, some messages are logically nested within other messages, reflecting implementation
details of the optimization passes.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the
scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and
-fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready
list information and unit/insn info. For n greater than two, it includes RTL at abort point,
control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable optimization passes. These
options are intended for use for debugging GCC. Compiler users should use regular options for
enabling/disabling passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the
compiler multiple times, the pass name should be appended with a sequential number starting from
1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the
compiler multiple times, the pass name should be appended with a sequential number starting from
1. range-list is a comma-separated list of function ranges or assembler names. Each range is a
number pair separated by a colon. The range is inclusive in both ends. If the range is trivial,
the number pair can be simplified as a single number. If the function's call graph node's uid
falls within one of the specified ranges, the pass is disabled for that function. The uid is
shown in the function header of a dump file, and the pass names can be dumped by using option
-fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the
compiler multiple times, the pass name should be appended with a sequential number starting from
1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the compiler configuration.
-fchecking=2 enables further internal consistency checking that might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers in generating certain symbol
names that have to be different in every compiled file. It is also used to place unique stamps in
coverage data files and the object files that produce them. You can use the -frandom-seed option to
produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's
converted to a number by computing CRC32).
The string should be different for every file you compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place them in the current directory and
name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i
and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler
now normally uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid
over writing an input source file with the same extension as an intermediate file. The corresponding
intermediate file may be obtained by renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that share a common base name
in different subdirectories or the same source file compiled for multiple output destinations, it is
likely that the different parallel compilers will interfere with each other, and overwrite the
temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary
files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves
like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this
is the compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself.
The second number is "system time", time spent executing operating system routines on behalf of the
program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like
this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program name, and the options passed to
the program are displayed, so that one can later tell what file was being compiled, and with which
options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if
file is "."), the name of the dump file is determined by appending ".gkd" to the compilation output
file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and
-fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables
-fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used
for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which
disables the dumping of the final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation
(rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG
to -w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by
-fcompare-debug, along with options to silence warnings, and omitting other options that would cause
the compiler to produce output to files or to standard output as a side effect. Dump files and
preserved temporary files are renamed so as to contain the ".gk" additional extension during the
second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped,
which makes it useful for little other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2
otherwise. The position of this argument in the command line does not matter; it takes effect after
all other options are processed, and it does so only once, no matter how many times it is given.
This is mainly intended to be used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about
each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is
5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format
as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of
this report vary from version to version. It is meant to be useful to GCC developers when processing
object files in LTO mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of Link Time Optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after
interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of
individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The
filename for the dump is made by appending .su to the auxname. auxname is generated from the name of
the output file, if explicitly specified and it is not an executable, otherwise it is the basename of
the source file. An entry is made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack statically: a fixed number of
bytes are allocated for the frame on function entry and released on function exit; no stack
adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the
static allocation described above, stack adjustments are made in the body of the function, for
example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the
amount of these adjustments is bounded at compile time and the second field is an upper bound of the
total amount of stack used by the function. If it is not present, the amount of these adjustments is
not bounded at compile time and the second field only represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of the compilation. This option is supported
only by the C++ front end, and the information is generally only useful to the G++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list
of name:lower_bound:upper_bound tuples which sets the lower and the upper bound of each debug counter
name. The lower_bound is optional and is zero initialized if not set. All debug counters have the
initial upper bound of "UINT_MAX"; thus "dbg_cnt" returns true always unless the upper bound is set
by this option. For example, with -fdbg-cnt=dce:2:4,tail_call:10, "dbg_cnt(dce)" returns true only
for third and fourth invocation. For "dbg_cnt(tail_call)" true is returned for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't
do anything else. With this option, GCC does not compile or link anything; it just prints the file
name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the
command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory
name is separated from the switches by ;, and each switch starts with an @ instead of the -, without
spaces between multiple switches. This is supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS
libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if
OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or
../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9
or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You
can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories
gcc searches---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file
or directory. To resolve this you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory
where you installed them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation. This is the target sysroot
specified either at configure time or using the --sysroot option, possibly with an extra suffix that
depends on compilation options. If no target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the
compiler is not configured with such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't do anything else. This is the
compiler version used in filesystem paths and specs. Depending on how the compiler has been
configured it can be just a single number (major version), two numbers separated by a dot (major and
minor version) or three numbers separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version---and don't do anything else. The output is always three numbers
separated by dots, major, minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is
being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for example, to allow you to compile for
a particular processor variant or ABI, or to control optimizations specific to that machine. By
convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-specific options, usually for
compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values are ilp32 for SysV-like data model
where int, long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits,
but long int and pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not
link-compatible; you must compile your entire program with the same ABI, and link with a compatible
set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from
using floating-point and Advanced SIMD registers but will not impose any restrictions on the
assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not
an aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its statically defined symbols must be within
1MB of each other. Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its statically defined symbols must be
within 4GB of each other. Programs can be statically or dynamically linked. This is the default
code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions about addresses and sizes of
sections. Programs can be statically linked only.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as
described in the architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behavior is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for a global
canary or sysreg for a canary in an appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which system register to use as base
register for reading the canary, and from what offset from that base register. There is no default
register or offset as this is entirely for use within the Linux kernel.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for a global
canary or sysreg for a canary in an appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which system register to use as base
register for reading the canary, and from what offset from that base register. There is no default
register or offset as this is entirely for use within the Linux kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.
This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires
binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves
inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate
instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum
workaround is made at link time and this will only pass the corresponding flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This option only has an effect if
-ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of
reciprocal square root results to about 16 bits for single precision and to 32 bits for double
precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results
to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies
-mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to
about 16 bits for single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track speculative execution through conditional
branches. The tracking state can then be used by the compiler when expanding calls to
"__builtin_speculation_safe_copy" to permit a more efficient code sequence to be generated.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will,
at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the
load/store-exclusive instructions that are present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later
revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used
directly. The same applies when using -mcpu= when the selected cpu supports the lse feature.
-march=name
Specify the name of the target architecture and, optionally, one or more feature modifiers. This
option has the form -march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a or
native.
The value armv8.5-a implies armv8.4-a and enables compiler support for the ARMv8.5-A architecture
extensions.
The value armv8.4-a implies armv8.3-a and enables compiler support for the ARMv8.4-A architecture
extensions.
The value armv8.3-a implies armv8.2-a and enables compiler support for the ARMv8.3-A architecture
extensions.
The value armv8.2-a implies armv8.1-a and enables compiler support for the ARMv8.2-A architecture
extensions.
The value armv8.1-a implies armv8-a and enables compiler support for the ARMv8.1-A architecture
extension. In particular, it enables the +crc, +lse, and +rdma features.
The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the
architecture of the host system. This option has no effect if the compiler is unable to recognize
the architecture of the host system,
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march
and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most
feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If
-march is specified without either of -mtune or -mcpu also being specified, the code is tuned to
perform well across a range of target processors implementing the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should tune the performance of the code.
Permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, ares, exynos-m1, emag, falkor, neoverse-e1,
neoverse-n1, neoverse-n2, neoverse-v1, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx,
octeontx81, octeontx83, a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110,
thunderxt83, thunderx2t99, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC should tune for
a big.LITTLE system.
Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host
system. This option has no effect if the compiler is unable to recognize the processor of the host
system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a
range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This
option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as
those available for -mtune. The permissible values for feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers
are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if
by -march) and to determine the target processor for which to tune for performance (as if by -mtune).
Where this option is used in conjunction with -march or -mtune, those options take precedence over
the appropriate part of this option.
-moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax,
semantics, and accepted values for string in this option are not guaranteed to be consistent across
releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in
debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools are accessed using a
single instruction and emitted after each function. This limits the maximum size of functions to
1MB. This is enabled by default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be applied. Permissible values are
none, which disables return address signing, non-leaf, which enables pointer signing for functions
which are not leaf functions, and all, which enables pointer signing for all functions. The default
value is none. This option has been deprecated by -mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf]|bti
Select the branch protection features to use. none is the default and turns off all types of branch
protection. standard turns on all types of branch protection features. If a feature has additional
tuning options, then standard sets it to its standard level. pac-ret[+leaf] turns on return address
signing to its standard level: signing functions that save the return address to memory (non-leaf
functions will practically always do this) using the a-key. The optional argument leaf can be used
to extend the signing to include leaf functions. bti turns on branch target identification
mechanism.
-mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of
the following options:
retbr
blr
In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS
hardening.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option only has an effect when SVE is
enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any
size of vector register and "vector-length specific" output that allows GCC to make assumptions about
the vector length when it is useful for optimization reasons. The possible values of bits are:
scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output.
At present -msve-vector-bits=128 also generates vector-length agnostic output. All other values
generate vector-length specific code. The behavior of these values may change in future releases and
no value except scalable should be relied on for producing code that is portable across different
hardware SVE vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
crc Enable CRC extension. This is on by default for -march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
fp Enable floating-point instructions. This is on by default for all possible values for options -march
and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by
default for all possible values for options -march and -mcpu.
sve Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point
instructions.
lse Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This
option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to
Armv8.2-A is not supported.
rcpc
Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the
assembler, enabling inline asm statements to use instructions from the RcPc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of
this option with architectures prior to Armv8.2-A is not supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this
option with architectures prior to Armv8.2-A is not supported.
profile
Enable the Statistical Profiling extension. This option is only to enable the extension at the
assembler level and does not affect code generation.
rng Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the
assembler level and does not affect code generation.
memtag
Enable the Armv8.5-a Memory Tagging Extensions. This option is only to enable the extension at the
assembler level and does not affect code generation.
sb Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at
the assembler level and does not affect code generation. This option is enabled by default for
-march=armv8.5-a.
ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the
extension at the assembler level and does not affect code generation. This option is enabled by
default for -march=armv8.5-a.
predres
Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to
enable the extension at the assembler level and does not affect code generation. This option is
enabled by default for -march=armv8.5-a.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which
implies nocrypto, noaes and nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants
that lack these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction generation. This can result in
increased instruction count, so this may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is
not guaranteed to produce consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This
is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when
two different small numbers are compared such that their difference is calculated as zero. The
default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the
eight bytes in the range "sp+0...sp+7" can be used by leaf functions without stack allocation.
Values other than 8 or 16 are untested and unlikely to work. Note also that this option changes the
ABI; compiling a program with a different stack offset than the libraries have been compiled with
generally does not work. This option can be useful if you want to evaluate if a different stack
offset would give you better code, but to actually use a different stack offset to build working
programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num
option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default is
-mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the
"b" / "bl" instructions, and therefore load the function address into a register before performing a
(otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" /
"bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses
for which -mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is
provided and expected at function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller and faster by avoiding
unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored when the function returns, and when
it calls other functions. This mode is useful for compiling libraries or other compilation units
you might want to incorporate into different programs with different prevailing FPU modes, and
the convenience of being able to use a single object file outweighs the size and speed overhead
for any extra mode switching that might be needed, compared with what would be needed with a more
specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with truncating (i.e. round towards zero)
rounding mode. That includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with round-to-nearest-or-even rounding
mode.
int This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or
integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-
increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as
preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that
this is an ABI change, even though many library function interfaces are unaffected if they don't use
SIMD vector modes in places that affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can give better register
allocation, but so far the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants and certain
bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a
fixed register, and none, which means that no register is used for this purpose. The default is
-m1reg-none.
AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for gpu are
fiji
Compile for GCN3 Fiji devices (gfx803).
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
-mstack-size=bytes
Specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware
that there may be many threads and limited memory available. The size of the stack allocation may
also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and
1MB otherwise.
ARC Options
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or
-mcpu=ARCEM is in effect.
-mjli-always
Force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling parameters for cpu. There are also
shortcut alias options available for backward compatibility and convenience. Supported values for
cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with
--with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist
instructions.
hs Compile for ARC HS CPU with no hardware extensions except the atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux registers.
-mea
Generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are
supported. This is always enabled for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for
-mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes only. Software floating-point code is
emitted by default, and this default can overridden by FPX options; -mspfp, -mspfp-compact, or
-mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension to implement atomic memory built-in
functions. Not available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can specify the option using either a string
or numeric value for multo. wlh1 is the default value. The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally
enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally
enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following instructions are additionally
enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled:
"mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values
for fpu are:
fpus
Enables support for single-precision floating-point hardware extensions.
fpud
Enables support for double-precision floating-point hardware extensions. The single-precision
floating-point extension is also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware extensions using double-precision
assist instructions. The single-precision floating-point extension is also enabled. This option
is only available for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware extensions using double-precision
assist instructions. The single-precision floating-point, square-root, and divide extensions are
also enabled. This option is only available for ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware extensions using double-precision
assist instructions. The single-precision floating-point and fused multiply and add hardware
extensions are also enabled. This option is only available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware extensions using double-precision
assist instructions. All single-precision floating-point hardware extensions are also enabled.
This option is only available for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-root and divide hardware extensions.
This option includes option fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused multiply and add hardware
extensions.
fpud_fma
Enables support for double-precision floating-point and fused multiply and add hardware
extensions. This option includes option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point hardware extensions.
fpud_all
Enables support for all single- and double-precision floating-point hardware extensions. Not
available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor automatically saves/restores on interrupt
entry and exit. register-range is specified as two registers separated by a dash. The register
range always starts with "r0", the upper limit is "fp" register. blink and lp_count are optional.
This option is only valid for ARC EM and ARC HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second register bank on entry to fast interrupt.
Fast interrupts are interrupts with the highest priority level P0. These interrupts save only PC and
STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this
option when you are using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8,
16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for width are 8, 16, 20, 24, 28 and 32
bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler does not
attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known
that the "lp_count" register can hold the required loop-counter value. Depending on the width
specified, the compiler and run-time library might continue to use the loop mechanism for various
needs. This option defines macro "__ARC_LPC_WIDTH__" with the value of width.
-mrf16
This option instructs the compiler to generate code for a 16-entry register file. This option
defines the "__ARC_RF16__" preprocessor macro.
-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump tables.
The following options are passed through to the assembler, and also define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol
"__Xdsp_packa". This option is deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the
preprocessor symbol "__Xdvbf". This option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store conditional extension. Also sets the
preprocessor symbol "__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This option is
deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This option is
deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also
sets the preprocessor symbol "__Xrtsc". This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the
preprocessor symbol "__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand instructions for telephony. Also
sets the preprocessor symbol "__Xtelephony". This option is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol
"__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to make an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux" emulation. This option is enabled by
default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling
is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof" emulation. This option is
enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when
profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the offset available for an
unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to
allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This
is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num
bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store
operations.
