Provided by: gcc-4.8-arm-linux-gnueabihf_4.8.5-4ubuntu1cross2_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 stage 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 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.
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 -no-canonical-prefixes -pipe -pass-exit-codes -x language -v
-### --help[=class[,...]] --target-help --version -wrapper @file -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 -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fopenmp -fms-extensions -fplan9-extensions -trigraphs
-traditional -traditional-cpp -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 -fcheck-new -fconstexpr-depth=n
-ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope
-fno-for-scope -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags
-fpermissive -fno-pretty-templates -frepo -fno-rtti -fstats
-ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -Wabi
-Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wliteral-suffix
-Wnarrowing -Wnoexcept -Wnon-virtual-dtor -Wreorder -Weffc++ -Wstrict-null-sentinel
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo
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 -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match
-Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall
-Waddress -Waggregate-return -Waggressive-loop-optimizations -Warray-bounds
-Wno-attributes -Wno-builtin-macro-redefined -Wc++-compat -Wc++11-compat -Wcast-align
-Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch
-Wno-cpp -Wno-deprecated -Wno-deprecated-declarations -Wdisabled-optimization
-Wno-div-by-zero -Wdouble-promotion -Wempty-body -Wenum-compare -Wno-endif-labels
-Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security
-Wformat-y2k -Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init
-Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration -Wimplicit-int
-Winit-self -Winline -Wmaybe-uninitialized -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len -Wunsafe-loop-optimizations
-Wlogical-op -Wlong-long -Wmain -Wmaybe-uninitialized -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs -Wno-mudflap -Wno-multichar
-Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpacked-bitfield-compat
-Wpadded -Wparentheses -Wpedantic-ms-format -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-to-int-cast -Wredundant-decls -Wno-return-local-addr -Wreturn-type
-Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion
-Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=len -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wsuggest-attribute=[pure|const|noreturn|format] -Wmissing-format-attribute -Wswitch
-Wswitch-default -Wswitch-enum -Wsync-nand -Wsystem-headers -Wtrampolines
-Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas
-Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wunused-but-set-parameter -Wunused-but-set-variable -Wuseless-cast
-Wvariadic-macros -Wvector-operation-performance -Wvla -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
-dletters -dumpspecs -dumpmachine -dumpversion -fsanitize=style -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-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes -fdump-statistics -fdump-tree-all
-fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-cfg -fdump-tree-alias
-fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n]
-fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n]
-fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiprop[-n] -fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect
-fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-forwprop[-n] -fdump-tree-fre[-n]
-fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n]
-fdump-final-insns=file -fcompare-debug[=opts] -fcompare-debug-second
-feliminate-dwarf2-dups -fno-eliminate-unused-debug-types
-feliminate-unused-debug-symbols -femit-class-debug-always -fenable-kind-pass
-fenable-kind-pass=range-list -fdebug-types-section -fmem-report-wpa -fmem-report
-fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs -fopt-info
-fopt-info-options[=file] -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose
-fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstack-usage -ftest-coverage
-ftime-report -fvar-tracking -fvar-tracking-assignments
-fvar-tracking-assignments-toggle -g -glevel -gtoggle -gcoff -gdwarf-version -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf
-gno-strict-dwarf -gvms -gxcoff -gxcoff+ -fno-merge-debug-strings
-fno-dwarf2-cfi-asm -fdebug-prefix-map=old=new -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -p -pg
-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]
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n] -falign-jumps[=n]
-falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-inc-dec
-fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcheck-data-deps -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 -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-pta -fipa-profile
-fipa-pure-const -fipa-reference -fira-algorithm=algorithm -fira-region=region
-fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fira-verbose=n -fivopts -fkeep-inline-functions
-fkeep-static-consts -floop-block -floop-interchange -floop-strip-mine
-floop-nest-optimize -floop-parallelize-all -flto -flto-compression-level
-flto-partition=alg -flto-report -fmerge-all-constants -fmerge-constants
-fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants fmudflap
-fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays
-fprofile-report -fprofile-correction -fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fprofile-use -fprofile-use=path -fprofile-values
-freciprocal-math -free -fregmove -frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math -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-insns
-fschedule-insns2 -fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fshrink-wrap
-fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-wide-types -fstack-protector -fstack-protector-all -fstrict-aliasing
-fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp -ftree-ch -ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop
-ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-loop-if-convert -ftree-loop-if-convert-stores -ftree-loop-im
-ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink -ftree-slsr
-ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize
-ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa
-fuse-ld=linker -fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os
-Ofast -Og
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H
-idirafter dir -include file -imacros file -iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir -imultilib dir -isysroot dir -M -MM -MF -MG
-MP -MQ -MT -nostdinc -P -fdebug-cpp -ftrack-macro-expansion -fworking-directory
-remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option -no-integrated-cpp
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s
-static -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iplugindir=dir -iquotedir -Ldir -specs=file -I- --sysroot=dir
--no-sysroot-suffix
Machine Dependent Options
AArch64 Options -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large -mstrict-align -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional
-mfix-cortex-a53-835769 -mno-fix-cortex-a53-835769 -march=name -mcpu=name
-mtune=name
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
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check
-mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant
-mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian
-mwords-little-endian -mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -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 -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 -mrestrict-it
AVR Options -mmcu=mcu -maccumulate-args -mbranch-cost=cost -mcall-prologues -mint8
-mno-interrupts -mrelax -mstrict-X -mtiny-stack -Waddr-space-convert
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
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
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 -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 -mbig-switch -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-big-switch -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
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -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 -mmmx
-msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -maes -mpclmul
-mfsgsbase -mrdrnd -mf16c -mfma -msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4
-mxop -mlzcnt -mbmi2 -mrtm -mlwp -mthreads -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg
-mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double
-mlong-double-64 -mlong-double-80 -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 -mlarge-data-threshold=num -msse2avx -mfentry
-m8bit-idiv -mavx256-split-unaligned-load -mavx256-split-unaligned-store
i386 and x86-64 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows -fno-set-stack-executable
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
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
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips64 -mips64r2 -mips16 -mno-mips16 -mflip-mips16
-minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls
-mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32
-mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float -mdsp
-mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -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 -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp
-mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r10000 -mno-fix-r10000 -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 -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
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 -mno-crt0
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy
-mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64
-mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap
-munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address
-mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RL78 Options -msim -mmul=none -mmul=g13 -mmul=rl78
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 -msingle-float -mdouble-float
-msimple-fpu -mstring -mno-string -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 -maltivec -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mspe -mno-spe
-mspe=yes -mspe=no -mpaired -mgen-cell-microcode -mwarn-cell-microcode -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes -mfloat-gprs=no
-mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme
-mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num -pthread -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 -mdirect-move -mno-direct-move
-mquad-memory -mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic
-mcompat-align-parm -mno-compat-align-parm
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 -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 -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 -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact
-m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas
-mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate
-mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range -mindexed-addressing
-mgettrcost=number -mpt-fixed -maccumulate-outgoing-args -minvalid-symbols
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranchdi
-mcmpeqdi -mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options -mimpure-text -mno-impure-text -pthreads -pthread
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 -munaligned-doubles
-mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis
-mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mcbcond -mno-cbcond -mfmaf -mno-fmaf
-mpopc -mno-popc -mfix-at697f -mfix-ut699
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 -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
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86-64 Options See i386 and x86-64 Options.
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 -mtarget-align -mno-target-align -mlongcalls
-mno-longcalls
zSeries Options See S/390 and zSeries Options.
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 -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,... -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-double -fshort-wchar -fverbose-asm
-fpack-struct[=n] -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack -fleading-underscore -ftls-model=model
-fstack-reuse=reuse_level -ftrapv -fwrapv -fbounds-check -fvisibility
-fstrict-volatile-bitfields -fsync-libcalls
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.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.d
D source code file.
file.di
D interface code file.
file.dd
D documentation code file.
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
java
-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).
-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.
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.
-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.
--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 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
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path
absolute when generating a relative prefix.
--version
Display the version number and copyrights of the invoked GCC.
-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 ....
-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. Note that this standard is not yet fully supported; 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. Support is incomplete and
experimental. The name c1x is deprecated.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features). This is the default for C
code.
gnu99
gnu9x
GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will
become the default. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. Support is incomplete and experimental. The name gnu1x
is deprecated.
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. This is the default for C++ code.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. Support for C++11 is still
experimental, and may change in incompatible ways in future releases. The name
c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. Support for C++11 is still experimental, and may change
in incompatible ways in future releases. The name gnu++0x is deprecated.
c++1y
The next revision of the ISO C++ standard, tentatively planned for 2017. Support
is highly experimental, and will almost certainly change in incompatible ways in
future releases.
gnu++1y
GNU dialect of -std=c++1y. 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.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including
4.3. In GCC versions 4.3 and later it changes the behavior of GCC 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 was first supported in GCC 4.3. 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.
