Provided by: verilator_4.028-1_amd64 
NAME
Verilator - Translate and simulate SystemVerilog code using C++/SystemC
SYNOPSIS
verilator --help
verilator --version
verilator --cc [options] [source_files.v]... [opt_c_files.cpp/c/cc/a/o/so]
verilator --sc [options] [source_files.v]... [opt_c_files.cpp/c/cc/a/o/so]
verilator --lint-only -Wall [source_files.v]...
DESCRIPTION
Verilator converts synthesizable (generally not behavioral) Verilog code, plus some
Synthesis, SystemVerilog and a small subset of Verilog AMS into C++ or SystemC code. It
is not a traditional simulator, but a compiler.
Verilator is invoked with parameters similar to GCC, Cadence Verilog-XL/NC-Verilog, or
Synopsys's VCS. It reads the specified Verilog code, lints it, and optionally adds
coverage and waveform tracing code. For C++ and SystemC formats, it outputs .cpp and .h
files.
The files created by Verilator are then compiled with C++. The user writes a little C++
wrapper file, which instantiates the top level module, and passes this filename on the
command line. These C files are compiled in C++, and linked with the Verilated files.
The resulting executable will perform the actual simulation.
To get started, jump down to "EXAMPLE C++ EXECUTION".
ARGUMENT SUMMARY
This is a short summary of the arguments to Verilator itself. See the detailed
descriptions in "VERILATION ARGUMENTS" for more information.
{file.v} Verilog package, module and top module filenames
{file.c/cc/cpp} Optional C++ files to compile in
{file.a/o/so} Optional C++ files to link in
+1364-1995ext+<ext> Use Verilog 1995 with file extension <ext>
+1364-2001ext+<ext> Use Verilog 2001 with file extension <ext>
+1364-2005ext+<ext> Use Verilog 2005 with file extension <ext>
+1800-2005ext+<ext> Use SystemVerilog 2005 with file extension <ext>
+1800-2009ext+<ext> Use SystemVerilog 2009 with file extension <ext>
+1800-2012ext+<ext> Use SystemVerilog 2012 with file extension <ext>
+1800-2017ext+<ext> Use SystemVerilog 2017 with file extension <ext>
--assert Enable all assertions
--autoflush Flush streams after all $displays
--bbox-sys Blackbox unknown $system calls
--bbox-unsup Blackbox unsupported language features
--bin <filename> Override Verilator binary
-CFLAGS <flags> C++ Compiler flags for makefile
--cc Create C++ output
--cdc Clock domain crossing analysis
--clk <signal-name> Mark specified signal as clock
--make <make-system> Generate scripts for specified make system
--compiler <compiler-name> Tune for specified C++ compiler
--converge-limit <loops> Tune convergence settle time
--coverage Enable all coverage
--coverage-line Enable line coverage
--coverage-toggle Enable toggle coverage
--coverage-user Enable SVL user coverage
--coverage-underscore Enable coverage of _signals
-D<var>[=<value>] Set preprocessor define
--debug Enable debugging
--debug-check Enable debugging assertions
--no-debug-leak Disable leaking memory in --debug mode
--debugi <level> Enable debugging at a specified level
--debugi-<srcfile> <level> Enable debugging a source file at a level
--default-language <lang> Default language to parse
+define+<var>=<value> Set preprocessor define
--dpi-hdr-only Only produce the DPI header file
--dump-defines Show preprocessor defines with -E
--dump-tree Enable dumping .tree files
--dump-treei <level> Enable dumping .tree files at a level
--dump-treei-<srcfile> <level> Enable dumping .tree file at a source file at a level
-E Preprocess, but do not compile
--error-limit <value> Abort after this number of errors
--exe Link to create executable
-F <file> Parse options from a file, relatively
-f <file> Parse options from a file
-FI <file> Force include of a file
-G<name>=<value> Overwrite toplevel parameter
--gdb Run Verilator under GDB interactively
--gdbbt Run Verilator under GDB for backtrace
--generate-key Create random key for --protect-key
--getenv <var> Get environment variable with defaults
--help Display this help
-I<dir> Directory to search for includes
--gate-stmts <value> Tune gate optimizer depth
--if-depth <value> Tune IFDEPTH warning
+incdir+<dir> Directory to search for includes
--inhibit-sim Create function to turn off sim
--inline-mult <value> Tune module inlining
-LDFLAGS <flags> Linker pre-object flags for makefile
--l2-name <value> Verilog scope name of the top module
--language <lang> Default language standard to parse
+libext+<ext>+[ext]... Extensions for finding modules
--lint-only Lint, but do not make output
--max-num-width <value> Maximum number width (default: 64K)
--MMD Create .d dependency files
--MP Create phony dependency targets
--Mdir <directory> Name of output object directory
--mod-prefix <topname> Name to prepend to lower classes
--no-clk <signal-name> Prevent marking specified signal as clock
--no-decoration Disable comments and symbol decorations
--no-pins64 Don't use vluint64_t's for 33-64 bit sigs
--no-skip-identical Disable skipping identical output
+notimingchecks Ignored
-O0 Disable optimizations
-O3 High performance optimizations
-O<optimization-letter> Selectable optimizations
-o <executable> Name of final executable
--no-order-clock-delay Disable ordering clock enable assignments
--output-split <statements> Split .cpp files into pieces
--output-split-cfuncs <statements> Split .cpp functions
--output-split-ctrace <statements> Split tracing functions
-P Disable line numbers and blanks with -E
--pins-bv <bits> Specify types for top level ports
--pins-sc-uint Specify types for top level ports
--pins-sc-biguint Specify types for top level ports
--pins-uint8 Specify types for top level ports
--pipe-filter <command> Filter all input through a script
--pp-comments Show preprocessor comments with -E
--prefix <topname> Name of top level class
--prof-cfuncs Name functions for profiling
--prof-threads Enable generating gantt chart data for threads
--protect-key <key> Key for symbol protection
--protect-ids Hash identifier names for obscurity
--protect-lib <name> Create a DPI protected library
--private Debugging; see docs
--public Debugging; see docs
--public-flat-rw Mark all variables, etc as public_flat_rw
-pvalue+<name>=<value> Overwrite toplevel parameter
--quiet-exit Don't print the command on failure
--relative-includes Resolve includes relative to current file
--no-relative-cfuncs Disallow 'this->' in generated functions
--report-unoptflat Extra diagnostics for UNOPTFLAT
--rr Run Verilator and record with rr
--savable Enable model save-restore
--sc Create SystemC output
--stats Create statistics file
--stats-vars Provide statistics on variables
-sv Enable SystemVerilog parsing
+systemverilogext+<ext> Synonym for +1800-2017ext+<ext>
--threads <threads> Enable multithreading
--threads-dpi <mode> Enable multithreaded DPI
--threads-max-mtasks <mtasks> Tune maximum mtask partitioning
--top-module <topname> Name of top level input module
--trace Enable waveform creation
--trace-depth <levels> Depth of tracing
--trace-coverage Enable tracing of coverage
--trace-fst Enable FST waveform creation
--trace-fst-thread Enable FST threaded waveform creation
--trace-max-array <depth> Maximum bit width for tracing
--trace-max-width <width> Maximum array depth for tracing
--trace-params Enable tracing of parameters
--trace-structs Enable tracing structure names
--trace-underscore Enable tracing of _signals
-U<var> Undefine preprocessor define
--unroll-count <loops> Tune maximum loop iterations
--unroll-stmts <stmts> Tune maximum loop body size
--unused-regexp <regexp> Tune UNUSED lint signals
-V Verbose version and config
-v <filename> Verilog library
+verilog1995ext+<ext> Synonym for +1364-1995ext+<ext>
+verilog2001ext+<ext> Synonym for +1364-2001ext+<ext>
--version Displays program version and exits
--vpi Enable VPI compiles
-Wall Enable all style warnings
-Werror-<message> Convert warnings to errors
-Wfuture-<message> Disable unknown message warnings
-Wno-<message> Disable warning
-Wno-context Disable source context on warnings
-Wno-fatal Disable fatal exit on warnings
-Wno-lint Disable all lint warnings
-Wno-style Disable all style warnings
-Wpedantic Warn on compliance-test issues
--x-assign <mode> Assign non-initial Xs to this value
--x-initial <mode> Assign initial Xs to this value
--x-initial-edge Enable initial X->0 and X->1 edge triggers
--xml-only Create XML parser output
--xml-output XML output filename
-y <dir> Directory to search for modules
This is a short summary of the arguments to simulation runtime Verilated arguments.
detailed descriptions in "SIMULATION RUNTIME ARGUMENTS" for more information.