-mno-sdata
Do not generate sdata references. This is the default for tool chains built for "arc-linux-uclibc"
and "arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg to generate compare-and-branch ("brcc")
instructions. It has no effect on generation of these instructions driven by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and only available for ARCv1 cores.
This option is deprecated.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution instructions.
Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction
lengths, and the support for conditional execution, the target-independent pass to generate
conditional execution is often lacking, so the ARC port has kept a special pass around that tries to
find more conditional execution generation opportunities after register allocation, branch
shortening, and delay slot scheduling have been done. This pass generally, but not always, improves
performance and code size, at the cost of extra compilation time, which is why there is an option to
switch it off. If you have a problem with call instructions exceeding their allowable offset range
because they are conditionalized, you should consider using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option is
deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because some optimizers then assume that
indexed stores exist, which is not the case.
-mlra
Enable Local Register Allocation. This is still experimental for ARC, so by default the compiler
uses standard reload (i.e. -mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using -Os), prologues and epilogues that have to save or restore a large
number of registers are often shortened by using call to a special function in libgcc; this is
referred to as a millicode call. As these calls can pose performance issues, and/or cause linking
issues when linking in a nonstandard way, this option is provided to turn on or off millicode call
generation.
-mcode-density-frame
This option enable the compiler to emit "enter" and "leave" instructions. These instructions are
only valid for CPUs with code-density feature.
-mmixed-code
Tweak register allocation to help 16-bit instruction generation. This generally has the effect of
decreasing the average instruction size while increasing the instruction count.
-mq-class
Enable q instruction alternatives. This is the default for -Os.
-mRcq
Enable Rcq constraint handling. Most short code generation depends on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly depends on this. This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values
for level are:
0 No size optimization. This level is deprecated and treated like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers are dropped.
3 In addition, optional data alignment is dropped, and the option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent
to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed,
branches without filled delay slot are preferably emitted unaligned and long, unless profiling
indicates that the probability for the branch to be taken is below probability. The default is
(REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility, but are now deprecated and will be
removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is
supported by configuring GCC to build "arceb-elf32" and "arceb-linux-uclibc" targets, for which big
endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now deprecated. Little-endian code
is supported by configuring GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for which little
endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and
ARC700-xmac respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and
iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even
if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer
with this option causes the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame. This option is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets. Without this option,
on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The
default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is
specified. In AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging of those instruction
with the instructions in the function's body. This means that all functions start with a
recognizable set of instructions (or in fact one of a choice from a small set of different function
prologues), and this information can be used to locate the start of functions inside an executable
piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations.
softfp allows the generation of code using hardware floating-point instructions, but still uses the
soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-
specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float
ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from
using floating-point and Advanced SIMD registers but will not impose any restrictions on the
assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard
configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a
little-endian processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for
little-endian images and is ignored. The default is dependent on the selected target architecture.
For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32.
BE32 format has been deprecated by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with
or instead of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z,
armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a,
armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and
iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are
recognized but support is deprecated: armv4.
Many of the architectures support extensions. These can be added by appending +extension to the
architecture name. Extension options are processed in order and capabilities accumulate. An
extension will also enable any necessary base extensions upon which it depends. For example, the
+crypto extension will always enable the +simd extension. The exception to the additive construction
is for extensions that are prefixed with +no...: these extensions disable the specified option and
any other extensions that may depend on the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the
+simd option is entirely disabled by the +nofp option that follows it.
Most extension names are generically named, but have an effect that is dependent upon the
architecture to which it is applied. For example, the +simd option can be applied to both armv7-a
and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for
armv7-a and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture. Architectures not mentioned do
not support any extensions.
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this
extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The extension
+vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not
supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and
ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The extension
+vfpv3-d16 can be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon
and +neon-vfpv3 can be used as aliases for this extension.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the half-
precision floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the half-
precision floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-
precision floating-point conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support for virtualization.
+fp The VFPv4 floating-point instructions, with 16 double-precision registers. The extension
+vfpv4-d16 can be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension
+neon-vfpv4 can be used as an alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-precision registers.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the half-
precision floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the half-
precision floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension
+neon-vfpv3 can be used as an alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-
precision floating-point conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point
instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing instructions. This also enables the
Advanced SIMD and floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension. This also enables the half-precision
floating-point extension and Advanced SIMD and floating-point instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point
instructions.
+dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the Dot Product extension and the
half-precision floating-point fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product
extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point
instructions as well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.5-a
+fp16
The half-precision floating-point data processing instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the Dot Product extension and the
half-precision floating-point fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product
extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point
instructions as well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used
as an alias for this extension.
+fp The VFPv3 floating-point instructions with 16 double-precision registers. The extension
+vfpv3-d16 can be used as an alias for this extension.
+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point instructions with 16 double-precision registers and
the half-precision floating-point conversion operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-precision registers and the half-
precision floating-point conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point instructions.
+fpv5
The single-precision FPv5 floating-point instructions.
+fp.dp
The single- and double-precision FPv5 floating-point instructions.
+nofp
Disable the floating-point extensions.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present,
this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM processor for which GCC should tune the performance
of the code. For some ARM implementations better performance can be obtained by using this option.
Permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, 0strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s,
arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e,
arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s,
arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12,
cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72,
cortex-a73, cortex-a75, cortex-a76, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8,
cortex-r52, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23,
cortex-m33, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312,
fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune the performance of the code for a
big.LITTLE system. Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within
architecture arch. The aim is to generate code that run well on the current most popular processors,
balancing between optimizations that benefit some CPUs in the range, and avoiding performance
pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models
come and go.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the
tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this
feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect
is unsuccessful the option has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the
target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for
performance (as if specified by -mtune). Where this option is used in conjunction with -march or
-mtune, those options take precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions. Where this is so the
architectural extensions are normally enabled by default. If implementations that lack the extension
exist, then the extension syntax can be used to disable those extensions that have been omitted. For
floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and
-mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if
-mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available
floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations: integer only, with just a
floating-point unit or with floating-point and Advanced SIMD. The default is to enable all the
instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the
SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp
Disable the DSP instructions on cortex-m33.
+nofp
Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e,
arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4,
cortex-m7 and cortex-m33. Disables the floating-point and SIMD instructions on generic-armv7-a,
cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-point instructions on cortex-r5,
cortex-r7, cortex-r8, cortex-r52 and cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7
and cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55,
cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers.
It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16,
vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the
extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See
-mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this
feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect
is unsuccessful the option has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is available on the target.
Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd,
vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16,
fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for
neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the compiler to select the floating-point
and Advanced SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that
floating-point operations are not generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because NEON hardware does not fully
implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the "target("fpu=")" function attributes or
pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none,
ieee, and alternative; the default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this
option. Permissible values are 8, 32 and 64. The default value varies for different toolchains.
For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the
underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of
the program. Different values are potentially incompatible. Code compiled with one value cannot
necessarily expect to work with code or libraries compiled with another value, if they exchange
information using structures or unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the
function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a
register and then performing a subroutine call on this register. This switch is needed if the target
function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine
call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is
that static functions, functions that have the "short_call" attribute, functions that are inside the
scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been
compiled within the current compilation unit are not turned into long calls. The exceptions to this
rule are that weak function definitions, functions with the "long_call" attribute or the "section"
attribute, and functions that are within the scope of a "#pragma long_calls" directive are always
turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as
does placing the function calls within the scope of a "#pragma long_calls_off" directive. Note these
switches have no effect on how the compiler generates code to handle function calls via function
pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for
each function. The runtime system is responsible for initializing this register with an appropriate
value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard PIC base case, the default is any
suitable register determined by compiler. For single PIC base case, the default is R9 if target is
EABI based or stack-checking is enabled, otherwise the default is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time. This
permits using PC-relative addressing operations to access data known to be in the data segment. For
non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will
enable -msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The
generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the
trace function then looks at location "pc - 12" and the top 8 bits are set, then we know that there
is a function name embedded immediately preceding this location and has length "((pc[-3]) &
0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The default for most
configurations is to generate code that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by using the "target("thumb")" and
"target("arm")" function attributes or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of
mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf
functions. (A leaf function is one that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf
functions. (A leaf function is one that does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which
switches to Thumb mode before executing the rest of the function. This allows these functions to be
called from non-interworking code. This option is not valid in AAPCS configurations because
interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of
whether the target code has been compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS
configurations because interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which
generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which uses the best available method for the
selected processor. The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and
gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS
models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for
shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require
new assembler, linker and library support. Initial and local exec TLS models are unaffected by this
option and always use the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by
default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when
-fpic or -fPIC is specified. This option conflicts with -mslow-flash-data.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination
and base registers are used. This option avoids generating these instructions. This option is
enabled by default when -mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16-
or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for
ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not
enabled then words in packed data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or
false, depending upon the setting of this option. If unaligned access is enabled then the
preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
Enables using Neon to handle scalar 64-bits operations. This is disabled by default since the cost of
moving data from core registers to Neon is high.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction. Therefore literal load is
minimized for better performance. This option is only supported when compiling for ARMv7 M-profile
and off by default. It conflicts with -mword-relocations.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is currently off which implies
divided syntax. This option has no impact on Thumb2. However, this may change in future releases of
GCC. Divided syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a
single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A
Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is an option used only for
regression testing of the compiler and not intended for ordinary use in compiling code. This option
is disabled by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in
debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF
object format give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE".
This option is only available when generating non-pic code for M-profile targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools
Engineering Specification", which can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/>.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu = "attiny22", "attiny26", "at90s2313",
"at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534",
"at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction. mcu =
"attiny13", "attiny13a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a",
"attiny2313", "attiny2313a", "attiny43u", "attiny44", "attiny44a", "attiny45", "attiny48",
"attiny441", "attiny461", "attiny461a", "attiny4313", "attiny84", "attiny84a", "attiny85",
"attiny87", "attiny88", "attiny828", "attiny841", "attiny861", "attiny861a", "ata5272",
"ata6616c", "at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = "at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu = "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory and with the "MOVW" instruction.
mcu = "attiny167", "attiny1634", "atmega8u2", "atmega16u2", "atmega32u2", "ata5505", "ata6617c",
"ata664251", "at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu = "atmega48", "atmega48a",
"atmega48p", "atmega48pa", "atmega48pb", "atmega8", "atmega8a", "atmega8hva", "atmega88",
"atmega88a", "atmega88p", "atmega88pa", "atmega88pb", "atmega8515", "atmega8535", "ata6285",
"ata6286", "ata6289", "ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b",
"at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = "atmega16", "atmega16a",
"atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4",
"atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165",
"atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a", "atmega168p",
"atmega168pa", "atmega168pb", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega32",
"atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4",
"atmega32u6", "atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega325pa", "atmega328", "atmega328p", "atmega328pb", "atmega329", "atmega329a",
"atmega329p", "atmega329pa", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa",
"atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega64", "atmega64a",
"atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640",
"atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2", "atmega645",
"atmega645a", "atmega645p", "atmega649", "atmega649a", "atmega649p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5795", "ata5790", "ata5790n",
"ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510", "ata5702m322",
"at90pwm161", "at90pwm216", "at90pwm316", "at90can32", "at90can64", "at90scr100", "at90usb646",
"at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu = "atmega128", "atmega128a",
"atmega128rfa1", "atmega128rfr2", "atmega1280", "atmega1281", "atmega1284", "atmega1284p",
"atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu =
"atmega256rfr2", "atmega2560", "atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = "atxmega8e5",
"atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4",
"atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory
visible in the RAM address space. mcu = "attiny202", "attiny204", "attiny212", "attiny214",
"attiny402", "attiny404", "attiny406", "attiny412", "attiny414", "attiny416", "attiny417",
"attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny1604",
"attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny3214", "attiny3216",
"attiny3217", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209",
"atmega4808", "atmega4809".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = "atxmega64a3",
"atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3",
"atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of
RAM. mcu = "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu = "atxmega128a3", "atxmega128a3u",
"atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b",
"atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3",
"atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu =
"atxmega128a1", "atxmega128a1u", "atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu = "attiny4", "attiny5",
"attiny9", "attiny10", "attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu =
"attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS instructions. This option has
only an effect on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable
Attributes,variable attribute.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing
function arguments once in function prologue/epilogue. Without this option, outgoing arguments are
pushed before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack
space might lead to smaller executables because arguments need not be removed from the stack after
such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that
get their arguments on the stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are
small, non-negative integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is
smaller.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo instruction supported by GNU
Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of
the AVR Function Attributes,,"no_gccisr" function attribute. This feature is activated per default
if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support
PR21683 ("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int"
is 1 byte, a "long" is 2 bytes, and "long long" is 4 bytes. Please note that this option does not
conform to the C standards, but it results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like attaching attribute AVR Function
Attributes,,"OS_task" to "main". It is activated per default if optimization is on.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if
applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and
the --relax option to the linker's command line.
Jump relaxing is performed by the linker because jump offsets are not known before code is located.
Therefore, the assembler code generated by the compiler is the same, but the instructions in the
executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs
below.
-mrmw
Assume that the device supports the Read-Modify-Write instructions "XCH", "LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory.
This option is used internally for multilib selection. It is not an optimization option, and you
don't need to set it by hand.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer
is zero. In general, you don't need to set this option by hand.
This option is used internally by the compiler to select and build multilibs for architectures "avr2"
and "avr25". These architectures mix devices with and without "SPH". For any setting other than
-mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this option from the compiler proper's
command line, because the compiler then knows if the device or architecture has an 8-bit stack
pointer and thus no "SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" is only used in
indirect, post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is
emulated by additional instructions. For example, loading a value with "X+const" addressing with a
small non-negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library "lib<mcu>.a".
-nodevicespecs
Don't add -specs=device-specs/specs-<mcu> to the compiler driver's command line. The user takes
responsibility for supplying the sub-processes like compiler proper, assembler and linker with
appropriate command line options.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting address space is not
contained in the incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as
word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space,
there is a special function register called "EIND" that serves as most significant part of the target
address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some
limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions or might read "EIND" directly in
order to emulate an indirect call/jump by means of a "RET" instruction.
* The compiler assumes that "EIND" never changes during the startup code or during the application. In
particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
* For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads
sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub
contains a direct jump to the desired address.
* Linker relaxation must be turned on so that the linker generates the stubs correctly in all
situations. See the compiler option -mrelax and the linker option --relax. There are corner cases
where the linker is supposed to generate stubs but aborts without relaxation and without a helpful
error message.
* The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a
setup with "EIND != 0", a custom linker script has to be used in order to place the sections whose
name start with ".trampolines" into the segment where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from
libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set up "EIND" early, for example by means of
initialization code located in section ".init3". Such code runs prior to general startup code that
initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets
"EIND" to the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line option.