-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))
-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.
-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 v3.0 <http://www.openmp.org/>. This option
implies -pthread, and thus is only supported on targets that have support for
-pthread.
-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.
-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++.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C
conformance) implies -trigraphs.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler.
They are now only supported with the -E switch. The preprocessor continues to support
a pre-standard mode. See the GNU CPP manual for details.
-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.
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 -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the
other options with any language supported by GCC.
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 2.
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.
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.
See also -Wabi.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in
the access control code.
-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).
-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.
-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++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside
the scope of the class in which they are declared. Friend functions were documented
to work this way in the old Annotated C++ Reference Manual, and versions of G++ before
4.1 always worked that way. However, in ISO C++ a friend function that is not
declared in an enclosing scope can only be found using argument dependent lookup.
This option causes friends to be injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future release 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.
-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.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement
is limited to the for loop itself, as specified by the C++ standard. If
-fno-for-scope is specified, the scope of variables declared in a for-init-statement
extends to the end of the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
If neither flag is given, the default is to follow the standard, but to allow and give
a warning for old-style code that would otherwise be invalid, or have different
behavior.
-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. -ansi implies
-fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e. by
use); only emit code for explicit instantiations.
-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.
-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.
-fstats
Emit statistics about front-end processing at the end of the compilation. This
information is generally only useful to the G++ development team.
-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.
-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:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions have linkage like inline functions; they just aren't
inlined by default.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral
C++ ABI. 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.
The known incompatibilities in -fabi-version=2 (the default) include:
• A template with a non-type template parameter of reference type is mangled
incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This is fixed in -fabi-version=3.
• SIMD vector types declared using "__attribute ((vector_size))" are mangled in a
non-standard way that does not allow for overloading of functions taking vectors
of different sizes.
The mangling is changed in -fabi-version=4.
The known incompatibilities in -fabi-version=1 include:
• Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data
into the same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ places "B::f2" into the same byte as "A::f1"; other compilers do
not. You can avoid this problem by explicitly padding "A" so that its size is a
multiple of the byte size on your platform; that causes G++ and other compilers to
lay out "B" identically.
• Incorrect handling of tail-padding for virtual bases. G++ does not use tail
padding when laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ does not place "B" into the tail-padding for "A"; other
compilers do. You can avoid this problem by explicitly padding "A" so that its
size is a multiple of its alignment (ignoring virtual base classes); that causes
G++ and other compilers to lay out "C" identically.
• Incorrect handling of bit-fields with declared widths greater than that of their
underlying types, when the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ makes the union too small by
the number of bits in an "int".
• Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ places the "A" base class of "C" at a nonzero offset; it should be placed at
offset zero. G++ mistakenly believes that the "A" data member of "B" is already
at offset zero.
• Names of template functions whose types involve "typename" or template template
parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
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.
-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.
-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.
This warning is enabled by default.
-Wnarrowing (C++ and Objective-C++ only)
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.
With -std=c++11, -Wno-narrowing suppresses the diagnostic required by the standard.
Note that this does not affect the meaning of well-formed code; narrowing conversions
are still considered ill-formed in SFINAE context.
-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.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual destructor, in
which case it is possible but unsafe to delete an instance of a derived class through
a pointer to the base class. This warning is also enabled if -Weffc++ is specified.
-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.
-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++1y. 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++, Second Edition book:
• Item 11: Define a copy constructor and an assignment operator for classes with
dynamically-allocated memory.
• Item 12: Prefer initialization to assignment in constructors.
• Item 14: Make destructors virtual in base classes.
• Item 15: Have "operator=" return a reference to *this.
• Item 23: Don't try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers' More
Effective C++ book:
• Item 6: Distinguish between prefix and postfix forms of increment and decrement
operators.
• Item 7: Never overload "&&", "||", or ",".
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-templatized friend functions are declared within a template.
Since the advent of explicit template specification support in G++, if the name of the
friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification
demands that the friend declare or define an ordinary, nontemplate function. (Section
14.5.3). Before G++ implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized function. Because this
non-conforming behavior is no longer the default behavior for G++,
-Wnon-template-friend allows the compiler to check existing code for potential trouble
spots and is on by default. This new compiler behavior can be turned off with
-Wno-non-template-friend, which keeps the conformant compiler code but disables the
helpful warning.
-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.
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++ and Java. 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.
-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. The
default is 72 characters for g++ and 0 for the rest of the front ends supported by
GCC. If n is zero, then no line-wrapping is done; each error message appears on a
single line.
-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.
-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.
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.
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.
-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
Like -Wpedantic, except that errors are produced rather than warnings.
-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 (only with -O2) -Wc++11-compat -Wchar-subscripts
-Wenum-compare (in C/ObjC; this is on by default in C++) -Wimplicit-int (C and
Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Wcomment
-Wformat -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized
-Wmissing-braces (only for C/ObjC) -Wnonnull -Wparentheses -Wpointer-sign -Wreorder
-Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -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 -Wempty-body -Wignored-qualifiers -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init
-Wsign-compare -Wtype-limits -Wuninitialized -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 a derived class's copy constructor.
-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.
-Wcomment
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.
-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-gen 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".
-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.
-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.)
NOTE: In Ubuntu 8.10 and later versions this option is enabled by default for C,
C++, ObjC, ObjC++. To disable, use -Wno-format-security, or disable all format
warnings with -Wformat=0. To make format security warnings fatal, specify
-Werror=format-security.
-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.
-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.
-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.
-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.
-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.
-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 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 ();
}
}
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.
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 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), and about a "return" statement with an expression in a function
whose return type is "void".
For C++, a function without return type always produces a diagnostic message, even
when -Wno-return-type is specified. The only exceptions are main and functions
defined in system headers.
This warning is enabled by -Wall.
-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.
-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.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program
(trigraphs within comments are not warned about). This warning is enabled by -Wall.
-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 variable or non-constant static variable is unused aside from
its declaration. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-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.
-Wmaybe-uninitialized
For an automatic 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 made optional
because GCC is not smart enough to see all the reasons why the code might be 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". These warnings as well are possible only in optimizing
compilation.
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.
-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 -fstrict-overflow is active. 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, with
-fstrict-overflow, 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 -fstrict-overflow is in
effect, 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.
-Wsuggest-attribute=[pure|const|noreturn|format]
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
Warn about functions that might be candidates for attributes "pure", "const" or
"noreturn". 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.
-Warray-bounds
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.
-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.
-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 and was not supported by GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable,
parameter, type, or class member (in 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.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The computation done
to determine the stack frame size is approximate and not conservative. The actual
requirements may be somewhat greater than len 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.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len bytes. 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
-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.
-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.
-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.
-Wbad-function-cast (C and Objective-C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if
"int malloc()" is cast to "anything *".
-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.
-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.
-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.
-Wclobbered
Warn for variables that might be changed by longjmp or vfork. This warning is also
enabled by -Wextra.
-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.
-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++ enumeral
mismatches in conditional expressions are also diagnosed and the warning is enabled by
default. In C this warning is enabled by -Wall.
-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. This warning is also enabled
by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.
-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.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions
if the argument uses "sizeof". This warning warns e.g. about "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));". 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.
-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.
-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-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 will declare 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-prototype 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 };
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.
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. 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-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.
-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.
-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.
-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.
-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 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In
existing C++ implementations, however, offsetof typically gives meaningful results
even when applied to certain kinds of non-POD types (such as a simple struct that
fails to be a POD type only by virtue of having a constructor). 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.
-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 can't 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 pedantic ISO C90 mode, or the GNU alternate syntax
when in pedantic ISO C99 mode. This is default. 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 will be 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 variable length array is used in the code. -Wno-vla prevents the -Wpedantic
warning of the variable length array.
-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.
-Wno-mudflap
Suppress warnings about constructs that cannot be instrumented by -fmudflap.
-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.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system's native format (stabs, COFF,
XCOFF, or DWARF 2). 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).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may
occasionally produce surprising results: 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 proves possible to debug optimized output. This makes it reasonable
to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more
than one debugging format.
-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.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive
format available (DWARF 2, stabs, or the native format if neither of those are
supported), including GDB extensions if at all possible.
-gpubnames
Generate dwarf .debug_pubnames and .debug_pubtypes sections.
-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 or SDB. On System V Release 4 systems this option requires the GNU
assembler.
-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.
-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 sections 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.
-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.
-gcoff
Produce debugging information in COFF format (if that is supported). This is the
format used by SDB on most System V systems prior to System V Release 4.
-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.
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The value of
version may be either 2, 3 or 4; the default version for most targets is 4.
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.
-grecord-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. See also -frecord-gcc-switches for another
way of storing compiler options into the object file. This is the default.