+verilator+debug Enable debugging
+verilator+debugi+<value> Enable debugging at a level
+verilator+help Display help
+verilator+prof+threads+file+I<filename> Set profile filename
+verilator+prof+threads+start+I<value> Set profile starting point
+verilator+prof+threads+window+I<value> Set profile duration
+verilator+rand+reset+I<value> Set random reset technique
+verilator+seed+I<value> Set random seed
+verilator+V Verbose version and config
+verilator+version Show version and exit
VERILATION ARGUMENTS
The following are the arguments that may be passed to Verilator itself.
{file.v}
Specifies the Verilog file containing the top module to be Verilated.
{file.c/.cc/.cpp/.cxx}
Specifies optional C++ files to be linked in with the Verilog code. The file path
should either be absolute, or relative to where the make will be executed from, or add
to your makefile's VPATH the appropriate directory to find the file.
If any C++ files are specified in this way, Verilator will include a make rule that
generates a module executable. Without any C++ files, Verilator will stop at the
module__ALL.a library, and presume you'll continue linking with make rules you write
yourself. See also the -CFLAGS option.
{file.a/.o/.so}
Specifies optional object or library files to be linked in with the Verilog code, as a
shorthand for -LDFLAGS "<file>". The file path should either be absolute, or relative
to where the make will be executed from, or add to your makefile's VPATH the
appropriate directory to find the file.
If any files are specified in this way, Verilator will include a make rule that uses
these files when linking the module executable. This generally is only useful when
used with the --exe option.
+1364-1995ext+ext
+1364-2001ext+ext
+1364-2005ext+ext
+1800-2005ext+ext
+1800-2009ext+ext
+1800-2012ext+ext
+1800-2017ext+ext
Specifies the language standard to be used with a specific filename extension, ext.
For compatibility with other simulators, see also the synonyms "+verilog1995ext+"ext,
"+verilog2001ext+"ext, and "+systemverilogext+"ext.
For any source file, the language specified by these options takes precedence over any
language specified by the "--default-language" or "--language" options.
These options take effect in the order they are encountered. Thus the following would
use Verilog 1995 for "a.v" and Verilog 2001 for "b.v".
verilator ... +1364-1995ext+v a.v +1364-2001ext+v b.v
These flags are only recommended for legacy mixed language designs, as the preferable
option is to edit the code to repair new keywords, or add appropriate
"`begin_keywords".
Note "`begin_keywords" is a SystemVerilog construct, which specifies only which the
set of keywords is to be recognized. Whatever set is chosen, the semantics will be
those of SystemVerilog. By contrast "+1364-1995ext+" etc. specify both the syntax and
semantics to be used.
--assert
Enable all assertions.
--autoflush
After every $display or $fdisplay, flush the output stream. This ensures that
messages will appear immediately but may reduce performance; for best performance call
"fflush(stdout)" occasionally in the main C loop. Defaults off, which will buffer
output as provided by the normal C stdio calls.
--bbox-sys
Black box any unknown $system task or function calls. System tasks will be simply
become no-operations, and system functions will be replaced by unsized zero.
Arguments to such functions will be parsed, but not otherwise checked. This prevents
errors when linting in the presence of company specific PLI calls.
--bbox-unsup
Black box some unsupported language features, currently UDP tables, the cmos and tran
gate primitives, deassign statements, and mixed edge errors. This may enable linting
the rest of the design even when unsupported constructs are present.
--bin filename
Rarely needed. Override the default filename for Verilator itself. When a dependency
(.d) file is created, this filename will become a source dependency, such that a
change in this binary will have make rebuild the output files.
-CFLAGS flags
Add specified C compiler flag to the generated makefiles. For multiple flags either
pass them as a single argument with space separators quoted in the shell ("-CFLAGS "-a
-b""), or use multiple -CFLAGS arguments ("-CFLAGS -a -CFLAGS -b").
When make is run on the generated makefile these will be passed to the C++ compiler
(gcc/g++/msvc++).
--cc
Specifies C++ without SystemC output mode; see also --sc.
--cdc
Experimental. Perform some clock domain crossing checks and issue related warnings
(CDCRSTLOGIC) and then exit; if warnings other than CDC warnings are needed make a
second run with --lint-only. Additional warning information is also written to the
file {prefix}__cdc.txt.
Currently only checks some items that other CDC tools missed; if you have interest in
adding more traditional CDC checks, please contact the authors.
--clk signal-name
Sometimes it is quite difficult for Verilator to distinguish clock signals from other
data signals. Occasionally the clock signals can end up in the checking list of
signals which determines if further evaluation is needed. This will heavily degrade
the performance of a Verilated model.
With --clk <signal-name>, user can specified root clock into the model, then Verilator
will mark the signal as clocker and propagate the clocker attribute automatically to
other signals derived from that. In this way, Verilator will try to avoid taking the
clocker signal into checking list.
Note signal-name is specified by the RTL hierarchy path. For example, v.foo.bar. If
the signal is the input to top-module, the directly the signal name. If you find it
difficult to find the exact name, try to use "/*verilator clocker*/" in RTL file to
mark the signal directly.
If clock signals are assigned to vectors and then later used individually, Verilator
will attempt to decompose the vector and connect the single-bit clock signals
directly. This should be transparent to the user.
--make make-system
Generates a script for the specified make system.
Supported make systems are gmake and cmake. Both can be specified. If no make system
is specified, gmake is assumed.
--compiler compiler-name
Enables tunings and workarounds for the specified C++ compiler.
clang
Tune for clang. This may reduce execution speed as it enables several workarounds
to avoid silly hardcoded limits in clang. This includes breaking deep structures
as for msvc as described below.
gcc Tune for GNU C++, although generated code should work on almost any compliant C++
compiler. Currently the default.
msvc
Tune for Microsoft Visual C++. This may reduce execution speed as it enables
several workarounds to avoid silly hardcoded limits in MSVC++. This includes
breaking deeply nested parenthesized expressions into sub-expressions to avoid
error C1009, and breaking deep blocks into functions to avoid error C1061.
--converge-limit loops
Rarely needed. Specifies the maximum number of runtime iterations before creating a
model failed to converge error. Defaults to 100.
--coverage
Enables all forms of coverage, alias for "--coverage-line --coverage-toggle
--coverage-user".
--coverage-line
Specifies basic block line coverage analysis code should be inserted.
Coverage analysis adds statements at each code flow change point, which are the
branches of IF and CASE statements, a super-set of normal Verilog Line Coverage. At
each such branch a unique counter is incremented. At the end of a test, the counters
along with the filename and line number corresponding to each counter are written into
logs/coverage.dat.
Verilator automatically disables coverage of branches that have a $stop in them, as it
is assumed $stop branches contain an error check that should not occur. A /*verilator
coverage_block_off*/ comment will perform a similar function on any code in that block
or below, or /*verilator coverage_on/coverage_off*/ will disable coverage around lines
of code.
Note Verilator may over-count combinatorial (non-clocked) blocks when those blocks
receive signals which have had the UNOPTFLAT warning disabled; for most accurate
results do not disable this warning when using coverage.
--coverage-toggle
Specifies signal toggle coverage analysis code should be inserted.
Every bit of every signal in a module has a counter inserted. The counter will
increment on every edge change of the corresponding bit.
Signals that are part of tasks or begin/end blocks are considered local variables and
are not covered. Signals that begin with underscores, are integers, or are very wide
(>256 bits total storage across all dimensions) are also not covered.
Hierarchy is compressed, such that if a module is instantiated multiple times,
coverage will be summed for that bit across ALL instantiations of that module with the
same parameter set. A module instantiated with different parameter values is
considered a different module, and will get counted separately.
Verilator makes a minimally-intelligent decision about what clock domain the signal
goes to, and only looks for edges in that clock domain. This means that edges may be
ignored if it is known that the edge could never be seen by the receiving logic. This
algorithm may improve in the future. The net result is coverage may be lower than
what would be seen by looking at traces, but the coverage is a more accurate
representation of the quality of stimulus into the design.
There may be edges counted near time zero while the model stabilizes. It's a good
practice to zero all coverage just before releasing reset to prevent counting such
behavior.
A /*verilator coverage_off/on */ comment pair can be used around signals that do not
need toggle analysis, such as RAMs and register files.
--coverage-underscore
Enable coverage of signals that start with an underscore. Normally, these signals are
not covered. See also --trace-underscore.
--coverage-user
Enables user inserted functional coverage. Currently, all functional coverage points
are specified using SVA which must be separately enabled with --assert.
For example, the following statement will add a coverage point, with the comment
"DefaultClock":
DefaultClock: cover property (@(posedge clk) cyc==3);
-Dvar=value
Defines the given preprocessor symbol, without allowing. Similar to +define; +define
is fairly standard across Verilog tools while -D is an alias for GCC compatibility.