-<C and C++ constructors/destructors called during startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through a symbol ("func_4" in
the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, "func_4" can be defined
in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers.
To access memory locations outside this 64 KiB range, the content of a "RAMP" register is used as high
part of the address: The "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY",
"RAMPZ" special function register, respectively, to get a wide address. Similarly, "RAMPD" is used
together with direct addressing.
* The startup code initializes the "RAMP" special function registers with zero.
* If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then
"RAMPZ" is set as needed before the operation.
* If the device supports RAM larger than 64 KiB and the compiler needs to change "RAMPZ" to accomplish
an operation, "RAMPZ" is reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that
SFR and initializes it with zero in case the ISR code might (implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read
from locations outside the 16-bit address range and change one of the "RAMP" registers, you must
reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of
features. Almost any of the following built-in macros are deduced from device capabilities and thus
triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the
-mmcu=mcu option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7",
respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with
-mmcu=atmega8 the macro is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the device's name. For example,
-mmcu=atmega8 defines the built-in macro "__AVR_ATmega8__", -mmcu=attiny261a defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device name as from
the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device
is that the latter is always lowercase.
If device is not a device but only a core architecture like avr51, this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's name. For example, with
-mmcu=atmega8 the macro is defined to "atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices with more than 8 KiB
of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than
128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program
memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler.
The definition of these macros is affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack
pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of
-mmcu=avr2 and -mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware
erratum. Skip instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only
defined if "__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc.
may use a different address as if addressed by an instruction to access RAM like "LD" or "STS". This
offset depends on the device architecture and has to be subtracted from the RAM address in order to
get the respective I/O address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*" instructions. The flash memory is
seen in the data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not
defined, this feature is not available. If defined, the address space is linear and there is no need
to put ".rodata" into RAM. This is handled by the default linker description file, and is currently
available for "avrtiny" and "avrxmega3". Even more convenient, there is no need to use address
spaces like "__flash" or features like attribute "progmem" and "pgm_read_*".
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure
option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514,
bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537,
bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561,
bf592.
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any
workarounds available for the targeted silicon revision are enabled. If sirevision is none, no
workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are
enabled. The "__SILICON_REVISION__" macro is defined to two hexadecimal digits representing the
major and minor numbers in the silicon revision. If sirevision is none, the "__SILICON_REVISION__"
is not defined. If sirevision is any, the "__SILICON_REVISION__" is defined to be 0xffff. If this
optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted
Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option
causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by
libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other
options, such as -mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save,
set up and restore frame pointers and makes an extra register available in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain speculative loads after
jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC
instructions too soon after conditional branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a
conditional branch.
-mlow64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits
into the low 64k of memory.
-mno-low64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in
place and shared libraries in an environment without virtual memory management. This option implies
-fPIC. With a bfin-elf target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that this library
or executable won't link against any other ID shared libraries. That allows the compiler to use
faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries. Slower code
is generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a
value of 0 generates more compact code; specifying other values forces the allocation of that number
to the current library but is no more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text
segment. This allows for execute in place in an environment without virtual memory management by
eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a
register and then performing a subroutine call on this register. This switch is needed if the target
function lies outside of the 24-bit addressing range of the offset-based version of subroutine call
instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior.
Note these switches have no effect on how the compiler generates code to handle function calls via
function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE floating-point
standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It
has no effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors. This option causes proper start
files and link scripts supporting multicore to be used, and defines the macro "__BFIN_MULTICORE". It
can only be used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core
programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is
used. In this model, the main function of Core B should be named as "coreb_main".
If this option is not used, the single-core application programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when using the one-application-per-core
programming model. Proper start files and link scripts are used to support Core A, and the macro
"__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using the one-application-per-core
programming model. Proper start files and link scripts are used to support Core B, and the macro
"__BFIN_COREB" is defined. When this option is used, "coreb_main" should be used instead of "main".
This option can only be used in conjunction with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files and link scripts are used to put the
application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM
before loading the application.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For
Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default
is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+,
c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the ".neardata" section, which is pointed to by register "B14".
Put small uninitialized global and static data in the ".bss" section, which is adjacent to the
".neardata" section. Put small read-only data into the ".rodata" section. The corresponding
sections used for large pieces of data are ".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections reserved for small data, and use addressing
relative to the "B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data.
Put all initialized global and static data in the ".fardata" section, and all uninitialized data in
the ".far" section. Put all constant data into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10
for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu,
where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the
set of available instructions. The choices for architecture-type are the same as for
-march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option
is active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has
the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of
the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test
instructions before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and
constants to be aligned for the maximum single data access size for the chosen CPU model. The
default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by
these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack frame,
writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit
alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame
are omitted and no return instructions or return sequences are generated in the code. Use this
option only together with visual inspection of the compiled code: no warnings or errors are generated
when call-saved registers must be saved, or storage for local variables needs to be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for
functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the
PLT. The default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a
simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data
at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
-msim
Links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates "sbit"/"cbit" instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or medium. medium is default. However, far
is not valid with -mcr16c, as the CR16C architecture does not support the far data model.
C-SKY Options
GCC supports these options when compiling for C-SKY V2 processors.
-march=arch
Specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and
ck810. The default is ck810.
-mcpu=cpu
Specify the C-SKY target processor. Valid values for cpu are: ck801, ck801t, ck802, ck802t, ck802j,
ck803, ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef,
ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1,
ck803er1, ck803ehr1, ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f,
ck810e, ck810et, ck810ef, ck810eft, ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv.
-mbig-endian
-EB
-mlittle-endian
-EL Select big- or little-endian code. The default is little-endian.
-mhard-float
-msoft-float
Select hardware or software floating-point implementations. The default is soft float.
-mdouble-float
-mno-double-float
When -mhard-float is in effect, enable generation of double-precision float instructions. This is
the default except when compiling for CK803.
-mfdivdu
-mno-fdivdu
When -mhard-float is in effect, enable generation of "frecipd", "fsqrtd", and "fdivd" instructions.
This is the default except when compiling for CK803.
-mfpu=fpu
Select the floating-point processor. This option can only be used with -mhard-float. Values for fpu
are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and
fpv2_divd (-mdouble-float -mdivdu).
-melrw
-mno-elrw
Enable the extended "lrw" instruction. This option defaults to on for CK801 and off otherwise.
-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the "interrupt" and "isr" function attributes.
-mmp
Enable multiprocessor instructions; the default is off.
-mcp
Enable coprocessor instructions; the default is off.
-mcache
Enable coprocessor instructions; the default is off.
-msecurity
Enable C-SKY security instructions; the default is off.
-mtrust
Enable C-SKY trust instructions; the default is off.
-mdsp
-medsp
-mvdsp
Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options
default to off.
-mdiv
-mno-div
Generate divide instructions. Default is off.
-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit instructions.
This option is ignored for CK801 where this is the required behavior, and it defaults to on for
CK802. For other targets, the default is off.
-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802,
or CK803, and is enabled by default for other processors.
-manchor
-mno-anchor
Generate code using global anchor symbol addresses.
-mpushpop
-mno-pushpop
Generate code using "push" and "pop" instructions. This option defaults to on.
-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm
Generate code using "stm" and "ldm" instructions. This option isn't supported on CK801 but is
enabled by default on other processors.
-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it to the assembler. This option is the
default and required for correct code generation on CK801 and CK802, and is optional on other
processors.
-mstack-size
-mno-stack-size
Emit ".stack_size" directives for each function in the assembly output. This option defaults to off.
-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.
-mbranch-cost=n
Set the branch costs to roughly "n" instructions. The default is 1.
-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue sequences. Using this option can result in code
that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or
backtraced. It is disabled by default.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single
architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple
-arch options are used; it does so by running the compiler or linker multiple times and joining the
results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify
the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to
override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only
permits instructions to be used that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries,
/usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive
subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library).
The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its
input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header
files. These directories are interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a
Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name
of a framework is the name of this directory excluding the .framework. Headers associated with the
framework are found in one of those two directories, with Headers being searched first. A
subframework is a framework directory that is in a framework's Frameworks directory. Includes of
subframework headers can only appear in a header of a framework that contains the subframework, or in
a sibling subframework header. Two subframeworks are siblings if they occur in the same framework.
A subframework should not have the same name as a framework; a warning is issued if this is violated.
Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to
support this. The standard frameworks can be found in /System/Library/Frameworks and
/Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework
denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference between this
-iframework and -F is that with -iframework the compiler does not warn about constructs contained
within header files found via dir. This option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values of
version include 10.1, 10.2, and 10.3.9.
If the compiler was built to use the system's headers by default, then the default for this option is
the system version on which the compiler is running, otherwise the default is to make choices that
are compatible with as many systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable.
This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By default "sizeof(bool)" is 4 when
compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on
x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Using this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o
files into already-running programs. -findirect-data and -ffix-and-continue are provided for
backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references
when the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will load the build output file being linked. See man
ld(1) for more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an executable when linking, using
the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or
-march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When
-msoft-float is specified, functions in libgcc.a are used to perform floating-point operations.
Unless they are replaced by routines that emulate the floating-point operations, or compiled in such
a way as to call such emulations routines, these routines issue floating-point operations. If you
are compiling for an Alpha without floating-point operations, you must ensure that the library is
built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point
registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies
-msoft-float. If the floating-point register set is not used, floating-point operands are passed in
integer registers as if they were integers and floating-point results are passed in $0 instead of
$f0. This is a non-standard calling sequence, so any function with a floating-point argument or
return value called by code compiled with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and
restore, any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is
mostly compliant with the IEEE floating-point standard. However, for full compliance, software
assistance is required. This option generates code fully IEEE-compliant code except that the
inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro
"_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to
correctly support denormalized numbers and exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this
option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting
code may execute significantly slower than the code generated by default. Since there is very little
code that depends on the inexact-flag, you should normally not specify this option. Other Alpha
compilers call this option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this
option -fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot
be disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like u, but the instructions are marked to be safe for software completion (see Alpha
architecture manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The
rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number
or towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha
architecture reference manual) controls the rounding mode in effect. The C library initializes
this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d
corresponds to round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without software
assistance it is impossible to recover from a floating trap and program execution normally needs to
be terminated. GCC can generate code that can assist operating system trap handlers in determining
the exact location that caused a floating-point trap. Depending on the requirements of an
application, different levels of precisions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which
program caused a floating-point exception.
f Function precision. The trap handler can determine the function that caused a floating-point
exception.
i Instruction precision. The trap handler can determine the exact instruction that caused a
floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you
also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function prologue of the generated assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller
constants in two or three instructions. If it cannot, it outputs the constant as a literal and
generates code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more
instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it
must relocate itself in memory before it can find the variables and constants in its own data
segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets.
The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or
that of the CPU on which GCC was built if none is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single
and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros.
Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark which relocations should apply to
which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of
the assembler when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata"
and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits
the size of the small data area to 64KB, but allows the variables to be directly accessed via a
single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs
that require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap
instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared
library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used,
the compiler can assume that all local symbols share the same $gp value, and thus reduce the number
of instructions required for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can
specify either the EV style name or the corresponding chip number. GCC supports scheduling
parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the
instruction set from the processor you specify. If you do not specify a processor type, GCC defaults
to the processor on which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the
host processor. -mcpu=native has no effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not
changed.
Native toolchains also support the value native, which selects the best architecture option for the
host processor. -mtune=native has no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the
application. This number is highly dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware
for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main
memory. Note that L3 is only valid for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code, but it does assume that all
symbolic values and addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need to include the simulator
library (libsim.a) on the linker command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime startup
and library to be linked. You must not use this option when generating programs that will run on
real hardware; you must provide your own runtime library for whatever I/O functions are needed.
-mlra
Enable Local Register Allocation. This is still experimental for FT32, so by default the compiler
uses standard reload.
-mnodiv
Do not use div and mod instructions.
-mft32b
Enable use of the extended instructions of the FT32B processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm
Do not generate code that reads program memory.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without
any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small
data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are
computed with 32 bits. With a bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It
has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for
shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is
present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only
sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the
global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3
instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be
used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC
and -fpic without -mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated.
This option is enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the
functions to be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option
only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs
to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled
by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated
code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and
*-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
-mmusl
Use the musl C library. This is the default on *-*-linux-*musl* targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When
linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this
option causes the preprocessor macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh
or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mexr
Extended registers are stored on stack before execution of function with monitor attribute. Default
option is -mexr. This option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution of function with monitor attribute.
Default option is -mno-exr. This option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H
and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on
2-byte boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0,
1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to
determine the proper architecture option for your machine. Code compiled for lower numbered
architectures runs on higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mcaller-copies
The caller copies function arguments passed by hidden reference. This option should be used with
care as it is not compatible with the default 32-bit runtime. However, only aggregates larger than
eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility purposes only.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner. This is necessary for compiling
kernels that perform lazy context switching of floating-point registers. If you use this option and
attempt to perform floating-point operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems
when compiling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster
indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that
performs faster indirect calls.
This option does not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges can be specified separated
by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This
is equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type
are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to
determine the proper scheduling option for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It
also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when
linking some programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the
predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are
available under HP-UX and HI-UX.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the
default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not
affect which ld is called; it only changes what parameters are passed to that ld. The ld that is
called is determined by the --with-ld configure option, GCC's program search path, and finally by the
user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This
option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a shared library and passes
+Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or
implicitly, with the HP linker. This option does not affect which ld is called; it only changes what
parameters are passed to that ld. The ld that is called is determined by the --with-ld configure
option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be
printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX
GCC, i.e. configured with hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker
generated stubs. The default is to generate long calls only when the distance from the call site to
the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set
by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes,
respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or
when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be
useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of
code being generated. The impact on systems that support long absolute calls, and long pic symbol-
difference or pc-relative calls should be relatively small. However, an indirect call is used on
32-bit ELF systems in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for
unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10
and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95
for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines
for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides
additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It
also affects the operational behavior of the C library. Thus, extreme care is needed in using this
option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore
the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this
capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on
HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive
version of libdld.sl. Thus, when the -static option is specified, special link options are needed to
resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the
-static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port,
the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to
prevent the GCC driver from adding these link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for
both the preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent
code, and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may make
assembler output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working
around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel
code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling
firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The
default is to use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful
when not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the
stop bit. This can improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges can be specified separated
by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced,
itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer
to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are
HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results in generation of "ld.a"
instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default setting is
disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results in generation of "ld.a"
instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default setting is
enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during region scheduling
(i.e. before reload). This results in generation of the "ld.s" instructions and the corresponding
check instructions "chk.s". The default setting is disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative
loads before reload. This is effective only with -msched-br-data-spec enabled. The default setting
is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative
loads after reload. This is effective only with -msched-ar-data-spec enabled. The default setting
is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative
loads. This is effective only with -msched-control-spec enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there are no other choices
at the moment. This makes the use of the data speculation much more conservative. The default
setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if there are no other
choices at the moment. This makes the use of the control speculation much more conservative. The
default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the instructions
priorities. This makes the use of the speculation a bit more conservative. The default setting is
disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the
same instruction group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory
insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number
in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are
preferred when the limit is reached, but memory operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for
the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to
be linked in which supports, for example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own runtime library for whatever I/O
functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-
registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code
into available registers, and the performance penalty of using memory instead of registers. Note
that all modules in a program must be compiled with the same value for this option. Because of that,
you must not use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the
"ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. This is the
default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3"
instructions to load their addresses), and assume all subroutines are reachable with the "bl"
instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3"
instructions to load their addresses), and assume subroutines may not be reachable with the "bl"
instruction (the compiler generates the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into one of ".data", ".bss", or ".rodata"
(unless the "section" attribute has been specified). This is the default.