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer attribute in DWARF
debugging information.
-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.
-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
-gcofflevel
-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, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the
program. Some debuggers support macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an
option -gdwarf that meant to generate debug information in version 1 of the DWARF
format (which is very different from version 2), and it would have been too confusing.
That debug format is long obsolete, but the option cannot be changed now. Instead use
an additional -glevel option to change the debug level for DWARF.
-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.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access instructions
will be instrumented to detect out-of-bounds and use-after-free bugs. See
<http://code.google.com/p/address-sanitizer/> for more details.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be
instrumented to detect data race bugs. See
<http://code.google.com/p/data-race-test/wiki/ThreadSanitizer> for more details.
-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 side-effect compiler outputs to files or to the standard output.
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.
-feliminate-dwarf2-dups
Compress DWARF 2 debugging information by eliminating duplicated information about
each symbol. This option only makes sense when generating DWARF 2 debugging
information with -gdwarf-2.
-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 2.
-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 2.
-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 2.
-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 in directory old, record debugging information describing them as
in new instead.
-fno-dwarf2-cfi-asm
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using
GAS ".cfi_*" directives.
-p Generate extra code to write profile information suitable for the analysis program
prof. You must use this option when compiling the source files you want data about,
and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis program
gprof. You must use this option when compiling the source files you want data about,
and you must also use it when linking.
-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.
-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.
-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. 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.
• 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. Also "fork" calls are detected and
correctly handled (double counting will not happen).
• 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.
-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 upper bound. counter-value-list is a comma-separated
list of name:value pairs which sets the upper bound of each debug counter name to
value. 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:10,tail_call:0, "dbg_cnt(dce)" returns true only for first
10 invocations.
-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
-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. The file names for most
of the dumps are made by appending a pass number and a word to the dumpname, and the
files are created in the directory of the output file. In case of =filename option,
the dump is output on the given file instead of the pass numbered dump files. Note
that the pass number is computed statically as passes get registered into the pass
manager. Thus the numbering is not related to the dynamic order of execution of
passes. In particular, a pass installed by a plugin could have a number over 200 even
if it executed quite early. dumpname 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. These switches may have different effects 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-regmove
Dump after the register move pass.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block
scheduling passes.
-fdump-rtl-see
Dump after sign 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
-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and
-fdump-rtl-split5 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 of each instruction is 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-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.
-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-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire translation unit to a file.
The file name is made by appending .tu to the source file name, and the file is
created in the same directory as the output file. If the -options form is used,
options controls the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual function table layout to a
file. The file name is made by appending .class to the source file name, and the file
is created in the same directory as the output file. If the -options form is used,
options controls the details of the dump as described for the -fdump-tree options.
-fdump-ipa-switch
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.
-fdump-passes
Dump 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-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. 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. In
case of =filename option, the dump is output on the given file instead of the auto
named dump files. 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).
notes
Enable other detailed optimization information (only available in certain passes).
=filename
Instead of an auto named dump file, output into the given file name. The file
names stdout and stderr are treated specially and are considered already open
standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr.
If two conflicting dump filenames are given for the same pass, then the latter
option overrides the earlier one.
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and note.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
gimple
Dump each function before and after the gimplification pass to a file. The file
name is made by appending .gimple to the source file name.
cfg Dump the control flow graph of each function to a file. The file name is made by
appending .cfg to the source file name.
ch Dump each function after copying loop headers. The file name is made by appending
.ch to the source file name.
ssa Dump SSA related information to a file. The file name is made by appending .ssa
to the source file name.
alias
Dump aliasing information for each function. The file name is made by appending
.alias to the source file name.
ccp Dump each function after CCP. The file name is made by appending .ccp to the
source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by appending .storeccp
to the source file name.
pre Dump trees after partial redundancy elimination. The file name is made by
appending .pre to the source file name.
fre Dump trees after full redundancy elimination. The file name is made by appending
.fre to the source file name.
copyprop
Dump trees after copy propagation. The file name is made by appending .copyprop
to the source file name.
store_copyprop
Dump trees after store copy-propagation. The file name is made by appending
.store_copyprop to the source file name.
dce Dump each function after dead code elimination. The file name is made by
appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The file name is made by
appending .mudflap to the source file name.
sra Dump each function after performing scalar replacement of aggregates. The file
name is made by appending .sra to the source file name.
sink
Dump each function after performing code sinking. The file name is made by
appending .sink to the source file name.
dom Dump each function after applying dominator tree optimizations. The file name is
made by appending .dom to the source file name.
dse Dump each function after applying dead store elimination. The file name is made
by appending .dse to the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline code. The file
name is made by appending .phiopt to the source file name.
forwprop
Dump each function after forward propagating single use variables. The file name
is made by appending .forwprop to the source file name.
copyrename
Dump each function after applying the copy rename optimization. The file name is
made by appending .copyrename to the source file name.
nrv Dump each function after applying the named return value optimization on generic
trees. The file name is made by appending .nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The file name is made
by appending .vect to the source file name.
slp Dump each function after applying vectorization of basic blocks. The file name is
made by appending .slp to the source file name.
vrp Dump each function after Value Range Propagation (VRP). The file name is made by
appending .vrp to the source file name.
all Enable all the available tree dumps with the flags provided in this option.
-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 options to select the dump details and
optimizations. If options is not specified, it defaults to all for details and optall
for optimization groups. If the filename is not specified, it defaults to stderr. Note
that the output filename will be overwritten in case of multiple translation units. If
a combined output from multiple translation units is desired, stderr should be used
instead.
The options can be divided into two groups, 1) options describing the verbosity of the
dump, and 2) options describing which optimizations should be included. The options
from both the groups can be freely mixed as they are non-overlapping. However, in case
of any conflicts, the latter options override the earlier options on the command line.
Though multiple -fopt-info options are accepted, only one of them can have =filename.
If other filenames are provided then all but the first one are ignored.
The dump verbosity has the following options
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 got successfully vectorized.
missed
Print information about missed optimizations. Individual passes control which
information to include in the output. For example,
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
will print information about missed optimization opportunities from vectorization
passes on stderr.
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 second set of options describes a group of optimizations and may include one or
more of the following.
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
vec Enable dumps from all vectorization optimizations.
For example,
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
will output information about missed optimizations as well as optimized locations from
all the inlining passes into inline.txt.
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. If options
is omitted, it defaults to all-optall, which means dump all available optimization
info from all the passes. In the following example, all optimization info is output on
to stderr.
gcc -O3 -fopt-info
Note that -fopt-info-vec-missed behaves the same as -fopt-info-missed-vec.
As another example, 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 the vec.miss is produced which cotaints
dumps from the vectorizer about missed opportunities.
-ftree-vectorizer-verbose=n
This option is deprecated and is implemented in terms of -fopt-info. Please use
-fopt-info-kind form instead, where kind is one of the valid opt-info options. It
prints additional optimization information. For n=0 no diagnostic information is
reported. If n=1 the vectorizer reports each loop that got vectorized, and the total
number of loops that got vectorized. If n=2 the vectorizer reports locations which
could not be vectorized and the reasons for those. For any higher verbosity levels all
the analysis and transformation information from the vectorizer is reported.
Note that the information output by -ftree-vectorizer-verbose option is sent to
stderr. If the equivalent form -fopt-info-options=filename is used then the output is
sent into filename instead.
-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 should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of
debugging output the scheduler prints. This information is written to standard error,
unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output
to the usual dump listing file, .sched1 or .sched2 respectively. However for n
greater than nine, the output is always printed to standard error.
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.
-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.
-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.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-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)---and don't do anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when
GCC itself is being built.)
-fno-eliminate-unused-debug-types
Normally, when producing DWARF 2 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 only enabled if an -O level is set on the command line. Otherwise
they are disabled, even if individual optimization flags are specified.
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 -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fguess-branch-probability -fif-conversion2 -fif-conversion -fipa-pure-const
-fipa-profile -fipa-reference -fmerge-constants -fsplit-wide-types -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-slsr
-ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere
with debugging.
-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: -fthread-jumps -falign-functions -falign-jumps -falign-loops
-falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks -fdevirtualize -fexpensive-optimizations -fgcse
-fgcse-lm -fhoist-adjacent-loads -finline-small-functions -findirect-inlining
-fipa-sra -foptimize-sibling-calls -fpartial-inlining -fpeephole2 -fregmove
-freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-interblock
-fsched-spec -fschedule-insns -fschedule-insns2 -fstrict-aliasing -fstrict-overflow
-ftree-switch-conversion -ftree-tail-merge -ftree-pre -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 -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload,
-ftree-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone options.
-O0 Reduce compilation time and make debugging produce the expected results. This is the
default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase
code size. It also performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps
-falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition
-fprefetch-loop-arrays
-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 -fno-protect-parens and -fstack-arrays.