--debug
Select the debug built image of Verilator (if available), and enable more internal
assertions (equivalent to "--debug-check"), debugging messages (equivalent to
"--debugi 4"), and intermediate form dump files (equivalent to "--dump-treei 3").
--debug-check
Rarely needed. Enable internal debugging assertion checks, without changing debug
verbosity. Enabled automatically when --debug specified.
--no-debug-leak
In --debug mode, by default Verilator intentionally leaks AstNode's instead of freeing
them, so that each node pointer is unique in the resulting tree files and dot files.
This option disables the leak. This may avoid out-of-memory errors when Verilating
large models in --debug mode.
Outside of --debug mode, AstNode's should never be leaked and this option has no
effect.
--debugi level
--debugi-srcfile level
Rarely needed - for developer use. Set internal debugging level globally to the
specified debug level (1-10) or set the specified Verilator source file to the
specified level (e.g. "--debugi-V3Width 9"). Higher levels produce more detailed
messages.
--default-language value
Select the language to be used by default when first processing each Verilog file.
The language value must be "1364-1995", "1364-2001", "1364-2005", "1800-2005",
"1800-2009", "1800-2012" or "1800-2017".
Any language associated with a particular file extension (see the various +langext+
options) will be used in preference to the language specified by --default-language.
The --default-language flag is only recommended for legacy code using the same
language in all source files, as the preferable option is to edit the code to repair
new keywords, or add appropriate "`begin_keywords". For legacy mixed language designs,
the various +langext+ options should be used.
If no language is specified, either by this flag or +langext+ options, then the latest
SystemVerilog language (IEEE 1800-2017) is used.
+define+var=value
+define+var=value+var2=value2...
Defines the given preprocessor symbol, or multiple symbols if separated by plusses.
Similar to -D; +define is fairly standard across Verilog tools while -D is an alias
for GCC compatibility.
--dpi-hdr-only
Only generate the DPI header file. This option has no effect on the name or location
of the file.
--dump-defines
With -E, suppress normal output, and instead print a list of all defines existing at
the end of pre-processing the input files. Similar to GCC "-dM" option. This also
gives you a way of finding out what is predefined in Verilator using the command:
touch foo.v ; verilator -E --dump-defines foo.v
--dump-tree
Rarely needed. Enable writing .tree debug files with dumping level 3, which dumps the
standard critical stages. For details on the format see the Verilator Internals
manual. --dump-tree is enabled automatically with --debug, so "--debug
--no-dump-tree" may be useful if the dump files are large and not desired.
--dump-treei level
--dump-treei-srcfile level
Rarely needed - for developer use. Set internal tree dumping level globally to a
specific dumping level or set the specified Verilator source file to the specified
tree dumping level (e.g. "--dump-treei-V3Order 9"). Level 0 disbles dumps and is
equivalent to "--no-dump-tree". Level 9 enables dumping of every stage.
-E Preprocess the source code, but do not compile, as with 'gcc -E'. Output is written
to standard out. Beware of enabling debugging messages, as they will also go to
standard out.
--error-limit value
After this number of errors are encountered during Verilator run, exit. Warnings are
not counted in this limit. Defaults to 50.
Does not affect simulation runtime errors, for those see +verilator+error+limit.
--exe
Generate an executable. You will also need to pass additional .cpp files on the
command line that implement the main loop for your simulation.
-F file
Read the specified file, and act as if all text inside it was specified as command
line parameters. Any relative paths are relative to the directory containing the
specified file. See also -f. Note -F is fairly standard across Verilog tools.
-f file
Read the specified file, and act as if all text inside it was specified as command
line parameters. Any relative paths are relative to the current directory. See also
-F. Note -f is fairly standard across Verilog tools.
The file may contain // comments which are ignored to the end of the line. Any $VAR,
$(VAR), or ${VAR} will be replaced with the specified environment variable.
-FI file
Force include of the specified C++ header file. All generated C++ files will insert a
#include of the specified file before any other includes. The specified file might be
used to contain define prototypes of custom VL_VPRINTF functions, and may need to
include verilatedos.h as this file is included before any other standard includes.
-Gname=value
Overwrites the given parameter of the toplevel module. The value is limited to basic
data literals:
Verilog integer literals
The standard verilog integer literals are supported, so values like 32'h8, 2'b00,
4 etc. are allowed. Care must be taken that the single quote (I') is properly
escaped in an interactive shell, e.g., as -GWIDTH=8\'hx.
C integer literals
It is also possible to use C integer notation, including hexadecimal (0x..), octal
(0..) or binary (0b..) notation.
Double literals
Double literals must contain a dot (.) and/or an exponent (e).
Strings
String must in double quotes (""). On the command line it is required to escape
them properly, e.g. as -GSTR="\"My String\"" or -GSTR='"My String"'.
--gate-stmts value
Rarely needed. Set the maximum number of statements that may be present in an
equation for the gate substitution optimization to inline that equation.
--gdb
Run Verilator underneath an interactive GDB (or VERILATOR_GDB environment variable
value) session. See also --gdbbt.
--gdbbt
If --debug is specified, run Verilator underneath a GDB process and print a backtrace
on exit, then exit GDB immediately. Without --debug or if GDB doesn't seem to work,
this flag is ignored. Intended for easy creation of backtraces by users; otherwise
see the --gdb flag.
--generate-key
Generate a true-random key suitable for use with --protect-key, print it, and exit
immediately.
--getenv variable
If the variable is declared in the environment, print it and exit immediately.
Otherwise, if it's built into Verilator (e.g. VERILATOR_ROOT), print that and exit
immediately. Otherwise, print a newline and exit immediately. This can be useful in
makefiles. See also -V, and the various *.mk files.
--help
Displays this message and program version and exits.
-Idir
See -y.
--if-depth value
Rarely needed. Set the depth at which the IFDEPTH warning will fire, defaults to 0
which disables this warning.
+incdir+dir
See -y.
--inhibit-sim
Rarely needed. Create a "inhibitSim(bool)" function to enable and disable evaluation.
This allows an upper level testbench to disable modules that are not important in a
given simulation, without needing to recompile or change the SystemC modules
instantiated.
--inline-mult value
Tune the inlining of modules. The default value of 2000 specifies that up to 2000 new
operations may be added to the model by inlining, if more than this number of
operations would result, the module is not inlined. Larger values, or a value < 1
will inline everything, will lead to longer compile times, but potentially faster
simulation runtimes. This setting is ignored for very small modules; they will always
be inlined, if allowed.
-LDFLAGS flags
Add specified C linker flags to the generated makefiles. For multiple flags either
pass them as a single argument with space separators quoted in the shell ("-LDFLAGS
"-a -b""), or use multiple -LDFLAGS arguments ("-LDFLAGS -a -LDFLAGS -b").
When make is run on the generated makefile these will be passed to the C++ linker (ld)
*after* the primary file being linked. This flag is called -LDFLAGS as that's the
traditional name in simulators; it's would have been better called LDLIBS as that's
the Makefile variable it controls. (In Make, LDFLAGS is before the first object,
LDLIBS after. -L libraries need to be in the Make variable LDLIBS, not LDFLAGS.)
--l2-name value
Instead of using the module name when showing Verilog scope, use the name provided.
This allows simplifying some Verilator-embedded modeling methodologies. Default is an
l2-name matching the top module. The default before 3.884 was "--l2-name v"
For example, the program "module t; initial $display("%m"); endmodule" will show by
default "t". With "--l2-name v" it will print "v".
--language value
A synonym for "--default-language", for compatibility with other tools and earlier
versions of Verilator.
+libext+ext+ext...
Specify the extensions that should be used for finding modules. If for example module
x is referenced, look in x.ext. Note +libext+ is fairly standard across Verilog
tools. Defaults to .v and .sv.
--lint-only
Check the files for lint violations only, do not create any other output.
You may also want the -Wall option to enable messages that are considered stylistic
and not enabled by default.
If the design is not to be completely Verilated see also the --bbox-sys and
--bbox-unsup options.
--max-num-width value
Set the maximum number literal width (e.g. in 1024'd22 this it the 1024). Defaults to
64K.
--MMD =item --no-MMD
Enable/disable creation of .d dependency files, used for make dependency detection,
similar to gcc -MMD option. By default this option is enabled for --cc or --sp modes.
--MP
When creating .d dependency files with --MMD, make phony targets. Similar to gcc -MP
option.
--Mdir directory
Specifies the name of the Make object directory. All generated files will be placed
in this directory. If not specified, "obj_dir" is used. The directory is created if
it does not exist and the parent directories exist; otherwise manually create the Mdir
before calling Verilator.