The small data area consists of sections ".sdata" and ".sbss". Objects may be explicitly put in the
small data area with the "section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to
reference them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to
reference them.
-G num
Put global and static objects less than or equal to num bytes into the small data or BSS sections
instead of the normal data or BSS sections. The default value of num is 8. The -msdata option must
be set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num
may or may not work; if it doesn't the linker gives an error message---incorrect code is not
generated.
-mdebug
Makes the M32R-specific code in the compiler display some statistics that might help in debugging
programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2,
then the opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0
and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is
_flush_cache, but a function call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on
which architecture was selected when the compiler was configured; the defaults for the most common
choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of
arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire
architectures are selected according to Freescale's ISA classification and the permissible values
are: isaa, isaaplus, isab and isac.
GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire target. The arch in
this macro is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but
that is optimized for a particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which
also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and
-march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. It also defines
"__mcf_family_family", where the value of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The
M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire
microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040
targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select
the same tuning decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0 architecture arch. It also
defines "mcarch" unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range
of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every
architecture in the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire microarchitecture uarch, where uarch
is one of the arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based
systems. It is equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306,
68307, 68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler is configured for 68010-based
systems. It is equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based
systems. It is equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based
systems. It is equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based
systems. It is equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the
68040. Use this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based
systems. It is equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by
software on the 68060. Use this option if your 68060 does not have code to emulate those
instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based
systems. It is equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332,
68333, 68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for
520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and
MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent
-mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of
the equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware
floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor
of that option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that
can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code
does use the 68881 instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code that
can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code
does use the 68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire
devices that have an FPU. It defines the macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__"
on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for
68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used
without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures.
Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by
-mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the
stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if
you use a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "rtd" instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if
you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but
not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables
on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on
32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the
expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types
differently than most published application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table.
At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing.
-fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher
processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text
segment. This allows for execute-in-place in an environment without virtual memory management. This
option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute-in-
place and shared libraries in an environment without virtual memory management. This option implies
-fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a
value of 0 generates more compact code; specifying other values forces the allocation of that number
to the current library, but is no more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more
than 8192 entries. This code is larger and slower than code generated without this option. On
M680x0 processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively
efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to
report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large
GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch
the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort
GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single
object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the
linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase the speed
of programs that contain functions that need a large amount of stack space, but they can also trigger
a segmentation fault if the stack is extended too much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute difference between two registers.
-mall-opts
Enables all the optional instructions---average, multiply, divide, bit operations, leading zero,
absolute difference, min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two registers.
-mbased=n
Variables of size n bytes or smaller are placed in the ".based" section by default. Based variables
use the $tp register as a base register, and there is a 128-byte limit to the ".based" section.
-mbitops
Enables the bit operation instructions---bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert
("bnotm"), and test-and-set ("tas").
-mc=name
Selects which section constant data is placed in. name may be tiny, near, or far.
-mclip
Enables the "clip" instruction. Note that -mclip is not useful unless you also provide -mminmax.
-mconfig=name
Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each
module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The
"MeP-Integrator" tool, not part of GCC, provides these configurations through this option; using this
option is the same as using all the corresponding command-line options. The default configuration is
default.
-mcop
Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the
coprocessor is normally enabled via the -mconfig= option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by -mall-opts.
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to
this section. Accesses to these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler does not currently generate these
itself, but this option is included for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based runtime.
-msim
Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and
tables.
-mtf
Causes all functions to default to the ".far" section. Without this option, functions default to the
".near" section.
-mtiny=n
Variables that are n bytes or smaller are allocated to the ".tiny" section. These variables use the
$gp base register. The default for this option is 4, but note that there's a 65536-byte limit to the
".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z,
where X is a major version, YY is the minor version, and Z is compatibility code. Example values are
v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time.
This allows data to be referenced by offset from start of text address instead of GOT since PC-
relative addressing is not supported.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called
xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader. This model uses startup file crt2.o which
does not contain a processor reset vector handler. This is suitable for transferring control on a
processor reset to the bootloader rather than the application.
novectors
for applications that do not require any of the MicroBlaze vectors. This option may be useful for
applications running within a monitoring application. This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a
particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3,
mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc,
4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1,
24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1,
1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k,
m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600,
r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000,
r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500,
xlr and xlp. The special value from-abi selects the most compatible architecture for the selected
ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture
option for the host processor. -march=native has no effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are
optional, and vr may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of
the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the
form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For
compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as
synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is "_MIPS_ARCH", which gives the
name of target architecture, as a string. The second has the form "_MIPS_ARCH_foo", where foo is the
capitalized value of "_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and
defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given above. In other words, it has the
full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved
architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is
given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and
the perceived cost of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC optimizes for the processor specified by -march. By using -march
and -mtune together, it is possible to generate code that runs on a family of processors, but
optimize the code for one particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march
ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it
makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and
"nomips16" attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of
mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with
MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code;
it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct
jumps unless GCC knows that the target of the jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS
ASE and are retained for backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you
select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see <http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits
wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the "mthc1" and
"mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same, but each scalar value is
passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar
floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers
also remains the same in that the even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit
registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates
that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be
interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64
extension but forbids the use of odd-numbered single-precision registers. This can be used in
conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to
interlink and run in the same process without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the
default for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked
into shared libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and
-fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for
locally-binding symbols. It can also use shorter GP initialization sequences and generate direct
calls to locally-defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the
GNU linker. However, the option does not affect the ABI of the final executable; it only affects the
ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This
option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without
-msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt
otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively
efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to
report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This works with very large GOTs,
although the code is also less efficient, since it takes three instructions to fetch the value of a
global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to
use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using
library calls instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform
any floating-point operations. This option is presently supported only by some bare-metal MIPS
configurations, where it may select a special set of libraries that lack all floating-point support
(including, for example, the floating-point "printf" formats). If code compiled with -mno-float
accidentally contains floating-point operations, it is likely to suffer a link-time or run-time
failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the
default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is
the default for processors that are known to support these registers. When using the o32 FPXX ABI,
-mno-odd-spreg is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data
with the "abs.fmt" and "neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment is selected. In this case these
instructions are considered arithmetic and avoided where correct operation is required and the input
operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-
point datum manually is used instead unless the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are
considered non-arithmetic and therefore operating correctly in all cases, including in particular
where the input operand is a NaN. These instructions are therefore always used for the respective
operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by
the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted
by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the
first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of
their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When
neither option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful
when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with
--with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see
the installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines
"__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when
generating 64-bit code and requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and
"nomicromips" attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
-mcrc
-mno-crc
Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV) instructions.
-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way
that the pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use
32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit
"long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is
smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This
option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate
shorter and faster references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num
bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in C.
-mlocal-sdata is the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try
rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build
large libraries with -mno-local-sdata, so that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section if the size of that
data is within the -G limit. -mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var
that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If
Var is defined by another module, you must either compile that module with a high-enough -G setting
or attach a "section" attribute to Var's definition. If Var is common, you must link the application
with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G
option. However, you may wish to build a library that supports several different small data limits.
You can do this by compiling the library with the highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see
-G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For
example, if the code is part of a library that might be used in a boot monitor, programs that call
boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is
usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data
section if possible, otherwise in data. This gives slightly slower code than the default, but
reduces the amount of RAM required when executing, and thus may be preferred for some embedded
systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in
conjunction with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections. There are three possible
settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must
not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read
Inhibit bit set. It is also useful on processors that can be configured to have a dual
instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads
to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are
configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superseded by -mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The
alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation
operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break
instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also,
some versions of the Linux kernel have a bug that prevents trap from generating the proper signal
("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store instructions to enable
load/store bonding. This option is enabled by default but only takes effect when the selected
architecture is known to support bonding.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy,
which allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more
efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer instructions. The default is -mimadd on
architectures that support "madd" and "msub" except for the 74k architecture where it was found to
generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available.
The default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is
calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be
undesirable in some circumstances. On other processors the result is numerically identical to the
equivalent computation using separate multiply, add, subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when
assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented
by the assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after
starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer
multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or
a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after
starting an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on
revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions.
-mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction
placed at the end of a short loop of six instructions or fewer and always schedule a "nop"
instruction there instead. The short loop bug under certain conditions causes loops to execute only
once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the
assembler rather than by GCC.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds are implemented by the assembler
rather than by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these
functions are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata
are handled by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather
than by GCC, although GCC avoids using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and
"dmacchi" instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1"
and "F2" floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K
processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and
speculatively executes instructions from the "taken" branch. It later aborts these instructions if
the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side
effects.
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a
speculatively-executed store may load the target memory into cache and mark the cache line as dirty,
even if the store itself is later aborted. If a DMA operation writes to the same area of memory
before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K
processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be
speculatively executed and that might have side effects even if aborted.
-mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that
aborted accesses to any byte in the following regions does not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed
safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC
honors this restriction for functions it compiles itself. It expects non-GCC functions (such as
hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might
have side effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have
side effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If
called, the function must take the same arguments as the common "_flush_func", that is, the address
of the memory range for which the cache is being flushed, the size of the memory range, and the
number 3 (to flush both caches). The default depends on the target GCC was configured for, but
commonly is either "_flush_func" or "__cpu_flush".
mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is
not guaranteed to produce consistent results across releases. A zero cost redundantly selects the
default, which is based on the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected
architecture. By default, Branch Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that
implement those architectures; for those, Branch Likely instructions are not be generated by default
because the MIPS32 and MIPS64 architectures specifically deprecate their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The default is
-mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch instructions will never be generated.
The -mcompact-branches=always option ensures that a compact branch instruction will be generated if
available. If a compact branch instruction is not available, a delay slot form of the branch will be
used instead.
This option is supported from MIPS Release 6 onwards.
The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available
in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a
compact branch will be chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some
processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we
can use both FP pipes. Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first
one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks
should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at
the expense of making it bigger. It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci"
instructions (if enabled) are generated when "__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can be overridden by configuring GCC with
--with-synci.
When compiling code for single processor systems, it is generally safe to use "synci". However, on
many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may
lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only
possible if the linker can resolve the destination at link time and if the destination is within
range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support
the ".reloc" assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this
optimization can be performed by the assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When
enabled, this option extends the usual "_mcount" interface with a new ra-address parameter, which has
type "intptr_t *" and is passed in register $12. "_mcount" can then modify the return address by
doing both of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions
will allocate 16 bytes on the stack for the called function to write out register arguments. When
enabled, this optimization will suppress the allocation of the frame header if it can be determined
that it is unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1", "swxc1", "ldxc1", "sdxc1" instructions.
Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand "madd.s", "madd.d" and related
instructions. Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no
matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon
register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are
seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load
instructions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the
default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the
"PREFIX" assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix
simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable
branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically
generates a request (handled by the assembler and the linker) for a constant to be set up in a global
register. The register is used for one or more base address requests within the range 0 to 255 from
the value held in the register. The generally leads to short and fast code, but the number of
different data items that can be addressed is limited. This means that a program that uses lots of
static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the
default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not
change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and "d0".
Otherwise, the pointer is returned only in "a0", and attempts to call such functions without a
prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to
disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches,
calls and absolute memory addresses. This option only has an effect when used on the command line
for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later.
This is the default. This option defines the preprocessor macro "__LIW__".
-mno-liw
Do not allow the compiler to generate Long Instruction Word instructions. This option defines the
preprocessor macro "__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later.
This is the default. This option defines the preprocessor macro "__SETLB__".
-mno-setlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the
preprocessor macro "__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-* configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally such constants are signed decimals, but
this option is available for testsuite and/or aesthetic purposes.
-mmcu=
Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name,
converted to upper case and pre- and post-fixed with __. This in turn is used by the msp430.h header
file to select an MCU-specific supplementary header file.
The option also sets the ISA to use. If the MCU name is one that is known to only support the 430
ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also
be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker command line. The script's name is
the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the
C preprocessor symbol "__XXX__" and cause the linker to search for a script called xxx.ld.
This option is also passed on to the assembler.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the MCU name specified by the -mmcu
option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult
option. It also toggles warnings about unrecognized MCU names. This option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is
deprecated. The -mmcu= option should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script. Overrides any scripts that would be
selected by the -mmcu= option.
-mlarge
Use large-model addressing (20-bit pointers, 32-bit "size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").
-mrelax
This option is passed to the assembler and linker, and allows the linker to perform certain
optimizations that cannot be done until the final link.
mhwmult=
Describes the type of hardware multiply supported by the target. Accepted values are none for no
hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs. 32bit for
the 16/32-bit multiply supported by later MCUs and f5series for the 16/32-bit multiply supported by
F5-series MCUs. A value of auto can also be given. This tells GCC to deduce the hardware multiply
support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if
the MCU name is not recognized then no hardware multiply support is assumed. "auto" is the default
setting.
Hardware multiplies are normally performed by calling a library routine. This saves space in the
generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline.
This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt
state when they finish. This makes them safe to use inside interrupt handlers as well as in normal
code.
-minrt
Enable the use of a minimum runtime environment - no static initializers or constructors. This is
intended for memory-constrained devices. The compiler includes special symbols in some objects that
tell the linker and runtime which code fragments are required.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data that do not have one of the
"lower", "upper", "either" or "section" attributes. Possible values are "lower", "upper", "either"
or "any". The first three behave like the corresponding attribute. The fourth possible value -
"any" - is the default. It leaves placement entirely up to the linker script and how it assigns the
standard sections (".text", ".data", etc) to the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning messages when a silicon errata
might need to be applied.