-Og Optimize debugging experience. -Og enables optimizations that do not interfere with
debugging. It 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.
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-default-inline
Do not make member functions inline by default merely because they are defined inside
the class scope (C++ only). Otherwise, when you specify -O, member functions defined
inside class scope are compiled inline by default; i.e., you don't need to add inline
in front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For
machines that must pop arguments after a function call, the compiler normally lets
arguments accumulate on the stack for several function calls and pops them all at
once.
Disabled at levels -O, -O2, -O3, -Os.
-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
Don't keep the frame pointer in a register for functions that don't need one. This
avoids the instructions to save, set up and restore frame pointers; it also makes an
extra register available in many functions. It also makes debugging impossible on
some machines.
On some machines, such as the VAX, this flag has no effect, because the standard
calling sequence automatically handles the frame pointer and nothing is saved by
pretending it doesn't exist. The machine-description macro "FRAME_POINTER_REQUIRED"
controls whether a target machine supports this flag.
Starting with GCC version 4.6, the default setting (when not optimizing for size) for
32-bit GNU/Linux x86 and 32-bit Darwin x86 targets has been changed to
-fomit-frame-pointer. The default can be reverted to -fno-omit-frame-pointer by
configuring GCC with the --enable-frame-pointer configure option.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-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 level -O2.
-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 level -O2.
-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 level -O3.
-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.
-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-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
Do not use "decrement and branch" instructions on a count register, but instead
generate a sequence of instructions that decrement a register, compare it against
zero, 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.
The default is -fbranch-count-reg.
-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.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array
dereferencing operations, some standard library string/heap functions, and some other
associated constructs with range/validity tests. Modules so instrumented should be
immune to buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate runtime library
(libmudflap), which is linked into a program if -fmudflap is given at link time. Run-
time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-
threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if
instrumentation should ignore pointer reads. This produces less instrumentation (and
therefore faster execution) and still provides some protection against outright memory
corrupting writes, but allows erroneously read data to propagate within a program.
-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.
-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.
-funsafe-loop-optimizations
This option tells the loop optimizer to assume that loop indices do not overflow, and
that loops with nontrivial exit condition are not infinite. This enables a wider
range of loop optimizations even if the loop optimizer itself cannot prove that these
assumptions are valid. If you use -Wunsafe-loop-optimizations, the compiler warns you
if it finds this kind of loop.
-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 "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data
element resides there. This 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 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.
Some targets, especially embedded ones, disable this option at all levels. Otherwise
it is enabled at all levels: -O0, -O1, -O2, -O3, -Os. Passes that use the 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.
-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 x86 at levels -O2, -O3.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other
simple instructions in order to maximize the amount of register tying. This is
especially helpful on machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-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.
-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.
-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.
-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
Don't schedule instructions across basic blocks. This is normally enabled by default
when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or
higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally enabled by
default 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.
-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.
-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.
-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.
-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-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.
-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.
-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 -O and higher. 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.
-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 side-effect free. 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
Perform loop interchange transformations on tree. Same as -floop-interchange. To use
this code transformation, GCC has to be configured with --with-ppl and --with-cloog to
enable the Graphite loop transformation infrastructure.
-floop-interchange
Perform loop interchange transformations on loops. Interchanging two nested loops
switches the inner and outer loops. For example, given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange transforms the loop as if it were written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches, because in Fortran, the
elements of an array are stored in memory contiguously by column, and the original
loop iterates over rows, potentially creating at each access a cache miss. This
optimization applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured with --with-ppl
and --with-cloog to enable the Graphite loop transformation infrastructure.
-floop-strip-mine
Perform loop strip mining transformations on loops. Strip mining splits a loop into
two nested loops. The outer loop has strides equal to the strip size and the inner
loop has strides of the original loop within a strip. The strip length can be changed
using the loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining transforms the loop as if it were written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured with --with-ppl
and --with-cloog to enable the Graphite loop transformation infrastructure.
-floop-block
Perform loop blocking transformations on loops. Blocking strip mines each loop in the
loop nest such that the memory accesses of the element loops fit inside caches. The
strip length can be changed using the loop-block-tile-size parameter. For example,
given a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking transforms the loop as if it were written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches, because the innermost loop
iterates over a smaller amount of data which can be kept in the caches. This
optimization applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured with --with-ppl
and --with-cloog 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 CLooG, 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.
-fcheck-data-deps
Compare the results of several data dependence analyzers. This option is used for
debugging the data dependence analyzers.
-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-if-convert-stores
Attempt to also if-convert conditional jumps containing memory writes. This
transformation can be unsafe for multi-threaded programs as it transforms conditional
memory writes into unconditional memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
is transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
-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
-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.
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.
-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.
-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 -O and higher.
-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 -O and higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler temporaries to
other variables at copy locations, usually resulting in variable names which more
closely resemble the original variables. This flag is enabled by default at -O and
higher.
-ftree-coalesce-inlined-vars
Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-
defined variables too, but only if they were inlined from other functions. It is a
more limited form of -ftree-coalesce-vars. This may harm debug information of such
inlined variables, but it will keep variables of the inlined-into function apart from
each other, such that they are more likely to contain the expected values in a
debugging session. This was the default in GCC versions older than 4.7.
-ftree-coalesce-vars
Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-
defined variables too, 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, including
inlined ones. This option is enabled by default.
-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 loop vectorization on trees. This flag is enabled by default at -O3.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O3 and
when -ftree-vectorize is enabled.
-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 will
guard 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 will
disable 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".
-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.
-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.
-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. This option makes
code larger, and may or may not make it run faster.
-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,
-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 level -O2.
-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.
-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-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.
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 -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,
for linkonce sections, for functions with a user-defined section attribute and on any
architecture that does not support named sections.
-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.
Also profile feedback must be available to make this option effective. See
-fprofile-arcs for details.
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.
-fstrict-overflow
Allow the compiler to assume strict signed overflow rules, depending on the language
being compiled. For C (and C++) this means that overflow when doing arithmetic with
signed numbers is undefined, which means that the compiler may assume that it does not
happen. This permits various optimizations. For example, the compiler assumes that
an expression like "i + 10 > i" is always true for signed "i". This assumption is
only valid if signed overflow is undefined, as the expression is false if "i + 10"
overflows when using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers overflows must be written
carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer semantics: given a
pointer to an object, if adding an offset to that pointer does not produce a pointer
to the same object, the addition is undefined. This permits the compiler to conclude
that "p + u > p" is always true for a pointer "p" and unsigned integer "u". This
assumption is only valid because pointer wraparound is undefined, as the expression is
false if "p + u" overflows using twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer signed overflow is
fully defined: it wraps. When -fwrapv is used, there is no difference between
-fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain types
of overflow are permitted. For example, if the compiler gets an overflow when doing
arithmetic on constants, the overflowed value can still be used with -fwrapv, but not
otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n
bytes. For instance, -falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 aligns to the next 32-byte boundary only if this
can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions
are not aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is
rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like
-falign-functions. This option can easily make code slower, because it must insert
dummy operations for when the branch target is reached in the usual flow of the code.
-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.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions.
If the loops are executed many times, this makes up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not
aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets
can only be reached by jumping, skipping up to n bytes like -falign-functions. In
this case, no dummy operations need be executed.
-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.
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.
Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors,
which is otherwise enabled at -O0 on some targets.
-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 needs to be specified at compile time and during
the final link. 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 only important thing to keep in mind is that to enable link-time optimizations the
-flto flag needs to be passed to both the compile and the link commands.
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.
Note that when a file is compiled with -flto, the generated object file is larger than
a regular object file because it contains GIMPLE bytecodes and the usual final code.
This means that object files with LTO information can be linked as normal object
files; if -flto is not passed to the linker, no interprocedural optimizations are
applied.
Additionally, the optimization flags used to compile individual files are not
necessarily related to those used at link time. For instance,
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the
resulting binary myprog is optimized at -O3. If, instead, the final binary is
generated without -flto, then myprog is not optimized.
When producing the final binary with -flto, 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.
There are some code generation flags preserved by GCC when generating bytecodes, as
they need to be used during the final link stage. Currently, the following options
are saved into the GIMPLE bytecode files: -fPIC, -fcommon and all the -m target flags.
At link time, these options are read in and reapplied. Note that the current
implementation makes no attempt to recognize conflicting values for these options. If
different files have conflicting option values (e.g., one file is compiled with -fPIC
and another isn't), the compiler simply uses the last value read from the bytecode
files. It is recommended, then, that you compile all the files participating in the
same link with the same options.
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.