--mod-prefix topname
Specifies the name to prepend to all lower level classes. Defaults to the same as
--prefix.
--no-clk signal-name
Prevent the specified signal from being marked as clock. See "--clk".
--no-decoration
When creating output Verilated code, minimize comments, whitespace, symbol names and
other decorative items, at the cost of greatly reduced readability. This may assist
C++ compile times. This will not typically change the ultimate model's performance,
but may in some cases.
--no-pins64
Backward compatible alias for "--pins-bv 33".
--no-relative-cfuncs
Disable 'this->' references in generated functions, and instead Verilator will
generate absolute references starting from 'vlTOPp->'. This prevents V3Combine from
merging functions from multiple instances of the same module, so it can grow the
instruction stream.
This is a work around for old compilers. Don't set this if your C++ compiler supports
__restrict__ properly, as GCC 4.5.x and newer do. For older compilers, test if this
switch gives you better performance or not.
Compilers which don't honor __restrict__ will suspect that 'this->' references and
'vlTOPp->' references may alias, and may write slow code with extra loads and stores
to handle the (imaginary) aliasing. Using only 'vlTOPp->' references allows these old
compilers to produce tight code.
--no-skip-identical =item --skip-identical
Rarely needed. Disables or enables skipping execution of Verilator if all source
files are identical, and all output files exist with newer dates. By default this
option is enabled for --cc or --sp modes only.
+notimingchecks
Ignored for compatibility with other simulators.
-O0 Disables optimization of the model.
-O3 Enables slow optimizations for the code Verilator itself generates (as opposed to
"-CFLAGS -O3" which effects the C compiler's optimization. -O3 may reduce simulation
runtimes at the cost of compile time. This currently sets --inline-mult -1.
-Ooptimization-letter
Rarely needed. Enables or disables a specific optimizations, with the optimization
selected based on the letter passed. A lowercase letter disables an optimization, an
upper case letter enables it. This is intended for debugging use only; see the source
code for version-dependent mappings of optimizations to -O letters.
-o executable
Specify the name for the final executable built if using --exe. Defaults to the
--prefix if not specified.
--no-order-clock-delay
Rarely needed. Disables a bug fix for ordering of clock enables with delayed
assignments. This flag should only be used when suggested by the developers.
--output-split statements
Enables splitting the output .cpp files into multiple outputs. When a C++ file
exceeds the specified number of operations, a new file will be created at the next
function boundary. In addition, any infrequently executed "cold" routines will be
placed into __Slow files. This accelerates compilation by as optimization can be
disabled on the routines in __Slow, and the remaining files can be compiled on
parallel machines. Using --output-split should have only a trivial impact on
performance. On one design --output-split 20000 resulted in splitting into
approximately one-minute-compile chunks.
Typically when using this, make with VM_PARALLEL_BUILDS=1, and use ccache.
--output-split-cfuncs statements
Enables splitting functions in the output .cpp files into multiple functions. When a
generated function exceeds the specified number of operations, a new function will be
created. With --output-split, this will enable GCC to compile faster, at a small loss
in performance that gets worse with decreasing split values. Note that this option is
stronger than --output-split in the sense that --output-split will not split inside a
function.
--output-split-ctrace statements
Enables splitting trace functions in the output .cpp files into multiple functions.
Defaults to same setting as --output-split-cfuncs.
-P With -E, disable generation of `line markers and blank lines, similar to GCC -P flag.
--pins64
Backward compatible alias for "--pins-bv 65". Note that's a 65, not a 64.
--pins-bv width
Specifies SystemC inputs/outputs of greater than or equal to width bits wide should
use sc_bv's instead of uint32/vluint64_t's. The default is "--pins-bv 65", and the
value must be less than or equal to 65. Versions before Verilator 3.671 defaulted to
"--pins-bv 33". The more sc_bv is used, the worse for performance. Use the
"/*verilator sc_bv*/" attribute to select specific ports to be sc_bv.
--pins-sc-uint
Specifies SystemC inputs/outputs of greater than 2 bits wide should use sc_uint
between 2 and 64. When combined with the "--pins-sc-biguint" combination, it results
in sc_uint being used between 2 and 64 and sc_biguint being used between 65 and 512.
--pins-sc-biguint
Specifies SystemC inputs/outputs of greater than 65 bits wide should use sc_biguint
between 65 and 512, and sc_bv from 513 upwards. When combined with the
"--pins-sc-uint" combination, it results in sc_uint being used between 2 and 64 and
sc_biguint being used between 65 and 512.
--pins-uint8
Specifies SystemC inputs/outputs that are smaller than the --pins-bv setting and 8
bits or less should use uint8_t instead of uint32_t. Likewise pins of width 9-16 will
use uint16_t instead of uint32_t.
--pipe-filter command
Rarely needed and experimental. Verilator will spawn the specified command as a
subprocess pipe, to allow the command to perform custom edits on the Verilog code
before it reaches Verilator.
Before reading each Verilog file, Verilator will pass the file name to the subprocess'
stdin with 'read_verilog "<filename>"'. The filter may then read the file and perform
any filtering it desires, and feeds the new file contents back to Verilator on stdout
with 'Content-Length'. Output to stderr from the filter feeds through to Verilator's
stdout and if the filter exits with non-zero status Verilator terminates. See the
t/t_pipe_filter test for an example.
To debug the output of the filter, try using the -E option to see preprocessed output.
--pp-comments
With -E, show comments in preprocessor output.
--prefix topname
Specifies the name of the top level class and makefile. Defaults to V prepended to
the name of the --top-module switch, or V prepended to the first Verilog filename
passed on the command line.
--prof-cfuncs
Modify the created C++ functions to support profiling. The functions will be
minimized to contain one "basic" statement, generally a single always block or wire
statement. (Note this will slow down the executable by ~5%.) Furthermore, the
function name will be suffixed with the basename of the Verilog module and line number
the statement came from. This allows gprof or oprofile reports to be correlated with
the original Verilog source statements. See also verilator_profcfunc.
--prof-threads
Enable gantt chart data collection for threaded builds.
Verilator will record the start and end time of each macro-task across a number of
calls to eval. (What is a macro-task? See the Verilator internals document.)
When profiling is enabled, the simulation runtime will emit a blurb of profiling data
in non-human-friendly form. The "verilator_gantt" script will transform this into a
nicer visual format and produce some related statistics.
--protect-key key
Specifies the private key for --protect-ids. For best security this key should be 16
or more random bytes, a reasonable secure choice is the output of "verilator
--generate-key". Typically, a key would be created by the user once for a given
protected design library, then every Verilator run for subsequent versions of that
library would be passed the same --protect-key. Thus, if the input Verilog is similar
between library versions (Verilator runs), the Verilated code will likewise be mostly
similar.
If --protect-key is not specified and a key is needed, Verilator will generate a new
key for every Verilator run. As the key is not saved, this is best for security, but
means every Verilator run will give vastly different output even for identical input,
perhaps harming compile times (and certainly thrashing any ccache).
--protect-ids
Hash any private identifiers (variable, module, and assertion block names that are not
on the top level) into hashed random-looking identifiers, resulting after compilation
in protected library binaries that expose less design information. This hashing uses
the provided or default --protect-key, see important details there.
Verilator will also create a {prefix}__idmap.xml file which contains the mapping from
the hashed identifiers back to the original identifiers. This idmap file is to be kept
private, and is to assist mapping any simulation runtime design assertions, coverage,
or trace information, which will report the hashed identifiers, back to the original
design's identifier names.
Using DPI imports/exports is allowed and generally relatively safe in terms of
information disclosed, which is limited to the DPI function prototyptes. Use of the
VPI is not recommended as many design details may be exposed, and an INSECURE warning
will be issued.
--protect-lib name
Produces C++, Verilog wrappers and a Makefile which can in turn produce a DPI library
which can be used by Verilator or other simulators along with the corresponding
Verilog wrapper. The Makefile will build both a static and dynamic version of the
library named libname.a and libname.so respectively. This is done because some
simulators require a dynamic library, but the static library is arguably easier to use
if possible. --protect-lib implies --protect-ids.
This allows for the secure delivery of sensitive IP without the need for encrypted RTL
(i.e. IEEE P1735). See examples/make_protect_lib in the distribution for a
demonstration of how to build and use the DPI library.
--private
Opposite of --public. Is the default; this option exists for backwards compatibility.
--public
This is only for historical debug use. Using it may result in mis-simulation of
generated clocks.
Declares all signals and modules public. This will turn off signal optimizations as
if all signals had a /*verilator public*/ comments and inlining. This will also turn
off inlining as if all modules had a /*verilator public_module*/, unless the module
specifically enabled it with /*verilator inline_module*/.