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2 between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within 512KB addressing space. The text segment
must be within 16MB addressing space.
medium
The data segment must be within 512KB while the read-only data segment can be within 4GB
addressing space. The text segment should be still within 16MB addressing space.
large
All the text and data segments can be within 4GB addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
-G num
Put global and static objects less than or equal to num bytes into the small data or BSS sections
instead of the normal data or BSS sections. The default value of num is 8.
-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The following option names are recognized:
none
Do not generate GP-relative accesses.
local
Generate GP-relative accesses for small data objects that are not external, weak, or
uninitialized common symbols. Also use GP-relative addressing for objects that have been
explicitly placed in a small data section via a "section" attribute.
global
As for local, but also generate GP-relative accesses for small data objects that are external,
weak, or common. If you use this option, you must ensure that all parts of your program
(including libraries) are compiled with the same -G setting.
data
Generate GP-relative accesses for all data objects in the program. If you use this option, the
entire data and BSS segments of your program must fit in 64K of memory and you must use an
appropriate linker script to allocate them within the addressable range of the global pointer.
all Generate GP-relative addresses for function pointers as well as data pointers. If you use this
option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you
must use an appropriate linker script to allocate them within the addressable range of the global
pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code.
Note that the Nios II ABI does not permit GP-relative accesses from shared libraries.
You may need to specify -mno-gpopt explicitly when building programs that include large amounts of
small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative
addressing may not be large enough to allow access to the entire small data section.
-mgprel-sec=regexp
This option specifies additional section names that can be accessed via GP-relative addressing. It
is most useful in conjunction with "section" attributes on variable declarations and a custom linker
script. The regexp is a POSIX Extended Regular Expression.
This option does not affect the behavior of the -G option, and the specified sections are in addition
to the standard ".sdata" and ".sbss" small-data sections that are recognized by -mgpopt.
-mr0rel-sec=regexp
This option specifies names of sections that can be accessed via a 16-bit offset from "r0"; that is,
in the low 32K or high 32K of the 32-bit address space. It is most useful in conjunction with
"section" attributes on variable declarations and a custom linker script. The regexp is a POSIX
Extended Regular Expression.
In contrast to the use of GP-relative addressing for small data, zero-based addressing is never
generated by default and there are no conventional section names used in standard linker scripts for
sections in the low or high areas of memory.
-mel
-meb
Generate little-endian (default) or big-endian (experimental) code, respectively.
-march=arch
This specifies the name of the target Nios II architecture. GCC uses this name to determine what
kind of instructions it can emit when generating assembly code. Permissible names are: r1, r2.
The preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the
targeted ISA level.
-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache by using I/O variants of the
instructions. The default is not to bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O variants of the load and store instructions.
The default is not to bypass the cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3
and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div" family of instructions by the compiler. The
default is to emit "mul" and not emit "div" and "mulx".
-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density)
instructions. Enabling these instructions also requires -march=r2. Since these instructions are
optional extensions to the R2 architecture, the default is not to emit them.
-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code
that uses insn. For example, -mcustom-fadds=253 generates custom instruction 253 for single-
precision floating-point add operations instead of the default behavior of using a library call.
The following values of insn are supported. Except as otherwise noted, floating-point operations are
expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators
or the equivalent GCC built-in functions.
Single-precision floating point:
fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.
fnegs
Unary negation.
fabss
Unary absolute value.
fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.
fmins, fmaxs
Floating-point minimum and maximum. These instructions are only generated if -ffinite-math-only
is specified.
fsqrts
Unary square root operation.
fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential functions. These instructions are only generated if
-funsafe-math-optimizations is also specified.
Double-precision floating point:
faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.
fnegd
Unary negation.
fabsd
Unary absolute value.
fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.
fmind, fmaxd
Double-precision minimum and maximum. These instructions are only generated if
-ffinite-math-only is specified.
fsqrtd
Unary square root operation.
fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential functions. These instructions are only generated
if -funsafe-math-optimizations is also specified.
Conversions:
fextsd
Conversion from single precision to double precision.
ftruncds
Conversion from double precision to single precision.
fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned integer types, with truncation towards zero.
round
Conversion from single-precision floating point to signed integer, rounding to the nearest
integer and ties away from zero. This corresponds to the "__builtin_lroundf" function when
-fno-math-errno is used.
floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to floating-point types.
In addition, all of the following transfer instructions for internal registers X and Y must be
provided to use any of the double-precision floating-point instructions. Custom instructions taking
two double-precision source operands expect the first operand in the 64-bit register X. The other
operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the
least significant half in source register src1 and the most significant half in src2. A custom
instruction that returns a double-precision result returns the most significant 32 bits in the
destination register and the other half in 32-bit register Y. GCC automatically generates the
necessary code sequences to write register X and/or read register Y when double-precision floating-
point instructions are used.
fwrx
Write src1 into the least significant half of X and src2 into the most significant half of X.
fwry
Write src1 into Y.
frdxhi, frdxlo
Read the most or least (respectively) significant half of X and store it in dest.
frdy
Read the value of Y and store it into dest.
Note that you can gain more local control over generation of Nios II custom instructions by using the
"target("custom-insn=N")" and "target("no-custom-insn")" function attributes or pragmas.
-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom instruction encodings (see -mcustom-insn
above). Currently, the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245
-mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
Custom instruction assignments given by individual -mcustom-insn= options override those given by
-mcustom-fpu-cfg=, regardless of the order of the options on the command line.
Note that you can gain more local control over selection of a FPU configuration by using the
"target("custom-fpu-cfg=name")" function attribute or pragma.
These additional -m options are available for the Altera Nios II ELF (bare-metal) target:
-mhal
Link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination
code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate
startup code provided by the HAL BSP.
-msmallc
Link with a limited version of the C library, -lsmallc, rather than Newlib.
-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to use when linking. This option is only useful
in conjunction with -mhal.
-msys-lib=systemlib
systemlib is the library name of the library that provides low-level system calls required by the C
library, e.g. "read" and "write". This option is typically used to link with a library provided by a
HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m32
-m64
Generate code for 32-bit or 64-bit ABI.
-misa=ISA-string
Generate code for given the specified PTX ISA (e.g. sm_35). ISA strings must be lower-case. Valid
ISA strings include sm_30 and sm_35. The default ISA is sm_30.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone instead of offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the default when any level of optimization is
selected.
-msoft-stack
Generate code that does not use ".local" memory directly for stack storage. Instead, a per-warp stack
pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length
arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to
access automatic variables from other threads, or with atomic instructions. This code generation
variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of
testing the compiler; to generate code suitable for linking into programs using OpenMP offloading,
use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all threads in each warp, while maintaining
memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD
regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally
executed (iff current lane index equals the master lane index), and the register being assigned is
copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master;
inside, each thread sees itself as the master. Shared memory array "int __nvptx_uni[]" stores all-
zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each
thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute the master
lane index.
-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and
selects corresponding multilib variant.
OpenRISC Options
These options are defined for OpenRISC:
-mboard=name
Configure a board specific runtime. This will be passed to the linker for newlib board library
linking. The default is "or1ksim".
-mnewlib
For compatibility, it's always newlib for elf now.
-mhard-div
Generate code for hardware which supports divide instructions. This is the default.
-mhard-mul
Generate code for hardware which supports multiply instructions. This is the default.
-mcmov
Generate code for hardware which supports the conditional move ("l.cmov") instruction.
-mror
Generate code for hardware which supports rotate right instructions.
-msext
Generate code for hardware which supports sign-extension instructions.
-msfimm
Generate code for hardware which supports set flag immediate ("l.sf*i") instructions.
-mshftimm
Generate code for hardware which supports shift immediate related instructions (i.e. "l.srai",
"l.srli", "l.slli", "1.rori"). Note, to enable generation of the "l.rori" instruction the -mror flag
must also be specified.
-msoft-div
Generate code for hardware which requires divide instruction emulation.
-msoft-mul
Generate code for hardware which requires multiply instruction emulation.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not
supported.) Implies -m45.
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40. Implies -msoft-float -mno-split.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10. Implies -msoft-float -mno-split.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-msplit
Target has split instruction and data space. Implies -m45.
-munix-asm
Use Unix assembler syntax.
-mdec-asm
Use DEC assembler syntax.
-mgnu-asm
Use GNU assembler syntax. This is the default.
-mlra
Use the new LRA register allocator. By default, the old "reload" allocator is used.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling parameters for array element type
ae_type. Supported values for ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated with this option runs on any of the
other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and
some types of operation (e.g., multiplication) do not work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the
default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance
of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in a load/store instruction, without
first loading it into a register. Typically, the use of this option generates larger programs, which
run faster than when the option isn't used. However, the results vary from program to program, so it
is left as a user option, rather than being permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can be generated, for
example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC
AE has no hardware support for byte-level memory operations, so all byte load/stores must be
synthesized from word load/store operations. This is inefficient and a warning is generated to
indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has
the necessary hardware support. This option disables these warnings.
PowerPC Options
These are listed under
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is
-mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-string contains two parts: the size of
integer types and the registers used for floating-point types. For example -march=rv64ifd
-mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that
floating-point values up to 64 bits wide are passed in F registers. Contrast this with
-march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D
extensions but only allows floating-point values up to 32 bits long to be passed in registers; or
-march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.
The default for this argument is system dependent, users who want a specific calling convention
should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64,
lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example,
-march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F
registers, but F registers are only 32 bits wide. There is also the ilp32e ABI that can only be used
with the rv32e architecture. This ABI is not well specified at present, and is subject to change.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square root instructions. This requires the F or
D extensions for floating-point registers. The default is to use them if the specified architecture
has these instructions.
-mdiv
-mno-div
Do or don't use hardware instructions for integer division. This requires the M extension. The
default is to use them if the specified architecture has these instructions.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include
rv64i, rv32g, rv32e, and rv32imaf.
-mtune=processor-string
Optimize the output for the given processor, specified by microarchitecture name. Permissible values
for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, and size.
When -mtune= is not specified, the default is rocket.
The size choice is not intended for use by end-users. This is used when -Os is specified. It
overrides the instruction cost info provided by -mtune=, but does not override the pipeline info.
This helps reduce code size while still giving good performance.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128-bits).
Warning: If you use this switch, then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a special section (on some targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The
default is to use fast inline prologues and epilogues.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set depending on whether the
processor we are optimizing for supports fast unaligned access or not.
-mcmodel=medlow
Generate code for the medium-low code model. The program and its statically defined symbols must lie
within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB.
Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and its statically defined symbols must be
within any single 2 GiB address range. Programs can be statically or dynamically linked.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic addresses. The
alternative is to use assembler macros instead, which may limit optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of instructions required to materialize
symbol addresses. The default is to take advantage of linker relaxations.
-memit-attribute
-mno-emit-attribute
Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature
requires at least binutils 2.32.
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to be used. The simplest is
"none", which uses software for both multiplication and division. This is the default. The "g13"
value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The
"g14" value selects the use of the multiplication and division instructions supported by the RL78/G14
(S3 core) parts. The value "rl78" is an alias for "g14" and the value "mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible
values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just
RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware
peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a
different calling convention.
If this option is set it also selects the type of hardware multiply support to use, unless this is
overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables
the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware
multiplications altogether.
Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the
command line does change the behavior of the toolchain since it also enables G14 hardware multiply
support. If these options are not specified on the command line then software multiplication
routines will be used even though the code targets the RL78 core. This is for backwards
compatibility with older toolchains which did not have hardware multiply and divide support.
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible
values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are provided for backwards
compatibility.
-mallregs
Allow the compiler to use all of the available registers. By default registers "r24..r31" are
reserved for use in interrupt handlers. With this option enabled these registers can be used in
ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The
default is -m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC registers. This is only
necessary if normal code might use the MDUC registers, for example because it performs multiplication
and division operations. The default is to ignore the MDUC registers as this makes the interrupt
handlers faster. The target option -mg13 needs to be passed for this to work as this feature is only
available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt
handler performs a multiplication or division operation or it calls another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the processor you are using.
The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather
than the options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC
to use the optional PowerPC architecture instructions in the Graphics group, including floating-point
select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction
implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture.
The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate
instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02
architecture. The -mpopcntd option allows GCC to generate the popcount instruction implemented on
the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd
option allows GCC to generate the FP round to integer instructions implemented on the POWER5+
processor and other processors that support the PowerPC V2.03 architecture. The -mcmpb option allows
GCC to generate the compare bytes instruction implemented on the POWER6 processor and other
processors that support the PowerPC V2.05 architecture. The -mmfpgpr option allows GCC to generate
the FP move to/from general-purpose register instructions implemented on the POWER6X processor and
other processors that support the extended PowerPC V2.05 architecture. The -mhard-dfp option allows
GCC to generate the decimal floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in
the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters for machine type
cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476,
476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970,
8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4,
power5, power5+, power6, power6x, power7, power8, power9, powerpc, powerpc64, powerpc64le, rs64, and
native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian),
64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that corresponds to the
host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not
recognize the processor.
The other options specify a specific processor. Code generated under those options runs best on that
processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64
-mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
The particular options set for any particular CPU varies between compiler versions, depending on what
setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual
hardware's capabilities. If you wish to set an individual option to a particular value, you may
specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at
present because AIX does not have full support for these options. You may still enable or disable
them individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture
type or register usage, as -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as
for -mcpu. If both are specified, the code generated uses the architecture and registers set by
-mcpu, but the scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of
4G in size. This is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is
only limited by the 64-bit address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in
functions that allow more direct access to the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics such as "vec_splat", "vec_extract",
and "vec_insert" match array element order corresponding to the endianness of the target. That is,
element zero identifies the leftmost element in a vector register when targeting a big-endian
platform, and identifies the rightmost element in a vector register when targeting a little-endian
platform.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-executable
".plt" and ".got" sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and ".got"
sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of
built-in functions that allow more direct access to the VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct access to the cryptographic
instructions that were added in version 2.07 of the PowerPC ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow direct access to the Hardware
Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions
can be fused together on power8 and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar instructions that were added in version
2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to
the vector instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory instructions. The
-mquad-memory option requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory instructions. The
-mquad-memory-atomic option requires use of 64-bit mode.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point and use either software
emulation for IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx, -mcpu=power7, -mcpu=power8), or -mcpu=power9 must be enabled to use
the IEEE 128-bit floating point support. The IEEE 128-bit floating point support only works on
PowerPC Linux systems.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and
disabled on other systems.