Another feature of LTO is that it is possible to apply interprocedural optimizations
on files written in different languages. This requires support in the language front
end. Currently, the C, C++ and Fortran front ends are capable of emitting GIMPLE
bytecodes, so something like this should work:
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; all you need to add is -flto to all the
compile and link commands.
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 enable this feature, use the flag -fuse-linker-plugin
at link time:
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.
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 will not
work with an older/newer version of GCC.
Link-time optimization does not work well with generation of debugging information.
Combining -flto with -g is currently experimental and expected to produce wrong
results.
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. The default
value for n is 1.
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.
This option is disabled by default.
-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.
-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.
-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.
-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.
The default is -ffat-lto-objects but this default is intended to change in future
releases when linker plugin enabled environments become more common.
-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.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-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.
-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.
-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", "-fvpt".
If path is specified, GCC looks at the path to find the profile feedback data files.
See -fprofile-dir.
-fprofile-use
-fprofile-use=path
Enable profile feedback directed optimizations, and optimizations generally profitable
only with profile feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt",
"-funroll-loops", "-fpeel-loops", "-ftracer", "-ftree-vectorize",
"ftree-loop-distribute-patterns"
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 -Wcoverage-mismatch.
Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the profile feedback data files.
See -fprofile-dir.
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 registers have more precision than the IEEE "float" and "double" types and the
processor does not support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that operations are carried out in
the precision of the registers and that 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.
-fexcess-precision=standard is not implemented for languages other than C, and has no
effect if -funsafe-math-optimizations or -ffast-math is specified. On the x86, it
also 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 -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans and -fcx-limited-range.
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.
-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.
-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 with -fprofile-generate and -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.
-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 and -fpeel-loops.
-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 with -fprofile-use.
-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 with -fprofile-use.
-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). It also turns on complete loop peeling (i.e. complete removal of
loops with small constant number of iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level
-O1
-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).
-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 and SPARC processors running Solaris 2 have linkers with such optimizations.
AIX may have these optimizations in the future.
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. You cannot use "gprof" on all systems if you specify this
option, and you may have problems with debugging if you specify both this option and
-g.
-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.
-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.
NOTE: In Ubuntu 6.10 and later versions this option is enabled by default for C, C++,
ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are
found.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-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 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. The default is 10.
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. The
default value is 5.
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. The default value is
8.
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. The default value is 8.
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. The default value is
20.
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++).
The default value is 400.
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. The default value is 40.
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this
threshold (in precent), the function can be inlined regardless 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. The default value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. The
default value is 100 which 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. The default is 10000.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. The
default value is 30 which limits unit growth to 1.3 times the original size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural
constant propagation. The default value is 10 which 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. The default value is 256 bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents.
The default value is 1000 which 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.
For functions declared inline, --param max-inline-insns-recursive is taken into
account. For functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled and --param max-inline-insns-
recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
For functions declared inline, --param max-inline-recursive-depth is taken into
account. For functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled and --param max-inline-recursive-
depth-auto is used. The default value is 8.
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 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). The default value is 10.
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. The default value is 10.
max-early-inliner-iterations
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
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility are
shared across multiple compilation units. The default value is 20.
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. The
default value is 0.
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. The default value is
10.
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. The default value is 3.
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. The default value
is 30.
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. The default value is 10.
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. The default value is 2.
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.
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.
omega-max-vars
The maximum number of variables in an Omega constraint system. The default value
is 128.
omega-max-geqs
The maximum number of inequalities in an Omega constraint system. The default
value is 256.
omega-max-eqs
The maximum number of equalities in an Omega constraint system. The default value
is 128.
omega-max-wild-cards
The maximum number of wildcard variables that the Omega solver is able to insert.
The default value is 18.
omega-hash-table-size
The size of the hash table in the Omega solver. The default value is 550.
omega-max-keys
The maximal number of keys used by the Omega solver. The default value is 500.
omega-eliminate-redundant-constraints
When set to 1, use expensive methods to eliminate all redundant constraints. The
default value is 0.
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.
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 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-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge has probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation
for profile feedback and one for compilation without. The value for compilation
with profile feedback needs to be more conservative (higher) in order to make
tracer effective.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers. The default is 10.
max-cse-insns
The maximum number of instructions CSE processes before flushing. The default is
1000.
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. The default
value is 100.
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. The default value is 500.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by the basic block reordering pass to decide whether to use unconditional
branch or duplicate the code on its destination. Code is duplicated when its
estimated size is smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is
available. It may be set to higher values than reorder-block-duplicate since
information about the hot spots is more accurate.
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. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock
scheduling. The default value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the
selective scheduler. The default value is 15.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock
scheduling. The default value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the
selective scheduler. The default value is 200.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock
speculative scheduling. The default value is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0 (the
default) disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.
The default value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative
insns are scheduled. The default value is 40.
sched-spec-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state
across it. The default value is 10.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory
locations. The default value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth
of search for available instructions. The default value is 50.
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. The default value is 2.
selsched-max-insns-to-rename
The maximum number of best instructions in the ready list that are considered for
renaming in the selective scheduler. The default value is 2.
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates. The
default value is 2.
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. The
default is 10000.
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. The default value is 256.
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.
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. The default is zero for -O0 and -O1, and 100 for -Os, -O2, and
-O3.
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 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
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. By default, this
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.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during SCCVN processing. If
this limit is hit, SCCVN processing for the whole function is not done and
optimizations depending on it are disabled. The default maximum SCC size is
10000.
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.
The default maxmimum number of queries is 1000.
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.
The default value of the parameter is 100.
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.
The default value of the parameter is 2000.
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. The default value of the
parameter is 2, which is the minimal number of registers needed by typical
instructions. This value is the best found from numerous experiments.
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. The default value of the parameter is 1000 for -O1 and 10000
for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dapendencies 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. The default value is 1000.
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. The default is 12.
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.
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. The default value is 10
parameters. A variable whose value is unknown at compilation time and defined
outside a SCoP is a parameter of the SCoP.
graphite-max-bbs-per-function
To avoid exponential effects in the detection of SCoPs, the size of the functions
analyzed by Graphite is bounded. The default value is 100 basic blocks.
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. The default value is 51 iterations.
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.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The
number of partitions should exceed the number of CPUs used for compilation. The
default value is 32.
lto-minpartition
Size of minimal partition for WHOPR (in estimated instructions). This prevents
expenses of splitting very small programs into too many partitions.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup
fails for an identifier. The default is 1000.
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. The
default value is 75. 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 stores paires that can be sunk. Set to 0 if
either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert)
is disabled. The default is 2.
allow-load-data-races
Allow optimizers to introduce new data races on loads. Set to 1 to allow,
otherwise to 0. This option is enabled by default unless implicitly set by the
-fmemory-model= option.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to 1 to allow,
otherwise to 0. This option is enabled by default unless implicitly set by the
-fmemory-model= option.
allow-packed-load-data-races
Allow optimizers to introduce new data races on packed data loads. Set to 1 to
allow, otherwise to 0. This option is enabled by default unless implicitly set by
the -fmemory-model= option.
allow-packed-store-data-races
Allow optimizers to introduce new data races on packed data stores. Set to 1 to
allow, otherwise to 0. This option is enabled by default unless implicitly set by
the -fmemory-model= option.
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. The default is 0.
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 will be considered when seeking
a basis for a new straight-line strength reduction candidate.
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.
-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.
-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 will be
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 will need to 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.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined
macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header files.
Directories named by -I are searched before the standard system include directories.
If the directory dir is a standard system include directory, the option is ignored to
ensure that the default search order for system directories and the special treatment
of system headers are not defeated . If dir begins with "=", then the "=" will be
replaced by the sysroot prefix; see --sysroot and -isysroot.
-o file
Write output to file. This is the same as specifying file as the second non-option
argument to cpp. gcc has a different interpretation of a second non-option argument,
so you must use -o to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this
is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a
change of sign in "#if" expressions. Note that many of the preprocessor's warnings
are on by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a
backslash-newline appears in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a
trigraph that would form an escaped newline (??/ at the end of a line) can, by
changing where the comment begins or ends. Therefore, only trigraphs that would form
escaped newlines produce warnings inside a comment.
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.
-Wtraditional
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 problematic
constructs which should be avoided.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive,
outside of defined. Such identifiers are replaced with zero.
-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 will also warn 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
CPP will report 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
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in
code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs.
This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be
rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding
bugs in your own code, therefore suppressed. If you are responsible for the system
library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left
out by default, since they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors.
This includes mandatory diagnostics that GCC issues without -pedantic but treats as
warnings.
-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 will still be 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 will appear in -MM
dependency output. This is a slight change in semantics from GCC versions 3.0 and
earlier.
-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 have sent
preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency
output file.