--public-flat-rw
Declares all variables, ports and wires public as if they had /*verilator
public_flat_rw*/ comments. This will make them VPI accessible by their flat name, but
not turn off module inlining. This is particularly useful in combination with --vpi.
This may also in some rare cases result in mis-simulation of generated clocks.
Instead of this global switch, marking only those signals that need public_flat_rw is
typically significantly better performing.
-pvalue+name=value
Overwrites the given parameter(s) of the toplevel module. See -G for a detailed
description.
--quiet-exit
When exiting due to an error, do not display the "Command Failed" message.
--relative-includes
When a file references an include file, resolve the filename relative to the path of
the referencing file, instead of relative to the current directory.
--report-unoptflat
Extra diagnostics for UNOPTFLAT warnings. This includes for each loop, the 10 widest
variables in the loop, and the 10 most fanned out variables in the loop. These are
candidates for splitting into multiple variables to break the loop.
In addition produces a GraphViz DOT file of the entire strongly connected components
within the source associated with each loop. This is produced irrespective of whether
--dump-tree is set. Such graphs may help in analyzing the problem, but can be very
large indeed.
Various commands exist for viewing and manipulating DOT files. For example the dot
command can be used to convert a DOT file to a PDF for printing. For example:
dot -Tpdf -O Vt_unoptflat_simple_2_35_unoptflat.dot
will generate a PDF Vt_unoptflat_simple_2_35_unoptflat.dot.pdf from the DOT file.
--rr
Run Verilator and record with rr. See: rr-project.org.
--savable
Enable including save and restore functions in the generated model.
The user code must create a VerilatedSerialize or VerilatedDeserialze object then
calling the << or >> operators on the generated model and any other data the process
needs saved/restored. These functions are not thread safe, and are typically called
only by a main thread.
For example:
void save_model(const char* filenamep) {
VerilatedSave os;
os.open(filenamep);
os << main_time; // user code must save the timestamp, etc
os << *topp;
}
void restore_model(const char* filenamep) {
VerilatedRestore os;
os.open(filenamep);
os >> main_time;
os >> *topp;
}
--sc
Specifies SystemC output mode; see also --cc.
--stats
Creates a dump file with statistics on the design in {prefix}__stats.txt.
--stats-vars
Creates more detailed statistics, including a list of all the variables by size (plain
--stats just gives a count). See --stats, which is implied by this.
-sv Specifies SystemVerilog language features should be enabled; equivalent to "--language
1800-2005". This option is selected by default, it exists for compatibility with
other simulators.
+systemverilogext+ext
A synonym for "+1800-2017ext+"ext.
--threads threads
--no-threads
With --threads 0 or --no-threads, the default, the generated model is not thread safe.
With --threads 1, the generated model is single threaded but may run in a
multithreaded environment. With --threads N, where N >= 2, the model is generated to
run multithreaded on up to N threads. See "MULTITHREADING".
--threads-dpi all
--threads-dpi none
--threads-dpi pure
When using --dpi with --threads, control what DPI tasks are thread safe.
With --threads-dpi all, enable Verilator to assume all DPI imports are threadsafe, and
to use thread-local storage for communication with DPI, potentially improving
performance. Any DPI libraries need appropriate mutexes to avoid undefined behavior.
With --threads-dpi none, Verilator assume DPI imports are not thread safe, and
Verilator will serialize calls to DPI imports by default, potentially harming
performance.
With --threads-dpi pure, the default, Verilator assumes DPI pure imports are
threadsafe, but non-pure DPI imports are not.
--threads-max-mtasks value
Rarely needed. When using --threads, specify the number of mtasks the model is to be
partitioned into. If unspecified, Verilator approximates a good value.
--top-module topname
When the input Verilog contains more than one top level module, specifies the name of
the top level Verilog module to become the top, and sets the default for if --prefix
is not used. This is not needed with standard designs with only one top. See also
the MULTITOP warning section.
--trace
Adds waveform tracing code to the model using VCD format. This overrides
"--trace-fst".
Verilator will generate additional {prefix}__Trace*.cpp files that will need to be
compiled. In addition verilated_vcd_sc.cpp (for SystemC traces) or
verilated_vcd_c.cpp (for both) must be compiled and linked in. If using the Verilator
generated Makefiles, these files will be added as source targets for you. If you're
not using the Verilator makefiles, you will need to add these to your Makefile
manually.
Having tracing compiled in may result in some small performance losses, even when
waveforms are not turned on during model execution.
--trace-coverage
With --trace and --coverage-*, enable tracing to include a traced signal for every
--coverage-line or --coverage-user inserted coverage point, to assist in debugging
coverage items. Note --coverage-toggle does not get additional signals added, as the
original signals being toggle-analyzed are already visible.
The added signal will be a 32-bit value which will increment on each coverage
occurrence. Due to this, this option may greatly increase trace file sizes and
simulation runtime.
--trace-depth levels
Specify the number of levels deep to enable tracing, for example --trace-level 1 to
only see the top level's signals. Defaults to the entire model. Using a small number
will decrease visibility, but greatly improve simulation runtime and trace file size.
--trace-fst
Enable FST waveform tracing in the model. This overrides "--trace" and
"--trace-fst-thread". See also "--trace-fst-thread".
--trace-fst-thread
Enable FST waveform tracing in the model, using a separate thread. This is typically
faster in simulation runtime but slower in total computes than "--trace-fst". This
overrides "--trace" and "--trace-fst".
--trace-max-array depth
Rarely needed. Specify the maximum array depth of a signal that may be traced.
Defaults to 32, as tracing large arrays may greatly slow traced simulations.
--trace-max-width width
Rarely needed. Specify the maximum bit width of a signal that may be traced.
Defaults to 256, as tracing large vectors may greatly slow traced simulations.
--no-trace-params
Disable tracing of parameters.
--trace-structs
Enable tracing to show the name of packed structure, union, and packed array fields,
rather than a single combined packed bus. Due to VCD file format constraints this may
result in significantly slower trace times and larger trace files.
--trace-underscore
Enable tracing of signals that start with an underscore. Normally, these signals are
not output during tracing. See also --coverage-underscore.
-Uvar
Undefines the given preprocessor symbol.
--unroll-count loops
Rarely needed. Specifies the maximum number of loop iterations that may be unrolled.
See also BLKLOOPINIT warning.
--unroll-stmts statements
Rarely needed. Specifies the maximum number of statements in a loop for that loop to
be unrolled. See also BLKLOOPINIT warning.
--unused-regexp regexp
Rarely needed. Specifies a simple regexp with * and ? that if a signal name matches
will suppress the UNUSED warning. Defaults to "*unused*". Setting it to "" disables
matching.
-V Shows the verbose version, including configuration information compiled into
Verilator. (Similar to perl -V.) See also --getenv.
-v filename
Read the filename as a Verilog library. Any modules in the file may be used to
resolve cell instantiations in the top level module, else ignored. Note -v is fairly
standard across Verilog tools.
+verilog1995ext+ext
+verilog2001ext+ext
Synonyms for "+1364-1995ext+"ext and "+1364-2001ext+"ext respectively
--version
Displays program version and exits.
--vpi
Enable use of VPI and linking against the verilated_vpi.cpp files.
-Wall
Enable all code style warnings, including code style warnings that are normally
disabled by default. Equivelent to "-Wwarn-lint -Wwarn-style". Excludes some
specialty warnings, i.e. IMPERFECTSCH.
-Werror-message
Convert the specified warning message into an error message. This is generally to
discourage users from violating important site-wide rules, for example
"-Werror-NOUNOPTFLAT".
-Wfuture-message
Rarely needed. Suppress unknown Verilator comments or warning messages with the given
message code. This is used to allow code written with pragmas for a later version of
Verilator to run under a older version; add -Wfuture- arguments for each message code
or comment that the new version supports which the older version does not support.
-Wno-message
Disable the specified warning message, or in some cases where noted here disable an
error. This will override any lint_on directives in the source, i.e. the warning will
still not be printed.
-Wno-context
Disable showing the suspected context of the warning message by quoting the source
text at the suspected location. This is used to appease tools which process the error
messages may get confused by lines from the original source.
-Wno-fatal
When warnings are detected, print them, but do not exit the simulator.
Having warning messages in builds is sloppy. It is strongly recommended you cleanup
your code, use inline lint_off, or use -Wno-... flags rather than using this option.