If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a 64-bit system, the IEEE
128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point
instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting
a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction
set, and disabled on other systems.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux).
The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any
PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and
generates code for PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for
each unique non-automatic variable reference in your program. GCC also places floating-point
constants in the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you
can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc
forces GCC to generate code to calculate the sum of an address and a constant at run time instead of
putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to
produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify
-mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you
specify this option, GCC produces code that is slower and larger but which uses extremely little TOC
space. You may wish to use this option only on files that contain less frequently-executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the
infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32
disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI.
Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the
stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long
double value is properly rounded when comparing values and converting to double. Use XL symbol names
for long double support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case
of calling a function that takes the address of its arguments with fewer arguments than declared.
IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a
subroutine is compiled without optimization. Because always storing floating-point arguments on the
stack is inefficient and rarely needed, this option is not enabled by default and only is necessary
when calling subroutines compiled by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing
with special startup code to enable the application to run. The system must have PE installed in the
standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option
to specify the appropriate directory location. The Parallel Environment does not support threads, so
the -mpe option and the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-
defined alignment of larger types, such as floating-point doubles, on their natural size-based
boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC
defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating-point
emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple
word instructions. These instructions are generated by default on POWER systems, and not generated
on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions
do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which
permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register
to the address of the calculated memory location. These instructions are generated by default. If
you use -mno-update, there is a small window between the time that the stack pointer is updated and
the address of the previous frame is stored, which means code that walks the stack frame across
interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These
instructions can incur a performance penalty on Power6 processors in certain situations, such as when
stepping through large arrays that cross a 16M boundary. This option is enabled by default when
targeting Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions.
These instructions are generated by default if hardware floating point is used. The machine-
dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on
the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting
those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464
and 476 processors. This instruction is generated by default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-
fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is
aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is
aligned to a 1-byte boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are
handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated to a different address
at run time. A simple embedded PowerPC system loader should relocate the entire contents of ".got2"
and 4-byte locations listed in the ".fixup" section, a table of 32-bit addresses generated by this
option. For this to work, all objects linked together must be compiled with -mrelocatable or
-mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be
relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of
-mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any
combination of the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a
global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The
-mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external
references are relocatable. The resulting code is suitable for applications, but not shared
libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for
each function. The runtime system is responsible for initializing this register with an appropriate
value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during
the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest,
or second-highest (respectively) priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction
scheduling. The argument dependence_type takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second scheduling pass. The
argument scheme takes one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's
grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as
needed to force an insn to a new group, according to the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an
insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the
March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is
the default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
-mtraceback=traceback_type
Select the type of traceback table. Valid values for traceback_type are full, part, and no.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are
altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not likely to work if your
system defaults to using IEEE extended-precision long double. If you change the long double type
from IEEE extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option.
Requires -mlong-double-128 to be enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not likely to work if
your system defaults to using IBM extended-precision long double. If you change the long double type
from IBM extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option.
Requires -mlong-double-128 to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit
Linux. Overriding the default ABI requires special system support and is likely to fail in
spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC
64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in
spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that
specify ABI variations in function parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are
properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped
call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point
values are passed in the floating-point registers in case the function takes variable arguments.
With -mprototype, only calls to prototyped variable argument functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the
standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi
extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary
Interface (EABI), which is a set of modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to
set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16-byte
boundary, no EABI initialization function is called from "main", and the -msdata option only uses
"r13" to point to a single small data area. The -meabi option is on by default if you configured GCC
using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in
the ".sdata2" section, which is pointed to by register "r2". Put small initialized non-"const"
global and static data in the ".sdata" section, which is pointed to by register "r13". Put small
uninitialized global and static data in the ".sbss" section, which is adjacent to the ".sdata"
section. The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi
option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the ".sdata" section,
which is pointed to by register "r13". Put small uninitialized global and static data in the ".sbss"
section, which is adjacent to the ".sdata" section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the ".sdata" section. Put small
uninitialized global data in the ".sbss" section. Do not use register "r13" to address small data
however. This is the default behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the ".data" section, and
all uninitialized data in the ".bss" section.
-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This is the default.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to num
bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The
default value is target-specific.
-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such as calls to "memcmp" or structure
compares) less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of
block compare is disabled. The default value is target-specific.
-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block compares that are less than or equal to
num bytes, but greater than the limit for non-loop inline block compare expansion. If the block
length is not constant, at most num bytes will be compared before "memcmp" is called to compare the
remainder of the block. The default value is target-specific.
-mstring-compare-inline-limit=num
Compare at most num string bytes with inline code. If the difference or end of string is not found
at the end of the inline compare a call to "strcmp" or "strncmp" will take care of the rest of the
comparison. The default is 64 bytes.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the
small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G
num switch is also passed to the linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language
output using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more expensive calling sequence is
required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current
location. A short call is generated if the compiler knows the call cannot be that far away. This
setting can be overridden by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On
these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the
GNU linker for 32-bit PowerPC systems as well.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls
using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-
endian) do not support inline PLT calls.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee, L42", plus a branch island (glue
code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker
prefers the first address and generates a "bl callee" if the PPC "bl" instruction reaches the callee
directly; otherwise, the linker generates "bl L42" to call the branch island. The branch island is
appended to the body of the calling function; it computes the full 32-bit address of the callee and
jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call,
and the Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue.
-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an inline PLT call sequence that supports
lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only
supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and are enabled by
default if the support is detected when configuring GCC, and, in the case of 32-bit PowerPC, if GCC
is configured with --enable-secureplt. -mpltseq code and -mbss-plt 32-bit PowerPC relocatable
objects may not be linked together.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The
relocation allows the linker to reliably associate function call with argument setup instructions for
TLS optimization, which in turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root estimate instructions
with additional Newton-Raphson steps to increase precision instead of doing a divide or square root
and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip
(or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the sequence is generally higher than the
throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to
2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated
list of options, which may be preceded by a "!" to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both single and double precision.
divf
Enable the single-precision reciprocal approximation instructions.
divd
Enable the double-precision reciprocal approximation instructions.
rsqrt
Enable the reciprocal square root approximation instructions for both single and double
precision.
rsqrtf
Enable the single-precision reciprocal square root approximation instructions.
rsqrtd
Enable the double-precision reciprocal square root approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for
the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision
reciprocal square root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates
than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8
automatically selects -mrecip-precision. The double-precision square root estimate instructions are
not generated by default on low-precision machines, since they do not provide an estimate that
converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type
supported at present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS)
libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to
"acosd2", "acosf4", "acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2",
"atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2",
"coshf4", "erfcd2", "erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2",
"expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2", "sinhf4",
"sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when generating code for power7. Both
-ftree-vectorize and -funsafe-math-optimizations must also be enabled. The MASS libraries must be
specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used
to optimize rounding of floating-point values to 64-bit integer and back to floating point. The
"friz" instruction does not return the same value if the floating-point number is too large to fit in
an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ("r11") when calling through a
pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving
the function address, TOC value to be loaded in register "r2", and static chain value to be loaded in
register "r11". The -mpointers-to-nested-functions is on by default. You cannot call through
pointers to nested functions or pointers to functions compiled in other languages that use the static
chain if you use -mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function
prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value
is not saved in the prologue, it is saved just before the call through the pointer. The
-mno-save-toc-indirect option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum alignment of 64 bits, for
compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit
boundary when that structure contained a member requiring 128-bit alignment. This is corrected in
more recent versions of GCC. This option may be used to generate code that is compatible with
functions compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported locations are global for global
canary or tls for per-thread canary in the TLS block (the default with GNU libc version 2.4 or
later).
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which register to use as base register for
reading the canary, and from what offset from that base register. The default for those is as
specified in the relevant ABI. -mstack-protector-guard-symbol=symbol overrides the offset with a
symbol reference to a canary in the TLS block.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The
default is -m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is
why the default is -m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled
for the RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU
hardware is not used for doubles if the -m64bit-doubles option is used.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically.
This is because the RX FPU instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600
and RX200 series hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL"
instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default
when this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to
store data in the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed into the small
data area. Using the small data area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does not overflow. Also when the
small data area is used one of the RX's registers (usually "r13") is reserved for use pointing to
this area, so it is no longer available for use by the compiler. This could result in slower and/or
larger code if variables are pushed onto the stack instead of being held in this register.
Note, common variables (variables that have not been initialized) and constants are not placed into
the small data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default
with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving
a register. It is up to the programmer to experiment and discover whether this feature is of benefit
to their program. See the description of the -mpid option for a description of how the actual
register to hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler.
This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not
generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX
instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be
used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it
can be beneficial to restrict the size of constants that are used in instructions. Constants that
are too big are instead placed into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size
are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the
size of a program by finding shorter versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The value N can be
between 0 and 4. A value of 1 means that register "r13" is reserved for the exclusive use of fast
interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and
"r11", and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not reserve
any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only
necessary if normal code might use the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this makes the interrupt handlers
faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant data is
done via an offset from a base address held in a register. This allows the location of constant data
to be determined at run time without requiring the executable to be relocated, which is a benefit to
embedded applications with tight memory constraints. Data that can be modified is not affected by
this option.
Note, using this feature reserves a register, usually "r13", for the constant data base address.
This can result in slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends upon whether the
-msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with
register "r13" and proceeding downwards, registers are allocated first to satisfy the requirements of
-mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data
area register to be "r8" if both -mint-register=4 and -mpid are specified on the command line.
By default this feature is not enabled. The default can be restored via the -mno-pid command-line
option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it
is compiling a file. The default is to issue a warning for each extra fast interrupt handler found,
as the RX only supports one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions "SMOVF", "SCMPU", "SMOVB",
"SMOVU", "SUNTIL" "SWHILE" and also the "RMPA" instruction. These instructions may prefetch data,
which is not safe to do if accessing an I/O register. (See section 12.2.7 of the RX62N Group User's
Manual for more information).
The default is to allow these instructions, but it is not possible for GCC to reliably detect all
circumstances where a string instruction might be used to access an I/O register, so their use cannot
be disabled automatically. Instead it is reliant upon the programmer to use the
-mno-allow-string-insns option if their program accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__",
otherwise it defines the symbol "__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions. This option can be used when code
size exceeds the range of "BSR" instructions. Note that -mno-jsr does not mean to not use "JSR" but
instead means that any type of branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used
with the "interrupt" function attribute. This attribute indicates a function intended to process fast
interrupts. GCC ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only
provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or
-mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point
operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point
operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions.
This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point
operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-
floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-
point hardware instructions. This is the default for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type
equivalent to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack
frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call
frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the
bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost
word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain;
however, use of the backchain for debugging purposes usually requires that the whole binary is built
with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not
supported. In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the
all fields of the 96/160 byte register save area only for their default purpose; unused fields still
take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the
top of the register save area; unused space is reused for other purposes, allowing for more efficient
use of the available stack space. However, when -mbackchain is also in effect, the topmost word of
the save area is always used to store the backchain, and the return address register is always saved
two words below the backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-
compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95
for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for
debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also,
note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only
works reliably if the total executable size does not exceed 64k. The default is to use the "basr"
instruction instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is
specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular
to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets
default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When
-mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is
not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default
is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions available with the
transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390
System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions available with the vector extension
facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector
type values with regard to alignment and calling conventions. In case vector type values are being
used in an ABI-relevant context a GAS .gnu_attribute command will be added to mark the resulting
binary with the ABI used. -mvx is enabled by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and builtins using instructions available
with the vector extension facility introduced with the IBM z13 machine generation. This option adds
support for vector to be used as a keyword to define vector type variables and arguments. vector is
only available when GNU extensions are enabled. It will not be expanded when requesting strict
standard compliance e.g. with -std=c99. In addition to the GCC low-level builtins -mzvector enables
a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In
order to make use of these builtins the header file vecintrin.h needs to be included. -mzvector is
disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When
-mno-mvcle is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print
debug information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system representing a certain processor
type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8,
z196/arch9, zEC12, z13/arch11, z14/arch12, and native.
The default is -march=z900.
Specifying native as cpu type can be used to select the best architecture option for the host
processor. -march=native has no effect if GCC does not recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The list of cpu-type values is the same as for -march. The default is the
value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating
system. This option is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions.
These instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time
check it doesn't need to be a real problem when the program runs. It is intended to identify
functions that most probably cause a stack overflow. It is useful to be used in an environment with
limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a
bad idea with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional instructions in the function
prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember
that the stack on S/390 grows downward). If the stack-guard option is omitted the smallest power of
2 larger than the frame size of the compiled function is chosen. These options are intended to be
used to help debugging stack overflow problems. The additionally emitted code causes only little
overhead and hence can also be used in production-like systems without greater performance
degradation. The given values have to be exact powers of 2 and stack-size has to be greater than
stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that
the stack starts at an address aligned to the value given by stack-size. The stack-guard option can
only be used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions
in the compilation unit. The funtion label is prepended with the given number of two-byte NOP
instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are
appended, using the largest NOP like instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the "hotpatch" attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store instructions.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit
is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are
used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by
default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by
default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not
used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-point operations are
used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any
DSP instructions at the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some
functions from the standard C library do not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available.
This option is the default for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where
the result of a comparison is unordered. By default -mieee is implicitly enabled. If
-ffinite-math-only is enabled -mno-ieee is implicitly set, which results in faster floating-point
greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying
either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function trampolines.
This option has no effect if -musermode is in effect and the selected code generation option (e.g.
-m4) does not allow the use of the "icbi" instruction. If the selected code generation option does
not allow the use of the "icbi" instruction, and -musermode is not in effect, the inlined code
manipulates the instruction cache address array directly with an associative write. This not only
requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB
and has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the
SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as a comma separated list. For details
on the atomic built-in functions see __atomic Builtins. The following models and parameters are
supported:
none
Disable compiler generated atomic sequences and emit library calls for atomic operations. This
is the default if the target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions.