-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 will set 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.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to
also list the files from the precompiled header's dependencies. If not specified only
the precompiled header would be 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.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do
with standards conformance or extensions; it merely selects which base syntax to
expect. If you give none of these options, cpp will deduce the language from the
extension of the source file: .c, .cc, .m, or .S. Some other common extensions for
C++ and assembly are also recognized. If cpp does not recognize the extension, it
will treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the
language and the standards conformance level. This option has been removed, because
it conflicts with the -l option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C
and C++ standards; others may be added in the future.
standard may be one of:
"c90"
"c89"
"iso9899:1990"
The ISO C standard from 1990. c90 is the customary shorthand for this version of
the standard.
The -ansi option is equivalent to -std=c90.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this
was known as C9X.
"iso9899:2011"
"c11"
"c1x"
The revised ISO C standard, published in December 2011. Before publication, this
was known as C1X.
"gnu90"
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"gnu11"
"gnu1x"
The 2011 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. 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"". This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the directories
you have specified with -I 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.)
-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.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the
standard system directories have been exhausted. dir is treated as a system include
directory. If dir begins with "=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
-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.
-isystem dir
Search dir for header files, after all directories specified by -I but before the
standard system directories. Mark it as a system directory, so that it gets the same
special treatment as is applied to the standard system directories. If dir begins
with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and
-isysroot.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not
searched for "#include <file>", before all directories specified by -I and before the
standard system directories. If dir begins with "=", then the "=" will be replaced by
the sysroot prefix; see --sysroot and -isysroot.
-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 experimental; in a
future version of GCC, it will be enabled by default for C99 and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-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.
-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.
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E, dumps debugging
information about location maps. Every token in the output is preceded by the dump of
the map its location belongs to. The dump of the map holding the location of a token
would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
-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 just as if no -ftrack-macro-expansion was present on
the command line. 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.
-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.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler
know the current working directory at the time of preprocessing. When this option is
enabled, the preprocessor will emit, after the initial linemarker, a second linemarker
with the current working directory followed by two slashes. GCC will use 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.
-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.
-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.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be
preceded by a space. Other characters are interpreted by the compiler proper, or
reserved for future versions of GCC, and so are silently ignored. If you specify
characters whose behavior conflicts, the result is undefined.
M 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
will show all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for
-fdump-rtl-mach.
D Like M 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.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
U Like D 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.
-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.
-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.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C
preprocessors.
-trigraphs
Process trigraph sequences. 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. By default, GCC ignores
trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi
options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit very short file
names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and
report the final form of the include path.
-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 ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal.
With two dashes, exit immediately.
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.
-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.)
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.
The linker searches a standard list of directories for the library, which is actually
a file named liblibrary.a. The linker then uses this file as if it had been specified
precisely by name.
The directories searched include several standard system directories plus any that you
specify with -L.
Normally the files found this way are library files---archive files whose members are
object files. The linker handles an archive file by scanning through it for members
which define symbols that have so far been referenced but not defined. But if the
file that is found is an ordinary object file, it is linked in the usual fashion. The
only difference between using an -l option and specifying a file name is that -l
surrounds library with lib and .a and searches several directories.
-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 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.
-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.)
-pie
Produce a 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.
-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 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++ and GCJ drivers automatically add -shared-libgcc whenever you build
a shared library or a main executable, because C++ and Java 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++ or GCJ driver, as appropriate for the languages used in the
program, 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-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.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts
of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header
files. This can be used to override a system header file, substituting your own
version, since these directories are searched before the 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). If you use more than one
-I option, the directories are scanned in left-to-right order; the standard system
directories come after.
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.
-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.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header
files only for the case of #include "file"; they are not searched for #include <file>,
otherwise just like -I.
-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 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 the -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 includes 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.
-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.
--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.
-I- This option has been deprecated. Please use -iquote instead for -I directories before
the -I- and remove the -I-. Any directories you specify with -I options before the
-I- option are searched only for the case of #include "file"; they are not searched
for #include <file>.
If additional directories are specified with -I options after the -I-, these
directories are searched for all #include directives. (Ordinarily all -I directories
are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the
current input file came from) as the first search directory for #include "file".
There is no way to override this effect of -I-. With -I. you can specify searching
the directory that is current when the compiler is invoked. That is not exactly the
same as what the preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files.
Thus, -I- and -nostdinc are independent.
Specifying Target Machine and Compiler Version
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 version other than the one that was installed
last.
Hardware Models and Configurations
Each target machine types can have its own special options, starting with -m, to choose
among various hardware models or configurations---for example, 68010 vs 68020, floating
coprocessor or none. A single installed version of the compiler can compile for any model
or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for
compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-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 registers.
-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 1GB of each other. Pointers are 64 bits. Programs can be statically
or dynamically linked. This model is not fully implemented and mostly treated as
small.
-mcmodel=small
Generate code for the small code model. The program and its statically defined
symbols must be within 4GB of each other. Pointers are 64 bits. 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. Pointers are 64 bits. Programs can be statically linked only.
-mstrict-align
Do not assume that unaligned memory references will be handled by the system.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behaviour is the
default.
-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.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This
will involve inserting a NOP instruction between memory instructions and 64-bit
integer multiply-accumulate instructions.
-march=name
Specify the name of the target architecture, optionally suffixed by one or more
feature modifiers. This option has the form -march=arch{+[no]feature}*, where the
only value for arch is armv8-a. The possible values for feature are documented in the
sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is used.
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.
-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 possible
values for cpu are generic, large. The possible values for feature are documented in
the sub-section below.
Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses this name to determine what kind of instructions it can emit when generating
assembly code.
-mtune=name
Specify the name of the processor to tune the performance for. The code will be tuned
as if the target processor were of the type specified in this option, but still using
instructions compatible with the target processor specified by a -mcpu= option. This
option cannot be suffixed by feature modifiers.
-march and -mcpu feature modifiers
Feature modifiers used with -march and -mcpu can be one the following:
crc Enable CRC extension.
crypto
Enable Crypto extension. This implies Advanced SIMD is enabled.
fp Enable floating-point instructions.
simd
Enable Advanced SIMD instructions. This implies floating-point instructions are
enabled. This is the default for all current possible values for options -march and
-mcpu=.
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
Preferrentially 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
-mnosplit-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.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-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.
-mapcs
This is a synonym for -mapcs-frame.
-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.
-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.
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate
code for a little-endian word order but a big-endian byte order. That is, a byte
order of the form 32107654. Note: this option should only be used if you require
compatibility with code for big-endian ARM processors generated by versions of the
compiler prior to 2.8. This option is now deprecated.
-march=name
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: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5e,
armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a, armv7-r,
armv7-m, armv7e-m armv8-a, armv8-a+crc, iwmmxt, iwmmxt2, ep9312.
-march=armv8-a+crc enables code generation for the ARMv8-A architecture together with
the optional CRC32 extensions.
-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: arm2, arm250, arm3, arm6,
arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70,
arm700, arm700i, arm710, arm710c, arm7100, arm720, arm7500, arm7500fe, arm7tdmi,
arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100,
strongarm1110, 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, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a15, cortex-a53, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-m4,
cortex-m3, cortex-m1, cortex-m0, cortex-m0plus, marvell-pj4, xscale, iwmmxt, iwmmxt2,
ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te.
-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=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
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.
Permissible names for this option are the same as those for -mtune.
-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: vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fp-armv8, neon-fp-armv8, and crypto-neon-fp-armv8.
If -msoft-float is specified this specifies the format of floating-point values.
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.
-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.
-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 will be 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.
-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.
-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.
-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 and all ARMv6-M architectures, and enabled for all other architectures. If
unaligned access is not enabled then words in packed data structures will be accessed
a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" will be 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" will also be
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.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8. IT blocks can
only contain a single 16-bit instruction from a select set of instructions. This
option is on by default for ARMv8 Thumb mode.
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@tie{}"avr2".
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "attiny22",
"attiny26", "at90c8534", "at90s2313", "at90s2323", "at90s2333", "at90s2343",
"at90s4414", "at90s4433", "at90s4434", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program memory and with the "MOVW"
instruction. mcu@tie{}= "ata5272", "ata6289", "attiny13", "attiny13a",
"attiny2313", "attiny2313a", "attiny24", "attiny24a", "attiny25", "attiny261",
"attiny261a", "attiny43u", "attiny4313", "attiny44", "attiny44a", "attiny45",
"attiny461", "attiny461a", "attiny48", "attiny84", "attiny84a", "attiny85",
"attiny861", "attiny861a", "attiny87", "attiny88", "at86rf401".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory.
mcu@tie{}= "at43usb355", "at76c711".