-Wno-lint
Disable all lint related warning messages, and all style warnings. This is equivalent
to "-Wno-ALWCOMBORDER -Wno-BSSPACE -Wno-CASEINCOMPLETE -Wno-CASEOVERLAP -Wno-CASEX
-Wno-CASEWITHX -Wno-CMPCONST -Wno-COLONPLUS -Wno-ENDLABEL -Wno-IMPLICIT -Wno-LITENDIAN
-Wno-PINCONNECTEMPTY -Wno-PINMISSING -Wno-SYNCASYNCNET -Wno-UNDRIVEN -Wno-UNSIGNED
-Wno-UNUSED -Wno-WIDTH" plus the list shown for Wno-style.
It is strongly recommended you cleanup your code rather than using this option, it is
only intended to be use when running test-cases of code received from third parties.
-Wno-style
Disable all code style related warning messages (note by default they are already
disabled). This is equivalent to "-Wno-DECLFILENAME -Wno-DEFPARAM -Wno-IMPORTSTAR
-Wno-INCABSPATH -Wno-PINCONNECTEMPTY -Wno-PINNOCONNECT -Wno-SYNCASYNCNET -Wno-UNDRIVEN
-Wno-UNUSED -Wno-VARHIDDEN".
-Wpedantic
Warn on any construct demanded by IEEE, and disable all Verilator extensions that may
interfere with IEEE compliance to the standard defined with --language (etc). Similar
to GCC's -Wpedantic. Rarely used, and intended only for strict compliance tests.
-Wwarn-message
Enables the specified warning message.
-Wwarn-lint
Enable all lint related warning messages (note by default they are already enabled),
but do not affect style messages. This is equivalent to "-Wwarn-ALWCOMBORDER
-Wwarn-BSSPACE -Wwarn-CASEINCOMPLETE -Wwarn-CASEOVERLAP -Wwarn-CASEX -Wwarn-CASEWITHX
-Wwarn-CMPCONST -Wwarn-COLONPLUS -Wwarn-ENDLABEL -Wwarn-IMPLICIT -Wwarn-LITENDIAN
-Wwarn-PINMISSING -Wwarn-REALCVT -Wwarn-UNSIGNED -Wwarn-WIDTH".
-Wwarn-style
Enable all code style related warning messages. This is equivalent to "-Wwarn
ASSIGNDLY -Wwarn-DECLFILENAME -Wwarn-DEFPARAM -Wwarn-INCABSPATH -Wwarn-PINNOCONNECT
-Wwarn-SYNCASYNCNET -Wwarn-UNDRIVEN -Wwarn-UNUSED -Wwarn-VARHIDDEN".
--x-assign 0
--x-assign 1
--x-assign fast (default)
--x-assign unique
Controls the two-state value that is replaced when an assignment to X is encountered.
"--x-assign fast", the default, converts all Xs to whatever is best for performance.
"--x-assign 0" converts all Xs to 0s, and is also fast. "--x-assign 1" converts all
Xs to 1s, this is nearly as fast as 0, but more likely to find reset bugs as active
high logic will fire. "--x-assign unique" will call a function to determine the
value, this allows randomization of all Xs to find reset bugs and is the slowest, but
safest for finding reset bugs in code.
If using --x-assign unique, you may want to seed your random number generator such
that each regression run gets a different randomization sequence. Use the system's
srand48() or for Windows srand() function to do this. You'll probably also want to
print any seeds selected, and code to enable rerunning with that same seed so you can
reproduce bugs.
Note. This option applies only to variables which are explicitly assigned to X in the
Verilog source code. Initial values of clocks are set to 0 unless --x-initial-edge is
specified. Initial values of all other state holding variables are controlled with
--x-initial.
--x-initial 0
--x-initial fast
--x-initial unique (default)
Controls the two-state value that is used to initialize variables that are not
otherwise initialized.
"--x-initial 0", initializes all otherwise uninitialized variables to zero.
"--x-initial unique", the default, initializes variables using a function, which
determines the value to use each initialization. This gives greatest flexibility and
allows finding reset bugs. See "Unknown states".
"--x-initial fast", is best for performance, and initializes all variables to a state
Verilator determines is optimal. This may allow further code optimizations, but will
likely hide any code bugs relating to missing resets.
Note. This option applies only to initial values of variables. Initial values of
clocks are set to 0 unless --x-initial-edge is specified.
--x-initial-edge
Enables emulation of event driven simulators which generally trigger an edge on a
transition from X to 1 ("posedge") or X to 0 ("negedge"). Thus the following code,
where "rst_n" is uninitialized would set "res_n" to "1'b1" when "rst_n" is first set
to zero:
reg res_n = 1'b0;
always @(negedge rst_n) begin
if (rst_n == 1'b0) begin
res_n <= 1'b1;
end
end
In Verilator, by default, uninitialized clocks are given a value of zero, so the above
"always" block would not trigger.
While it is not good practice, there are some designs that rely on X X 0 triggering a
"negedge", particularly in reset sequences. Using --x-initial-edge with Verilator will
replicate this behavior. It will also ensure that X X 1 triggers a "posedge".
Note. Some users have reported that using this option can affect convergence, and that
it may be necessary to use --converge-limit to increase the number of convergence
iterations. This may be another indication of problems with the modeled design that
should be addressed.
--xml-only
Create XML output only, do not create any other output.
The XML format is intended to be used to leverage Verilator's parser and elaboration
to feed to other downstream tools. Be aware that the XML format is still evolving;
there will be some changes in future versions.
--xml-output filename
Filename for XML output file. Using this option automatically sets --xml-only.
-y dir
Add the directory to the list of directories that should be searched for include files
or libraries. The three flags -y, +incdir and -I have similar effect; +incdir and +y
are fairly standard across Verilog tools while -I is an alias for GCC compatibility.
Verilator defaults to the current directory ("-y .") and any specified --Mdir, though
these default paths are used after any user specified directories. This allows '-y
"$(pwd)"' to be used if absolute filenames are desired for error messages instead of
relative filenames.
SIMULATION RUNTIME ARGUMENTS
The following are the arguments that may be passed to a Verilated executable, provided
that executable calls Verilated::commandArgs().
All simulation runtime arguments begin with +verilator, so that the user's executable may
skip over all +verilator arguments when parsing its command line.
+verilator+debug
Enable simulation runtime debugging. Equivalent to +verilator+debugi+4.
+verilator+debugi+value
Enable simulation runtime debugging at the provided level.
+verilator+error+limit+value
Set number of non-fatal errors (e.g. assertion failures) before exiting simulation
runtime. Also affects number of $stop calls needed before exit. Defaults to 1.
+verilator+help
Display help and exit.
+verilator+prof+threads+file+filename
When using --prof-threads at simulation runtime, the filename to dump to. Defaults to
"profile_threads.dat".
+verilator+prof+threads+start+value
When using --prof-threads at simulation runtime, Verilator will wait until $time is at
this value, then start the profiling warmup, then capturing. Generally this should be
set to some time that is well within the normal operation of the simulation, i.e.
outside of reset. If 0, the dump is disabled. Defaults to 1.
+verilator+prof+threads+window+value
When using --prof-threads at simulation runtime, after $time reaches
+verilator+prof+threads+start, Verilator will warm up the profiling for this number of
eval() calls, then will capture the profiling of this number of eval() calls.
Defaults to 2, which makes sense for a single-clock-domain module where it's typical
to want to capture one posedge eval() and one negedge eval().
+verilator+rand+reset+value
When a model was Verilated using "-x-initial unique", sets the simulation runtime
initialization technique. 0 = Reset to zeros. 1 = Reset to all-ones. 2 = Randomize.
See "Unknown states".
+verilator+seed+value
For $random and "-x-initial unique", set the simulation runtime random seed value. If
zero or not specified picks a value from the system random number generator.
+verilator+V
Shows the verbose version, including configuration information.
+verilator+version
Displays program version and exits.
EXAMPLE C++ EXECUTION
We'll compile this example into C++.
mkdir test_our
cd test_our
cat >our.v <<'EOF'
module our;
initial begin $display("Hello World"); $finish; end
endmodule
EOF
cat >sim_main.cpp <<'EOF'
#include "Vour.h"
#include "verilated.h"
int main(int argc, char** argv, char** env) {
Verilated::commandArgs(argc, argv);
Vour* top = new Vour;
while (!Verilated::gotFinish()) { top->eval(); }
delete top;
exit(0);
}
EOF
See the README in the source kit for various ways to install or point to Verilator
binaries. In brief, if you are running Verilator that came from your operating system (as
an RPM), or did a "make install" to place Verilator into your default path, you do not
need anything special in your environment, and should not have VERILATOR_ROOT set.