The generated atomic sequences require additional support from the interrupt/exception handling
code of the system and are only suitable for SH3* and SH4* single-core systems. This option is
enabled by default when the target is "sh*-*-linux*" and SH3* or SH4*. When the target is SH4A,
this option also partially utilizes the hardware atomic instructions "movli.l" and "movco.l" to
create more efficient code, unless strict is specified.
soft-tcb
Generate software atomic sequences that use a variable in the thread control block. This is a
variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated
atomic sequences require additional support from the interrupt/exception handling code of the
system and are only suitable for single-core systems. When using this model, the gbr-offset=
parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily disable interrupts by setting "SR.IMASK =
1111". This model works only when the program runs in privileged mode and is only suitable for
single-core systems. Additional support from the interrupt/exception handling code of the system
is not required. This model is enabled by default when the target is "sh*-*-linux*" and SH1* or
SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and "movco.l" instructions only. This is
only available on SH4A and is suitable for multi-core systems. Since the hardware instructions
support only 32 bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit
accesses. Code compiled with this option is also compatible with other software atomic model
interrupt/exception handling systems if executed on an SH4A system. Additional support from the
interrupt/exception handling code of the system is not required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable in the thread control block
structure that should be used by the generated atomic sequences when the soft-tcb model has been
selected. For other models this parameter is ignored. The specified value must be an integer
multiple of four and in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic models, even if they are compatible, and
makes the compiler generate atomic sequences of the specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending on the particular
hardware and software configuration this can degrade overall performance due to the operand cache
line flushes that are implied by the "tas.b" instruction. On multi-core SH4A processors the "tas.b"
instruction must be used with caution since it can result in data corruption for certain cache
configurations.
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead
of the Procedure Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code. Specifying -musermode also implies
-mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default
when the target is "sh*-*-linux*". If the target is SH1* or SH2* -musermode has no effect, since
there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division operations. strategy can be one of:
call-div1
Calls a library function that uses the single-step division instruction "div1" to perform the
operation. Division by zero calculates an unspecified result and does not trap. This is the
default except for SH4, SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in double precision floating point.
Division by zero causes a floating-point exception. This is the default for SHcompact with FPU.
Specifying this for targets that do not have a double precision FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for small divisors and the "div1" instruction
with case distinction for larger divisors. Division by zero calculates an unspecified result and
does not trap. This is the default for SH4. Specifying this for targets that do not have
dynamic shift instructions defaults to "call-div1".
When a division strategy has not been specified the default strategy is selected based on the current
target. For SH2A the default strategy is to use the "divs" and "divu" instructions instead of
library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call.
Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack
frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affects the
name used in the call division strategies, and the compiler still expects the same sets of
input/output/clobbered registers as if this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges can be specified separated
by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate
more branch-free code if possible. If not specified the value is selected depending on the processor
type that is being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch instructions "bt" and "bf" are fast.
If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is
enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by
specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if
a suitable instruction cannot be found. By default this option is disabled. It can be enabled to
work around hardware bugs as found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions.
These instructions are generated by default if hardware floating point is used. The machine-
dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations.
The option -mfsca must be used in combination with -funsafe-math-optimizations. It is enabled by
default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even
if -funsafe-math-optimizations is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root
approximations. The option -mfsrra must be used in combination with -funsafe-math-optimizations and
-ffinite-math-only. It is enabled by default when generating code for SH4A. Using -mno-fsrra
disables reciprocal square root approximations even if -funsafe-math-optimizations and
-ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can
result in faster code on the SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris
assembler. This is only necessary when object files use ISA extensions not supported by the current
machine, but check at runtime whether or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when
linking a shared object. Using this option, you can link position-dependent code into a shared
object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections"
linker error message. However, the necessary relocations trigger copy-on-write, and the shared
object is not actually shared across processes. Instead of using -mimpure-text, you should compile
all source code with -fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4
ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then
treated as an allocable register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You
should compile libraries and system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single
register window model. This model is compatible with the regular register window model. The local
registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on
the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf
functions). This is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point instructions.
The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word
floating-point instructions. They all invoke a trap handler for one of these instructions, and then
the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this
is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the
default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained
in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte
alignment. Specifying this option avoids some rare compatibility problems with code generated by
other compilers. It is not the default because it results in a performance loss, especially for
floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the "casa"
instruction emitted for the LEON3 processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This
enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place
of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates
the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that
their resulting code is not directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in functions returning structures or
unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode.
-mlra
-mno-lra
Enable Local Register Allocation. This is the default for SPARC since GCC 7 so -mno-lra needs to be
passed to get old Reload.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type
cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3,
leon3v7, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara,
niagara2, niagara3, niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best
architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the
processor.
Default instruction scheduling parameters are used for values that select an architecture and not an
implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC
architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602
chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference
from V7 code is that the compiler emits the integer multiply and integer divide instructions which
exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it
for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This
adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite
but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930
chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally
optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This
adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions
which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes
it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit
integer and floating-point move instructions, 3 additional floating-point condition code registers
and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for
the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it
for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler
additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler
additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally
optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes
it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for
Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction
set or register set that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are
those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon,
leon3, leon3v7, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2,
niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also
be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the
global and out registers are considered 64 bits wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set
extensions. The default is -mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual
Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual
Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such
instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual
Instruction Set extensions. The default is -mvis4 when targeting a cpu that supports such
instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3, -mvis2 and -mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual
Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC
Architecture 2017. The default is -mvis4b when targeting a cpu that supports such instructions, such
as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-
Condition instructions. The default is -mcbcond when targeting a CPU that supports such
instructions, such as Niagara-4 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-
point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such
as Niagara-3 and later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to
Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU supporting the
architecture versions V8 or V9 with FPU except -mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction.
The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and
later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry
instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as
Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which
corresponds to erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data cache nullify errata of
the UT699 processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of the GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer
to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of
memory. Programs can be statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of
memory, the text and data segments must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the
text and data segments must be less than 2GB in size and the data segment must be located within
2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments
must be less than 2GB in size, both starting anywhere in memory (determined at link time). The
global register %g4 points to the base of the data segment. Programs are statically linked and
PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in
the processor's "PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by
-2047 which must be added back when making stack frame references. This is the default in 64-bit
mode. Otherwise, assume no such offset is present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By default, GCC gives an error when it
generates code that requires a dynamic relocation. -mno-error-reloc disables the error, -mwarn-reloc
generates a warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be reordered with respect to loads and
stores of the memory that is being accessed. With -munsafe-dma you must use the "volatile" keyword
to protect memory accesses, but that can lead to inefficient code in places where the memory is known
to not change. Rather than mark the memory as volatile, you can use -msafe-dma to tell the compiler
to treat the DMA instructions as potentially affecting all memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or
probably-taken branches. A hint is not generated closer than 8 instructions away from its branch.
There is little reason to disable them, except for debugging purposes, or to make an object a little
bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never larger than 18 bits. With
-mlarge-mem code is generated that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-style main function interface (which
has an unconventional parameter list). With -mstdmain, GCC links your program against startup code
that assumes a C99-style interface to "main", including a local copy of "argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator cannot use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges can be specified separated
by a comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address space accessed via the "__ea" named address
space qualifier are either 32 or 64 bits wide. The default is 32 bits. As this is an ABI-changing
option, all object code in an executable must be compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the generic address space. This
enables explicit type casts between "__ea" and generic pointer as well as implicit conversions of
generic pointers to "__ea" pointers. The default is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links to an executable and selects a
software-managed cache for accessing variables in the "__ea" address space with a particular cache
size. Possible options for cache-size are 8, 16, 32, 64 and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler links to an executable and selects
whether atomic updates to the software-managed cache of PPU-side variables are used. If you use
atomic updates, changes to a PPU variable from SPU code using the "__ea" named address space
qualifier do not interfere with changes to other PPU variables residing in the same cache line from
PPU code. If you do not use atomic updates, such interference may occur; however, writing back cache
lines is more efficient. The default behavior is to use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts NOPs to increase dual issue when it expects it to increase performance. n
can be a value from 0 to 10. A smaller n inserts fewer NOPs. 10 is the default, 0 is the same as
-mno-dual-nops. Disabled with -Os.
-mhint-max-nops=n
Maximum number of NOPs to insert for a branch hint. A branch hint must be at least 8 instructions
away from the branch it is affecting. GCC inserts up to n NOPs to enforce this, otherwise it does
not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be within 256 instructions of the
branch it is affecting. By default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts the
"hbrp" instruction to make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on
those systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the
output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for direct calls is limited to 500M in either
direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range.
-mcmodel=large
Generate code for the large model. There is no limitation on call distance, pc-relative addresses,
or absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer
to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the
only supported behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always
loads the function's address into a register, and calls indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy
pointer into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is
on by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of
a function. The external functions are slower, but use less code space if more than one function
saves the same number of registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and
-mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register
"ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte
references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register
"gp" points to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor constant "__v850e3v5__" is
defined if this option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constant "__v850e2v3__" is
defined if this option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constant "__v850e2__" is defined
if this option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants "__v850e1__" and
"__v850e__" are defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant "__v850e__" is defined if
this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined
then a default target processor is chosen and the relevant __v850*__ preprocessor constant is
defined.
The preprocessor constants "__v850" and "__v851__" are always defined, regardless of which processor
variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3
and v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by
default when the GCC ABI is in use. If "CALLT" instructions are being generated then the C
preprocessor symbol "__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions. This option is only
significant when the target architecture is V850E2V3 or higher. If hardware floating point
instructions are being generated then the C preprocessor symbol "__FPU_OK__" is defined, otherwise
the symbol "__NO_FPU__" is defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default
when the e3v5 architecture is selected because its use is still experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is the default. With this version of
the ABI the following rules apply:
* Integer sized structures and unions are returned via a memory pointer rather than a register.
* Large structures and unions (more than 8 bytes in size) are passed by value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-
line option is not supported.
When this version of the ABI is enabled the C preprocessor symbol "__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of the ABI the following
rules apply:
* Integer sized structures and unions are returned in register "r10".
* Large structures and unions (more than 8 bytes in size) are passed by reference.
* Functions are aligned to 32-bit boundaries, unless optimizing for size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but not enabled by default.
When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned on 8-byte boundaries. The default
is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the
C preprocessor symbol "__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain
about out of range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler. This setting is the
default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX
cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU assembler is being used.
-mg Output code for G-format floating-point numbers instead of D-format.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on an MCM target should be linked with this
option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on
the target under the control of the GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run on the simulator should be linked with
option. This causes libraries libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type
cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium
architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only
difference from GR5 code is that the compiler will generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction
set or register set that the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no restrictions on the access to general
registers. This is the default.
-muser-mode
Generate code for the user mode, where the access to some general registers is forbidden: on the GR5,
registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are
affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error)
codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64
or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables
"pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware
are listed with the other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches
from the former to the latter. It also defines the preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than static libraries. The options -static
and -shared can also be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for
compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility
with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely
tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that
may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies
-mtune=cpu-type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time by determining the processor type
of the compiling machine. Using -march=native enables all instruction subsets supported by the
local machine (hence the result might not run on different machines). Using -mtune=native
produces code optimized for the local machine under the constraints of the selected instruction
set.
x86-64
A generic CPU with 64-bit extensions.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686
family chips. When used with -mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction
set support. Used by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3
instruction set support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT
instruction set support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES and PCLMUL instruction set support.
sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AVX, AES and PCLMUL instruction set support.
ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
haswell
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED ADCX and PREFETCHW
instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW,
CLFLUSHOPT, XSAVEC and XSAVES instruction set support.
bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set
support.
silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AES, PREFETCHW, PCLMUL and RDRND instruction set support.
goldmont
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT and FSGSBASE instruction
set support.
goldmont-plus
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX and UMIP instruction set support.
tremont
Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID,
SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support.
knl Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
knm Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, PREFETCHWT1, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW, AVX5124FMAPS and
AVX512VPOPCNTDQ instruction set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD
instruction set support.
cannonlake
Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA and UMIP instruction set support.
icelake-client
Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG,
AVX512VNNI, VPCLMULQDQ, VAES instruction set support.
icelake-server
Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG,
AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support.
cascadelake
Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and
AVX512VNNI instruction set support.
tigerlake
Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VBMI, AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ, AVX512BITALG,
AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI, MOVDIR64B and CLWB instruction set
support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD
Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!,
enhanced 3DNow! and 64-bit instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX,
SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4,
AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM,
F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM,
F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
bdver4
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2,
TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
znver1
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2,
F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE,
MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and
64-bit instruction set extensions.
znver2
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2,
,CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT,
POPCNT, and 64-bit instruction set extensions.)
btver1
CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX,
SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE,
F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and
64-bit instruction set extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set
support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this
chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is
implemented for this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is
implemented for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented
for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented
for this chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is
implemented for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is
implemented for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2
instruction set support. (No scheduling is implemented for this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No
scheduling is implemented for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No
scheduling is implemented for this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No
scheduling is implemented for this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support.
(No scheduling is implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set
support. (No scheduling is implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set
support. (No scheduling is implemented for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. While picking a specific cpu-type schedules things appropriately for that
particular chip, the compiler does not generate any code that cannot run on the default machine type
unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu
then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices
for cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on
which your code will run, then you should use the corresponding -mtune or -march option instead
of -mtune=generic. But, if you do not know exactly what CPU users of your application will have,
then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change.
Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option
will change to reflect the processors that are most common at the time that version of GCC is
released.
There is no -march=generic option because -march indicates the instruction set the compiler can
use, and there is no generic instruction set applicable to all processors. In contrast, -mtune
indicates the processor (or, in this case, collection of processors) for which the code is
optimized.
intel
Produce code optimized for the most current Intel processors, which are Haswell and Silvermont
for this version of GCC. If you know the CPU on which your code will run, then you should use
the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your
application performs better on both Haswell and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of this option will change.
Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option
will change to reflect the most current Intel processors at the time that version of GCC is
released.
There is no -march=intel option because -march indicates the instruction set the compiler can
use, and there is no common instruction set applicable to all processors. In contrast, -mtune
indicates the processor (or, in this case, collection of processors) for which the code is
optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the majority of chips and emulated
otherwise. Code compiled with this option runs almost everywhere. The temporary results are
computed in 80-bit precision instead of the precision specified by the type, resulting in
slightly different results compared to most of other chips. See -ffloat-store for more detailed
description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE instruction set. This instruction set
is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and
Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision
arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later
version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic
too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE
extensions and make this option effective. For the x86-64 compiler, these extensions are enabled
by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical
instability problems of 387 code, but may break some existing code that expects temporaries to be
80 bits.
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice
for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively doubles the amount of
available registers, and on chips with separate execution units for 387 and SSE the execution
resources too. Use this option with care, as it is still experimental, because the GCC register
allocator does not model separate functional units well, resulting in unstable performance.
-masm=dialect
Output assembly instructions using selected dialect. Also affects which dialect is used for basic
"asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att.
Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the
case where the result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual
C compiler are used, but this cannot be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387 register stack, some
floating-point opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU
register, even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this
option to avoid generating those instructions. This option is overridden when -march indicates that
the target CPU always has an FPU and so the instruction does not need emulation. These instructions
are not generated unless you also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary
or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs
somewhat faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types are aligned
differently than the published application binary interface specifications for the x86-32 and are not
binary compatible with structures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The x86-32 application binary interface
specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or 16-byte
boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double" to a 16-byte boundary by padding the "long double" with an
additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long
double" is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for
a "long double".