"avr31"
"Classic" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega103",
"at43usb320".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory and with
the "MOVW" instruction. mcu@tie{}= "ata5505", "atmega16u2", "atmega32u2",
"atmega8u2", "attiny1634", "attiny167", "at90usb162", "at90usb82".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "ata6285",
"ata6286", "atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega8",
"atmega8a", "atmega8hva", "atmega8515", "atmega8535", "atmega88", "atmega88a",
"atmega88p", "atmega88pa", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3",
"at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory.
mcu@tie{}= "ata5790", "ata5790n", "ata5795", "atmega16", "atmega16a",
"atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1",
"atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p",
"atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa",
"atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega169", "atmega169a",
"atmega169p", "atmega169pa", "atmega26hvg", "atmega32", "atmega32a", "atmega32c1",
"atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6",
"atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa",
"atmega328", "atmega328p", "atmega329", "atmega329a", "atmega329p", "atmega329pa",
"atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406",
"atmega48hvf", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64m1",
"atmega64rfa2", "atmega64rfr2", "atmega640", "atmega644", "atmega644a",
"atmega644p", "atmega644pa", "atmega645", "atmega645a", "atmega645p",
"atmega6450", "atmega6450a", "atmega6450p", "atmega649", "atmega649a",
"atmega649p", "atmega6490", "atmega6490a", "atmega6490p", "at90can32",
"at90can64", "at90pwm161", "at90pwm216", "at90pwm316", "at90scr100", "at90usb646",
"at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega128",
"atmega128a", "atmega128rfa1", "atmega1280", "atmega1281", "atmega1284",
"atmega1284p", "at90can128", "at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128@tie{}KiB of program
memory. mcu@tie{}= "atmega2560", "atmega2561".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to 64@tie{}KiB of program memory.
mcu@tie{}= "atmxt112sl", "atmxt224", "atmxt224e", "atmxt336s", "atxmega16a4",
"atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16x1", "atxmega32a4",
"atxmega32a4u", "atxmega32c4", "atxmega32d4", "atxmega32e5", "atxmega32x1".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program
memory. mcu@tie{}= "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1",
"atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program
memory and more than 64@tie{}KiB of RAM. mcu@tie{}= "atxmega64a1",
"atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of program memory. mcu@tie{}=
"atmxt540s", "atmxt540sreva", "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@tie{}KiB of program memory and more than
64@tie{}KiB of RAM. mcu@tie{}= "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only.
mcu@tie{}= "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
-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 to 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.
-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.
-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 "--relax" option to the
linker command line when the linker is called.
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.
-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
will add or remove 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@tie{}bits of the stack pointer.
-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.
"EIND" and Devices with more than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}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@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}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" implicitely 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 will generate the stubs
correctly an all situaltion. See the compiler option "-mrelax" and the linler 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@tie{}KiB range that can be accessed
with 16-bit pointers. To access memory locations outside this 64@tie{}KiB range, the
contentent 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@tie{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@tie{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, 102, 104, 105, 106, 107
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
"avrxmega2", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu
specifies a device, this built-in macro is set accordingly. For example, with
"-mmcu=atmega8" the macro will be 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 will
not be defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the 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
at least 16@tie{}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@tie{}KiB of program memory. This also means that the program
counter (PC) is 3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is the case for devices with up to
128@tie{}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_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@tie{}address.
"__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. The option -fomit-frame-pointer removes the frame
pointer for all functions, which might make debugging harder.
-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.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire
program fits into the low 64k of memory.
-mno-low-64k
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.
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.
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* and
*-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* 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.
-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.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by modifying
the return pointer for the function call to be the target of the conditional jump.
-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.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 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.
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.
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.
corei7
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and
SSE4.2 instruction set support.
corei7-avx
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, AVX, AES and PCLMUL instruction set support.
core-avx-i
Intel Core CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
core-avx2
Intel Core CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C
instruction set support.
atom
Intel Atom CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3
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, 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, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM 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.)
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 an
extra choice 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.
-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 i386 compiler.
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 i386 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.
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. Supported choices are intel or
att (the default). 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.
-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 can't 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 the
default on FreeBSD, OpenBSD and NetBSD. 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 386 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 i386 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
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 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.
-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@tie{}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@tie{}1) follow
the ABI used by other x86 targets, which is the default on Solaris@tie{}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 Intel 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 will lead 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 will be 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
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4.1
-mno-sse4.1
-msse4.2
-mno-sse4.2
-msse4
-mno-sse4
-mavx
-mno-avx
-mavx2
-mno-avx2
-maes
-mno-aes
-mpclmul
-mno-pclmul
-mfsgsbase
-mno-fsgsbase
-mrdrnd
-mno-rdrnd
-mf16c
-mno-f16c
-mfma
-mno-fma
-msse4a
-mno-sse4a
-mfma4
-mno-fma4
-mxop
-mno-xop
-mlwp
-mno-lwp
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
-mbmi
-mbmi2
-mno-bmi
-mno-bmi2
-mlzcnt
-mno-lzcnt
-mrtm
-mtbm
-mno-tbm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP,
LWP, ABM, BMI, BMI2, LZCNT, RTM or 3DNow! extended instruction sets. 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.
-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.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions. "CMPXCHG16B" allows
for atomic operations on 128-bit double quadword (or oword) data types. This is
useful for high-resolution counters that can be updated by multiple processors (or
cores). This instruction is generated as part of atomic built-in functions: see
__sync Builtins or __atomic Builtins for details.
-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 were 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".
-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 -finite-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", "vmldLog102",
"vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2",
"vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2",
"vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "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 a specific function by using
the function attribute ms_abi/sysv_abi.
-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.
-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.
-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.
-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.
These -m switches are supported in addition to the above on x86-64 processors in 64-bit
environments.
-m32
-m64
-mx32
Generate code for a 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.
-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.
i386 and x86-64 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 .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 i386 and x86-64 Options for standard options.
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 2 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 is 'disable'.
-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 is 'enable'.
-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 is
'disable'.
-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 is 'enable'.
-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 is 'enable'.
-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 is 'enable'.
-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 is 'disable'.
-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 is 'disable'.
-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 is 'disable'.
-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.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the default.
-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.
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 runtime 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, 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.
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, mips64 and mips64r2. 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, loongson2e, loongson2f, loongson3a, m4k, octeon, octeon+,
octeon2, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, 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.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-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-mips16
-mno-interlink-mips16
Require (do not require) that non-MIPS16 code be link-compatible with MIPS16 code.
For example, non-MIPS16 code cannot jump directly to MIPS16 code; it must either use a
call or an indirect jump. -minterlink-mips16 therefore disables direct jumps unless
GCC knows that the target of the jump is not MIPS16.
-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 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, but all 64 bits
are saved.
-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.
-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.
-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.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE 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.
-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.
-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-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.
-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
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.
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__.
-mnoliw
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__.
-mnosetlb
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.
-mno-crt0
Do not link in the C run-time initialization object file.
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.)
-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.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation
only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target
other than pdp11-*-bsd.
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
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g13
-mmul=rl78
Specifies the type of hardware multiplication support to be used. The default is
"none", which uses software multiplication functions. The "g13" option is for the
hardware multiply/divide peripheral only on the RL78/G13 targets. The "rl78" option
is for the standard hardware multiplication defined in the RL78 software manual.
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, powerpc, powerpc64, powerpc64le, and rs64.
-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.
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 -msingle-float -mdouble-float -msimple-fpu -mstring
-mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mdirect-move -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic
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.
-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, rather than -maltivec=le or -maltivec=be, the element order
for Altivec intrinsics such as "vec_splat", "vec_extract", and "vec_insert" will 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.
-maltivec=be
Generate Altivec instructions using big-endian element order, regardless of whether
the target is big- or little-endian. This is the default when targeting a big-endian
platform.
The element order is used to interpret element numbers in Altivec intrinsics such as
"vec_splat", "vec_extract", and "vec_insert". By default, these will match array
element order corresponding to the endianness for the target.
-maltivec=le
Generate Altivec instructions using little-endian element order, regardless of whether
the target is big- or little-endian. This is the default when targeting a little-
endian platform. This option is currently ignored when targeting a big-endian
platform.
The element order is used to interpret element numbers in Altivec intrinsics such as
"vec_splat", "vec_extract", and "vec_insert". By default, these will match array
element order corresponding to the endianness for the target.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-mgen-cell-microcode
Generate Cell microcode instructions.
-mwarn-cell-microcode
Warn when a Cell microcode instruction is emitted. An example of a Cell microcode
instruction is a variable shift.
-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.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd instructions.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-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.
-mdirect-move
-mno-direct-move
Generate code that uses (does not use) the instructions to move data between the
general purpose registers and the vector/scalar (VSX) registers 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.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point operations on the
general-purpose registers for architectures that support it.
The argument yes or single enables the use of single-precision floating-point
operations.