However, if you installed Verilator from sources and want to run Verilator out of where
you compiled Verilator, you need to point to the kit:
# See above; don't do this if using an OS-distributed Verilator
export VERILATOR_ROOT=/path/to/where/verilator/was/installed
export PATH=$VERILATOR_ROOT/bin:$PATH
Now we run Verilator on our little example.
verilator -Wall --cc our.v --exe sim_main.cpp
We can see the source code under the "obj_dir" directory. See the FILES section below for
descriptions of some of the files that were created.
ls -l obj_dir
We then can compile it
make -j -C obj_dir -f Vour.mk Vour
(Verilator included a default compile rule and link rule, since we used --exe and passed a
.cpp file on the Verilator command line. You can also write your own compile rules, as
we'll show in the SYSTEMC section.)
And now we run it
obj_dir/Vour
And we get as output
Hello World
- our.v:2: Verilog $finish
Really, you're better off writing a Makefile to do all this for you. Then, when your
source changes it will automatically run all of these steps; to aid this Verilator can
create a makefile dependency file. See the examples directory in the distribution.
EXAMPLE SYSTEMC EXECUTION
This is an example similar to the above, but using SystemC.
mkdir test_our_sc
cd test_our_sc
cat >our.v <<'EOF'
module our (clk);
input clk; // Clock is required to get initial activation
always @ (posedge clk)
begin $display("Hello World"); $finish; end
endmodule
EOF
cat >sc_main.cpp <<'EOF'
#include "Vour.h"
int sc_main(int argc, char** argv) {
Verilated::commandArgs(argc, argv);
sc_clock clk ("clk", 10, 0.5, 3, true);
Vour* top;
top = new Vour("top");
top->clk(clk);
while (!Verilated::gotFinish()) { sc_start(1, SC_NS); }
delete top;
exit(0);
}
EOF
See the README in the source kit for various ways to install or point to Verilator
binaries. In brief, if you are running Verilator that came from your operating system (as
an RPM), or did a "make install" to place Verilator into your default path, you do not
need anything special in your environment, and should not have VERILATOR_ROOT set.
However, if you installed Verilator from sources and want to run Verilator out of where
you compiled Verilator, you need to point to the kit:
# See above; don't do this if using an OS-distributed Verilator
export VERILATOR_ROOT=/path/to/where/verilator/was/installed
export PATH=$VERILATOR_ROOT/bin:$PATH
Now we run Verilator on our little example.
verilator -Wall --sc our.v
We then can compile it
cd obj_dir
make -j -f Vour.mk Vour__ALL.a
make -j -f Vour.mk ../sc_main.o verilated.o
And link with SystemC. Note your path to the libraries may vary, depending on the
operating system.
export SYSTEMC_LIBDIR=/path/to/where/libsystemc.a/exists
export LD_LIBRARY_PATH=$SYSTEMC_LIBDIR:$LD_LIBRARY_PATH
# Might be needed if SystemC 2.3.0
export SYSTEMC_CXX_FLAGS=-pthread
g++ -L$SYSTEMC_LIBDIR ../sc_main.o Vour__ALL*.o verilated.o \
-o Vour -lsystemc
And now we run it
cd ..
obj_dir/Vour
And we get the same output as the C++ example:
Hello World
- our.v:2: Verilog $finish
Really, you're better off using a Makefile to do all this for you. Then, when your source
changes it will automatically run all of these steps. See the examples directory in the
distribution.
BENCHMARKING & OPTIMIZATION
For best performance, run Verilator with the "-O3 --x-assign fast --x-initial fast
--noassert" flags. The -O3 flag will require longer compile times, and "--x-assign fast
--x-initial fast" may increase the risk of reset bugs in trade for performance; see the
above documentation for these flags.
If using Verilated multithreaded, use "numactl" to ensure you are using non-conflicting
hardware resources. See "MULTITHREADING".
Minor Verilog code changes can also give big wins. You should not have any UNOPTFLAT
warnings from Verilator. Fixing these warnings can result in huge improvements; one user
fixed their one UNOPTFLAT warning by making a simple change to a clock latch used to gate
clocks and gained a 60% performance improvement.
Beyond that, the performance of a Verilated model depends mostly on your C++ compiler and
size of your CPU's caches.
By default, the lib/verilated.mk file has optimization turned off. This is for the
benefit of new users, as it improves compile times at the cost of simulation runtimes. To
add optimization as the default, set one of three variables, OPT, OPT_FAST, or OPT_SLOW
lib/verilated.mk. Or, use the -CFLAGS and/or -LDFLAGS option on the verilator command
line to pass the flags directly to the compiler or linker. Or, just for one run, pass
them on the command line to make:
make OPT_FAST="-O2 -fno-stack-protector" -f Vour.mk Vour__ALL.a
OPT_FAST specifies optimizations for those programs that are part of the fast path, mostly
code that is executed every cycle. OPT_SLOW specifies optimizations for slow-path files
(plus tracing), which execute only rarely, yet take a long time to compile with
optimization on. OPT specifies overall optimization and affects all compiles, including
those OPT_FAST and OPT_SLOW control. For best results, use OPT="-O2", and link with
"-static". Nearly the same results can be had with much better compile times with
OPT_FAST="-O1 -fstrict-aliasing". Higher optimization such as "-O3" may help, but gcc
compile times may be excessive under O3 on even medium sized designs. Alternatively, some
larger designs report better performance using "-Os".
Unfortunately, using the optimizer with SystemC files can result in compiles taking
several minutes. (The SystemC libraries have many little inlined functions that drive the
compiler nuts.)
For best results, use GCC 3.3 or newer. GCC 3.2 and earlier have optimization bugs around
pointer aliasing detection, which can result in 2x performance losses.
If you will be running many simulations on a single compile, investigate feedback driven
compilation. With GCC, using -fprofile-arcs, then -fbranch-probabilities will yield
another 15% or so.
Modern compilers also support link-time optimization (LTO), which can help especially if
you link in DPI code. To enable LTO on GCC, pass "-flto" in both compilation and link.
Note LTO may cause excessive compile times on large designs.
If you are using your own makefiles, you may want to compile the Verilated code with
-DVL_INLINE_OPT=inline. This will inline functions, however this requires that all cpp
files be compiled in a single compiler run.
You may uncover further tuning possibilities by profiling the Verilog code. Use
Verilator's --prof-cfuncs, then GCC's -g -pg. You can then run either oprofile or gprof
to see where in the C++ code the time is spent. Run the gprof output through
verilator_profcfunc and it will tell you what Verilog line numbers on which most of the
time is being spent.
When done, please let the author know the results. I like to keep tabs on how Verilator
compares, and may be able to suggest additional improvements.
FILES
All output files are placed in the output directory name specified with the -Mdir option,
or "obj_dir" if not specified.
Verilator creates the following files in the output directory:
For --make gmake, it creates:
{prefix}.mk // Make include file for compiling
{prefix}_classes.mk // Make include file with class names
For --make cmake, it creates:
{prefix}.cmake // CMake include script for compiling
For -cc and -sc mode, it also creates:
{prefix}.cpp // Top level C++ file
{prefix}.h // Top level header
{prefix}__Slow{__n}.cpp // Constructors and infrequent cold routines
{prefix}{__n}.cpp // Additional top C++ files (--output-split)
{prefix}{each_verilog_module}.cpp // Lower level internal C++ files
{prefix}{each_verilog_module}.h // Lower level internal header files
{prefix}{each_verilog_module}{__n}.cpp // Additional lower C++ files (--output-split)
In certain debug and other modes, it also creates:
{prefix}.xml // XML tree information (--xml)
{prefix}__Dpi.cpp // DPI import and export wrappers
{prefix}__Dpi.h // DPI import and export declarations
{prefix}__Inlines.h // Inline support functions
{prefix}__Syms.cpp // Global symbol table C++
{prefix}__Syms.h // Global symbol table header
{prefix}__Trace__Slow{__n}.cpp // Wave file generation code (--trace)
{prefix}__Trace{__n}.cpp // Wave file generation code (--trace)
{prefix}__cdc.txt // Clock Domain Crossing checks (--cdc)
{prefix}__stats.txt // Statistics (--stats)
{prefix}__idmap.txt // Symbol demangling (--protect-ids)
It also creates internal files that can be mostly ignored:
{mod_prefix}_{each_verilog_module}{__n}.vpp // Post-processed verilog
{prefix}__ver.d // Make dependencies (-MMD)
{prefix}__verFiles.dat // Timestamps for skip-identical
{prefix}{misc}.dot // Debugging graph files (--debug)
{prefix}{misc}.tree // Debugging files (--debug)
After running Make, the C++ compiler may produce the following:
verilated{misc}.d // Intermediate dependencies
verilated{misc}.o // Intermediate objects
{mod_prefix}{misc}.d // Intermediate dependencies
{mod_prefix}{misc}.o // Intermediate objects
{prefix} // Final executable (w/--exe argument)
{prefix}__ALL.a // Library of all Verilated objects
{prefix}__ALLboth.cpp // Include of classes for single compile
{prefix}__ALLcls.cpp // Include of user classes for single compile
{prefix}__ALLsup.cpp // Include of support files for single compile
{prefix}{misc}.d // Intermediate dependencies
{prefix}{misc}.o // Intermediate objects
ENVIRONMENT
LD_LIBRARY_PATH
A generic Linux/OS variable specifying what directories have shared object (.so)
files. This path should include SystemC and any other shared objects needed at
simultion runtime.