Warning: if you override the default value for your target ABI, this changes the size of structures
and arrays containing "long double" variables, as well as modifying the function calling convention
for functions taking "long double". Hence they are not binary-compatible with code compiled without
that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type
equivalent to the "double" type. This is the default for 32-bit Bionic C library. A size of 128 bits
makes the "long double" type equivalent to the "__float128" type. This is the default for 64-bit
Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures
and arrays containing "long double" variables, as well as modifying the function calling convention
for functions taking "long double". Hence they are not binary-compatible with code compiled without
that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type are compat uses increased alignment
value compatible uses GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and
cacheline uses increased alignment value to match the cache line size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data
section. This value must be the same across all objects linked into the binary, and defaults to
65535.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "ret num" instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function
attribute "stdcall". You can also override the -mrtd option by using the function attribute "cdecl".
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot
use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to
pass arguments, and at most 3 registers can be used. You can control this behavior for a specific
function by using the function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can
control this behavior for a specific function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris 8 and 9 and
VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions
(starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on
Solaris 10 and later. Only use this option if you need to remain compatible with existing code
produced by those previous compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands
of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the
significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds
the significands of results of floating-point operations to 64 bits (extended double precision),
which is the default. When this option is used, floating-point operations in higher precisions are
not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some
programs by 2% or more. Note that some mathematical libraries assume that extended-precision
(80-bit) floating-point operations are enabled by default; routines in such libraries could suffer
significant loss of accuracy, typically through so-called "catastrophic cancellation", when this
option is used to set the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue
and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that
keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE
compatibility. See also the attribute "force_align_arg_pointer", applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary.
Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in
controlled environment where stack space is important limitation. This option leads to wrong code
when functions compiled with 16 byte stack alignment (such as functions from a standard library) are
called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access
traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects
(including x87 long double and __int128), leading to wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and
startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If
-mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values should be aligned to an 8-byte boundary
(see -malign-double) or suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that
required by any value stored on the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary
from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It
is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is
sensitive to stack space usage, such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-mavx512vpopcntdq
-mavx5124fmaps
-mavx512vnni
-mavx5124vnniw
-mcldemote
These switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A,
SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ,
AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA,
PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced
3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM,
MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B,
AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or CLDEMOTE extended instruction sets. Each
has a corresponding -mno- option to disable use of these instructions.
These extensions are also available as built-in functions: see x86 Built-in Functions, for details of
the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387
instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX
equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code, even without
-mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In particular, the file containing the CPU
detection code should be compiled without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance tuning features and default
settings. The names can be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code generation features. feature-list is a
comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is
turned on if it is not preceded with ^, otherwise, it is turned off. -mtune-ctrl=feature-list is
intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can
potentially result in compiler ICEs or runtime errors.
-mno-default
This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and
-mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of functions that use string
instructions. String instructions depend on the DF flag to select between autoincrement or
autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some
operating systems violate this specification by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong
direction mode when string instructions are used. This option can be enabled by default on 32-bit
x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld"
instructions can be suppressed with the -mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of
the function to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper"
intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the
auto-vectorizer.
-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in instructions instead of default on the
selected platform.
none
No extra limitations applied to GCC other than defined by the selected platform.
128 Prefer 128-bit vector width for instructions.
256 Prefer 256-bit vector width for instructions.
512 Prefer 512-bit vector width for instructions.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to implement compare-
and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of
data structures exceeding one machine word in size. The compiler uses this instruction to implement
__sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is
always used.
-msahf
This option enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs
with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the
"LAHF" and "SAHF" instructions which are supported by AMD64. These are load and store instructions,
respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize
"fmod", "drem", and "remainder" built-in functions; see Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to implement "__builtin_bswap32" and
"__builtin_bswap64".
-mshstk
The -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement
Technology (CET).
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS"
and "RSQRTPS") with an additional Newton-Raphson step to increase precision instead of "DIVSS" and
"SQRTSS" (and their vectorized variants) for single-precision floating-point arguments. These
instructions are generated only when -funsafe-math-optimizations is enabled together with
-ffinite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher
than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased
by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS") already with
-ffast-math (or the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized
single-float division and vectorized "sqrtf(x)" already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated
list of options, which may be preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square
root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values
for type are svml for the Intel short vector math library and acml for the AMD math core library. To
use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an
SVML or ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2", "vmldTanh2",
"vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2",
"vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104",
"vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding
function type when -mveclibabi=svml is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp",
"__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf",
"__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding function type
when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention. Permissible values are sysv for the ABI used on
GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI
when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this
behavior for specific functions by using the function attributes "ms_abi" and "sysv_abi".
-mforce-indirect-call
Force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it
generates more precise timing information for function calls.
-mmanual-endbr
Insert ENDBR instruction at function entry only via the "cf_check" function attribute. This is useful
when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must
consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these
registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using
-mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc
to perform these saves and restores, thus reducing function size at the cost of a few extra
instructions.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the
conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that
cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as
method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is computed in the function
prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling
and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a
notable increase in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception
handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-
thread exception-handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays
out the structure differently than the way GCC normally does. Particularly when moving packed data
between functions compiled with GCC and the native Microsoft compiler (either via function call or as
data in a file), it may be necessary to access either format.
This option is enabled by default for Microsoft Windows targets. This behavior can also be
controlled locally by use of variable or type attributes. For more information, see x86 Variable
Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field
packing. The padding and alignment of members of structures and whether a bit-field can straddle a
storage-unit boundary are determine by these rules:
1. Structure members are stored sequentially in the order in which they are
declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement
for all data except structures, unions, and arrays is either the size of the object or the
current packing size (specified with either the "aligned" attribute or the "pack" pragma),
whichever is less. For structures, unions, and arrays, the alignment requirement is the largest
alignment requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation
unit if the integral types are the same size and if the next bit-field fits into the current
allocation unit without crossing the boundary imposed by the common alignment requirements of the
bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-field. If the zero-length bit-field were
removed, "t1"'s size would be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field, "foo", and the
alignment of the zero-length bit-field is greater than the member that follows it, "bar", "bar"
is aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4.
For "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the
size of the structure.
Taking this into account, it is important to note the following:
1. If a zero-length bit-field follows a normal bit-field, the type of the
zero-length bit-field may affect the alignment of the structure as whole. For example, "t2"
has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of
type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This switch reduces code size and
improves performance in case the destination is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is known to be aligned to least a
4-byte boundary. This enables more inlining and increases code size, but may improve performance of
code that depends on fast "memcpy", "strlen", and "memset" for short lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small blocks and a
library call for large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm to use for inlining string
operations. The allowed values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if "__builtin_memcpy" should be inlined and what
inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-
separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy,
max_size specifies the max byte size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be "-1". The max_size of the triplets in the list must be specified in
increasing order. The minimal byte size for alg is 0 for the first triplet and "max_size + 1" of the
preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to control "__builtin_memset"
expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save,
set up, and restore frame pointers and makes an extra register available in leaf functions. The
option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make
debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for
32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is
valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns
this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86
architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each
profiling call. This is useful for automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful
when they should be patched in later dynamically. This is likely only useful together with
-mrecord-mcount.
-minstrument-return=type
Instrument function exit in -pg -mfentry instrumented functions with call to specified function. This
only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types
are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop.
-mrecord-return
-mno-record-return
Generate a __return_loc section pointing to all return instrumentation code.
-mfentry-name=name
Set name of __fentry__ symbol called at function entry for -pg -mfentry functions.
-mfentry-section=name
Set name of section to record -mrecord-mcount calls (default __mcount_loc).
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can
be used to skip setting up RAX register when there are no variable arguments passed in vector
registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when
passing variable arguments, the impacts of this option are callees may waste some stack space,
misbehave or jump to a random location. GCC 4.4 or newer don't have those issues, regardless the RAX
register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit
integer divide. This option generates a run-time check. If both dividend and divisor are within
range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for global
canary or tls for per-thread canary in the TLS block (the default). This option has effect only when
-fstack-protector or -fstack-protector-all is specified.
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use
as base register for reading the canary, and from what offset from that base register. The default
for those is as specified in the relevant ABI.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This prevents the compiler from using
floating-point, vector, mask and bound registers.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default is keep, which keeps indirect call and jump
unmodified. thunk converts indirect call and jump to call and return thunk. thunk-inline converts
indirect call and jump to inlined call and return thunk. thunk-extern converts indirect call and
jump to external call and return thunk provided in a separate object file. You can control this
behavior for a specific function by using the function attribute "indirect_branch".
Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and
-mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model.
Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external
thunk can be made to enable control-flow check.
-mfunction-return=choice
Convert function return with choice. The default is keep, which keeps function return unmodified.
thunk converts function return to call and return thunk. thunk-inline converts function return to
inlined call and return thunk. thunk-extern converts function return to external call and return
thunk provided in a separate object file. You can control this behavior for a specific function by
using the function attribute "function_return".
Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external
thunk can be made to enable control-flow check.
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and
-mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model.
-mindirect-branch-register
Force indirect call and jump via register.
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets "int", "long", and
pointer types to 32 bits, and generates code that runs on any i386 system.
The -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits, and generates code for
the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and
-mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types to 32 bits, and generates code for the x86-64
architecture.
The -m16 option is the same as -m32, except for that it outputs the ".code16gcc" assembly directive
at the beginning of the assembly output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to
be turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is
a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt
handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag
-mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB
of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This
is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space.
This model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of the address space.
Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put
into large data or BSS sections and can be located above 2GB. Programs can be statically or
dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions about addresses and sizes of
sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is
the default address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is
the default address mode for 32-bit and x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be generated, by instructing the linker to set
the PE header subsystem type required for console applications. This option is available for Cygwin
and MinGW targets and is enabled by default on those targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link
library---is to be generated, enabling the selection of the required runtime startup object and entry
point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute
should be ignored.
-mthread
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be
used.
-municode
This option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be
predefined, and chooses Unicode-capable runtime startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft
Windows predefined macros are to be set in the pre-processor, but does not influence the choice of
runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be
generated by instructing the linker to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for the stack used
by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft
Windows, as there the User32 API, which is used to set executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only
section is put into the ".data" section. This is a necessary for older runtimes not supporting
modification of ".rdata" sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE
file format that permits the correct alignment of COMMON variables should be used when generating
code. It is enabled by default if GCC detects that the target assembler found during configuration
supports the feature.
See also under x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16"
instruction is currently not a standard option from Tensilica. When enabled, "CONST16" instructions
are always used in place of the standard "L32R" instructions. The use of "CONST16" is enabled by
default only if the "L32R" instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for
the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE
754-compliant results are required: the fused multiply add/subtract instructions do not round the
intermediate result, thereby producing results with more bits of precision than specified by the IEEE
standard. Disabling fused multiply add/subtract instructions also ensures that the program output is
not sensitive to the compiler's ability to combine multiply and add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to
guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile
to omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC),
this option disables PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The default is -mno-text-section-literals,
which places literals in a separate section in the output file. This allows the literal pool to be
placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object
files to remove redundant literals and improve code size. With -mtext-section-literals, the literals
are interspersed in the text section in order to keep them as close as possible to their references.
This may be necessary for large assembly files. Literals for each function are placed right before
that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The default is -mno-auto-litpools, which
places literals in a separate section in the output file unless -mtext-section-literals is used.
With -mauto-litpools the literals are interspersed in the text section by the assembler. Compiler
does not produce explicit ".literal" directives and loads literals into registers with "MOVI"
instructions instead of "L32R" to let the assembler do relaxation and place literals as necessary.
This option allows assembler to create several literal pools per function and assemble very big
functions, which may not be possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to
reduce branch penalties at the expense of some code density. The assembler attempts to widen density
instructions to align branch targets and the instructions following call instructions. If there are
not enough preceding safe density instructions to align a target, no widening is performed. The
default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions
like "LOOP", which the assembler always aligns, either by widening density instructions or by
inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls
unless it can determine that the target of a direct call is in the range allowed by the call
instruction. This translation typically occurs for calls to functions in other source files.
Specifically, the assembler translates a direct "CALL" instruction into an "L32R" followed by a
"CALLX" instruction. The default is -mno-longcalls. This option should be used in programs where
the call target can potentially be out of range. This option is implemented in the assembler, not
the compiler, so the assembly code generated by GCC still shows direct call instructions---look at
the disassembled object code to see the actual instructions. Note that the assembler uses an
indirect call for every cross-file call, not just those that really are out of range.
zSeries Options
These are listed under
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by
specifying directories or prefixes to use when searching for various kinds of files. Some are used to
specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take
precedence over places specified using environment variables, which in turn take precedence over those
specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information which allows GCC
to work with different national conventions. GCC inspects the locale categories LC_CTYPE and
LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value
supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the
character boundaries in a string; this is needed for some multibyte encodings that contain quote and
escape characters that are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES;
otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none
of these variables are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to
hold the output of one stage of compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver.
See the documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by
the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you
can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the
pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places
for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed
compiler. In many cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header
files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more
precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified
prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before
it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the
configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header
files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the
directories thus specified when searching for subprograms, if it cannot find the subprograms using
GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured
as a native compiler, GCC tries the directories thus specified when searching for special linker
files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories
when searching for ordinary libraries for the -l option (but directories specified with -L come
first).
LANG
This variable is used to pass locale information to the compiler. One way in which this information
is used is to determine the character set to be used when character literals, string literals and
comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the
following values for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as
defined by the default locale to recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in
which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and
determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost
all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths
given with -I options on the command line. This environment variable is used regardless of which
language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated.
Each specifies a list of directories to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working
directory. Empty elements can appear at the beginning or end of a path. For instance, if the value
of CPATH is ":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-
system header files processed by the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to
that file, guessing the target name from the source file name. Or the value can have the form file
target, in which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an
optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not
ignored, so it implies -M rather than -MM. However, the dependence on the main input file is
omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the
current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become
reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding
leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of
@command{date +%s} on GNU/Linux and other systems that support the %s extension in the "date"
command.
The value should be a known timestamp such as the last modification time of the source or package and
it should be set by the build process.
BUGS
For instructions on reporting bugs, see <file:///usr/share/doc/gcc-9/README.Bugs>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing
to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not
necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), dbx(1) and the Info entries for
gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to
GCC.
COPYRIGHT
Copyright (c) 1988-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with
the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license
is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.