The argument double enables the use of single and double-precision floating-point
operations.
The argument no disables floating-point operations on the general-purpose registers.
This option is currently only available on the MPC854x.
-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.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations.
-mdouble-float implies -msingle-float.
-msimple-fpu
Do not generate "sqrt" and "div" instructions for hardware floating-point unit.
-mfpu=name
Specify type of floating-point unit. Valid values for name are sp_lite (equivalent to
-msingle-float -msimple-fpu), dp_lite (equivalent to -mdouble-float -msimple-fpu),
sp_full (equivalent to -msingle-float), and dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
-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.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store
string word instructions to save multiple registers and do small block moves. These
instructions are generated by default on POWER systems, and not generated on PowerPC
systems. Do not use -mstring 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.
-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, spe, no-spe, ibmlongdouble, ieeelongdouble, elfv1,
elfv2.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI,
instead it adds the SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Book-E SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is a PowerPC
32-bit SYSV ABI option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is a PowerPC
32-bit Linux ABI option.
-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.
-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.
-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.
-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 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.
-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.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for
both the preprocessor and linker.
-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,
-finite-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 the -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.
In this version of the compiler, the -mcompat-align-parm is the default, except when
using the Linux ELFv2 ABI.
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.
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 2 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.
-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 g5, g6, z900, z990, z9-109,
z9-ec, z10, z196, and zEC12. When generating code using the instructions available on
z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
-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.
-m5-32media
Generate 32-bit code for SHmedia.
-m5-32media-nofpu
Generate 32-bit code for SHmedia in such a way that the floating-point unit is not
used.
-m5-64media
Generate 64-bit code for SHmedia.
-m5-64media-nofpu
Generate 64-bit code for SHmedia in such a way that the floating-point unit is not
used.
-m5-compact
Generate code for SHcompact.
-m5-compact-nofpu
Generate code for SHcompact in such a way that the floating-point unit is not used.
-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
implcit 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 will also
partially utilize 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 will also be 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 though they
would be compatible, and will make 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. For SHmedia
strategy can be one of:
fp Performs the operation in floating point. This has a very high latency, but needs
only a few instructions, so it might be a good choice if your code has enough
easily-exploitable ILP to allow the compiler to schedule the floating-point
instructions together with other instructions. Division by zero causes a
floating-point exception.
inv Uses integer operations to calculate the inverse of the divisor, and then
multiplies the dividend with the inverse. This strategy allows CSE and hoisting
of the inverse calculation. Division by zero calculates an unspecified result,
but does not trap.
inv:minlat
A variant of inv where, if no CSE or hoisting opportunities have been found, or if
the entire operation has been hoisted to the same place, the last stages of the
inverse calculation are intertwined with the final multiply to reduce the overall
latency, at the expense of using a few more instructions, and thus offering fewer
scheduling opportunities with other code.
call
Calls a library function that usually implements the inv:minlat strategy. This
gives high code density for "m5-*media-nofpu" compilations.
call2
Uses a different entry point of the same library function, where it assumes that a
pointer to a lookup table has already been set up, which exposes the pointer load
to CSE and code hoisting optimizations.
inv:call
inv:call2
inv:fp
Use the inv algorithm for initial code generation, but if the code stays
unoptimized, revert to the call, call2, or fp strategies, respectively. Note that
the potentially-trapping side effect of division by zero is carried by a separate
instruction, so it is possible that all the integer instructions are hoisted out,
but the marker for the side effect stays where it is. A recombination to
floating-point operations or a call is not possible in that case.
inv20u
inv20l
Variants of the inv:minlat strategy. In the case that the inverse calculation is
not separated from the multiply, they speed up division where the dividend fits
into 20 bits (plus sign where applicable) by inserting a test to skip a number of
operations in this case; this test slows down the case of larger dividends.
inv20u assumes the case of a such a small dividend to be unlikely, and inv20l
assumes it to be likely.
For targets other than SHmedia 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 will default 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 will
default to "call-div1".
When a division strategy has not been specified the default strategy will be 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 and inv: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 can not 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.
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only
safe if the hardware and/or OS implement 32-bit wrap-around semantics for the indexed
addressing mode. The architecture allows the implementation of processors with 64-bit
MMU, which the OS could use to get 32-bit addressing, but since no current hardware
implementation supports this or any other way to make the indexed addressing mode safe
to use in the 32-bit ABI, the default is -mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the "gettr" instruction to number. The default is 2 if
-mpt-fixed is in effect, 100 otherwise.
-mpt-fixed
Assume "pt*" instructions won't trap. This generally generates better-scheduled code,
but is unsafe on current hardware. The current architecture definition says that
"ptabs" and "ptrel" trap when the target anded with 3 is 3. This has the
unintentional effect of making it unsafe to schedule these instructions before a
branch, or hoist them out of a loop. For example, "__do_global_ctors", a part of
libgcc that runs constructors at program startup, calls functions in a list which is
delimited by -1. With the -mpt-fixed option, the "ptabs" is done before testing
against -1. That means that all the constructors run a bit more quickly, but when the
loop comes to the end of the list, the program crashes because "ptabs" loads -1 into a
target register.
Since this option is unsafe for any hardware implementing the current architecture
specification, the default is -mno-pt-fixed. Unless specified explicitly with
-mgettrcost, -mno-pt-fixed also implies -mgettrcost=100; this deters register
allocation from using target registers for storing ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler
are always valid to load with "movi"/"shori"/"ptabs" or "movi"/"shori"/"ptrel", but
with assembler and/or linker tricks it is possible to generate symbols that cause
"ptabs" or "ptrel" to trap. This option is only meaningful when -mno-pt-fixed is in
effect. It prevents cross-basic-block CSE, hoisting and most scheduling of symbol
loads. The default is -mno-invalid-symbols.
-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 will try to prefer 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.
-mcbranchdi
Enable the "cbranchdi4" instruction pattern.
-mcmpeqdi
Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is in effect.
-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.
Solaris 2 Options
These -m options are supported on Solaris 2:
-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
Add support for multithreading using the POSIX threads library. This option sets
flags for both the preprocessor and linker. This option does not affect the thread
safety of object code produced by the compiler or that of libraries supplied with it.
-pthread
This is a synonym for -pthreads.
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. The default is
-mno-user-mode.
-mno-faster-structs
-mfaster-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.
-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 and niagara4.
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
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.
-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 and niagara4. 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.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of compare-and-branch
instructions, as defined in the Sparc Architecture 2011. The default is -mcbcond when
targeting a cpu that supports such instructions, such as niagara-4 and later.
-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
instructions, such as Niagara-2 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-
Add Floating-point extensions. The default is -mfmaf when targeting a cpu that
supports such instructions, such as Niagara-3 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.
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.
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__" will be 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__" will be defined, otherwise the symbol "__NO_FPU__" will be
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 "doubles" 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__" will be 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.
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.
x86-64 Options
These are listed under
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
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.
-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
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.
-fbounds-check
For front ends that support it, generate additional code to check that indices used to
access arrays are within the declared range. This is currently only supported by the
Java and Fortran front ends, where this option defaults to true and false
respectively.
-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 will 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.
-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. This
option is enabled by default for the Java front end, as required by the Java language
specification.
-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 2 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-double
Use the same size for "double" as for "float".
Warning: the -fshort-double 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, controls the placement of uninitialized global variables. Unix C compilers
have traditionally permitted multiple definitions of such variables in different
compilation units by placing the variables in a common block. 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 place uninitialized global variables in the data section of the object
file, rather than generating them as common blocks. This has the effect that if the
same variable is declared (without "extern") in two different compilations, you get a
multiple-definition error when you link them. In this case, you must compile with
-fcommon instead. 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 declarations 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.
-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 and 32k
on the m68k and RS/6000. The 386 has no such limit.)
Position-independent code requires special support, and therefore works only on
certain machines. For the 386, 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 the 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 generated position independent code
can be only linked into executables. Usually these options are used when -pie GCC
option is used during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the
value 1 for -fpie and 2 for -fPIE.
-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.
-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.
-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.
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-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.
-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 i386 and x86_64 back ends 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.
-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".
The default without -fpic is "initial-exec"; with -fpic the default is
"global-dynamic".
-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---this
causes the same behavior as previous versions of GCC.
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 <http://people.redhat.com/~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.
If the target requires strict alignment, and honoring the field type would require
violating this alignment, a warning is issued. If the field has "packed" attribute,
the access is done without honoring the field type. If the field doesn't have
"packed" attribute, the access is done honoring the field type. In both cases, GCC
assumes that the user knows something about the target hardware that it is unaware of.
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.
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 can't
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 can't 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.
BUGS
For instructions on reporting bugs, see <file:///usr/share/doc/gcc-4.8/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), adb(1), dbx(1),
sdb(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) 1988-2015 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.