OBJCACHE
Optionally specifies a caching or distribution program to place in front of all runs
of the C++ Compiler. For example, "objcache --read --write", or "ccache". If using
distcc or icecc/icecream, they would generally be run under either objcache or ccache;
see the documentation for those programs.
SYSTEMC
Deprecated. Used only if SYSTEMC_INCLUDE or SYSTEMC_LIBDIR is not set. If set,
specifies the directory containing the SystemC distribution. If not specified, it
will come from a default optionally specified at configure time (before Verilator was
compiled).
SYSTEMC_ARCH
Deprecated. Used only if SYSTEMC_LIBDIR is not set. Specifies the architecture name
used by the SystemC kit. This is the part after the dash in the lib-{...} directory
name created by a 'make' in the SystemC distribution. If not set, Verilator will try
to intuit the proper setting, or use the default optionally specified at configure
time (before Verilator was compiled).
SYSTEMC_CXX_FLAGS
Specifies additional flags that are required to be passed to GCC when building the
SystemC model. System 2.3.0 may need this set to "-pthread".
SYSTEMC_INCLUDE
If set, specifies the directory containing the systemc.h header file. If not
specified, it will come from a default optionally specified at configure time (before
Verilator was compiled), or computed from SYSTEMC/include.
SYSTEMC_LIBDIR
If set, specifies the directory containing the libsystemc.a library. If not specified,
it will come from a default optionally specified at configure time (before Verilator
was compiled), or computed from SYSTEMC/lib-SYSTEMC_ARCH.
VERILATOR_BIN
If set, specifies an alternative name of the "verilator" binary. May be used for
debugging and selecting between multiple operating system builds.
VERILATOR_COVERAGE_BIN
If set, specifies an alternative name of the "verilator_coverage binary". May be used
for debugging and selecting between multiple operating system builds.
VERILATOR_GDB
If set, the command to run when using the --gdb option, such as "ddd". If not
specified, it will use "gdb".
VERILATOR_ROOT
Specifies the directory containing the distribution kit. This is used to find the
executable, Perl library, and include files. If not specified, it will come from a
default optionally specified at configure time (before Verilator was compiled). It
should not be specified if using a pre-compiled Verilator RPM as the hardcoded value
should be correct.
CONNECTING TO C++
Verilator creates a .h and .cpp file for the top level module and all modules under it.
See the examples directory in the kit for examples.
After the modules are completed, there will be a module.mk file that may be used with Make
to produce a module__ALL.a file with all required objects in it. This is then linked with
the user's top level to create the simulation executable.
The user must write the top level of the simulation. Here's a simple example:
#include <verilated.h> // Defines common routines
#include <iostream> // Need std::cout
#include "Vtop.h" // From Verilating "top.v"
Vtop *top; // Instantiation of module
vluint64_t main_time = 0; // Current simulation time
// This is a 64-bit integer to reduce wrap over issues and
// allow modulus. You can also use a double, if you wish.
double sc_time_stamp () { // Called by $time in Verilog
return main_time; // converts to double, to match
// what SystemC does
}
int main(int argc, char** argv) {
Verilated::commandArgs(argc, argv); // Remember args
top = new Vtop; // Create instance
top->reset_l = 0; // Set some inputs
while (!Verilated::gotFinish()) {
if (main_time > 10) {
top->reset_l = 1; // Deassert reset
}
if ((main_time % 10) == 1) {
top->clk = 1; // Toggle clock
}
if ((main_time % 10) == 6) {
top->clk = 0;
}
top->eval(); // Evaluate model
cout << top->out << endl; // Read a output
main_time++; // Time passes...
}
top->final(); // Done simulating
// // (Though this example doesn't get here)
delete top;
}
Note signals are read and written as member variables of the lower module. You call the
eval() method to evaluate the model. When the simulation is complete call the final()
method to wrap up any SystemVerilog final blocks, and complete any assertions.
CONNECTING TO SYSTEMC
Verilator will convert the top level module to a SC_MODULE. This module will plug
directly into a SystemC netlist.
The SC_MODULE gets the same pinout as the Verilog module, with the following type
conversions: Pins of a single bit become bool. Pins 2-32 bits wide become uint32_t's.
Pins 33-64 bits wide become sc_bv's or vluint64_t's depending on the --no-pins64 switch.
Wider pins become sc_bv's. (Uints simulate the fastest so are used where possible.)
Lower modules are not pure SystemC code. This is a feature, as using the SystemC pin
interconnect scheme everywhere would reduce performance by an order of magnitude.
DIRECT PROGRAMMING INTERFACE (DPI)
Verilator supports SystemVerilog Direct Programming Interface import and export
statements. Only the SystemVerilog form ("DPI-C") is supported, not the original
Synopsys-only DPI.
DPI Example
In the SYSTEMC example above, if you wanted to import C++ functions into Verilog, put in
our.v:
import "DPI-C" function integer add (input integer a, input integer b);
initial begin
$display("%x + %x = %x", 1, 2, add(1,2));
endtask
Then after Verilating, Verilator will create a file Vour__Dpi.h with the prototype to call
this function:
extern int add (int a, int b);
From the sc_main.cpp file (or another .cpp file passed to the Verilator command line, or
the link), you'd then:
#include "svdpi.h"
#include "Vour__Dpi.h"
int add(int a, int b) { return a+b; }
DPI System Task/Functions
Verilator extends the DPI format to allow using the same scheme to efficiently add system
functions. Simply use a dollar-sign prefixed system function name for the import, but
note it must be escaped.
export "DPI-C" function integer \$myRand;
initial $display("myRand=%d", $myRand());
Going the other direction, you can export Verilog tasks so they can be called from C++:
export "DPI-C" task publicSetBool;
task publicSetBool;
input bit in_bool;
var_bool = in_bool;
endtask
Then after Verilating, Verilator will create a file Vour__Dpi.h with the prototype to call
this function:
extern bool publicSetBool(bool in_bool);
From the sc_main.cpp file, you'd then:
#include "Vour__Dpi.h"
publicSetBool(value);
Or, alternatively, call the function under the design class. This isn't DPI compatible
but is easier to read and better supports multiple designs.
#include "Vour__Dpi.h"
Vour::publicSetBool(value);
// or top->publicSetBool(value);
Note that if the DPI task or function accesses any register or net within the RTL, it will
require a scope to be set. This can be done using the standard functions within svdpi.h,
after the module is instantiated, but before the task(s) and/or function(s) are called.
For example, if the top level module is instantiated with the name "dut" and the name
references within tasks are all hierarchical (dotted) names with respect to that top level
module, then the scope could be set with
#include "svdpi.h"
...
svSetScope(svGetScopeFromName("dut"));
(Remember that Verilator adds a "V" to the top of the module hierarchy.)
Scope can also be set from within a DPI imported C function that has been called from
Verilog by querying the scope of that function. See the sections on DPI Context Functions
and DPI Header Isolation below and the comments within the svdpi.h header for more
information.
DPI Display Functions
Verilator allows writing $display like functions using this syntax:
import "DPI-C" function void
\$my_display(input string formatted /*verilator sformat*/ );
The /*verilator sformat*/ indicates that this function accepts a $display like format
specifier followed by any number of arguments to satisfy the format.
DPI Context Functions
Verilator supports IEEE DPI Context Functions. Context imports pass the simulator
context, including calling scope name, and filename and line number to the C code. For
example, in Verilog:
import "DPI-C" context function int dpic_line();
initial $display("This is line %d, again, line %d\n", `line, dpic_line());
This will call C++ code which may then use the svGet* functions to read information, in
this case the line number of the Verilog statement that invoked the dpic_line function:
int dpic_line() {
// Get a scope: svScope scope = svGetScope();
const char* scopenamep = svGetNameFromScope(scope);
assert(scopenamep);
const char* filenamep = "";
int lineno = 0;
if (svGetCallerInfo(&filenamep, &lineno)) {
printf("dpic_line called from scope %s on line %d\n",
scopenamep, lineno);
return lineno;
} else {
return 0;
}
}
See the IEEE Standard for more information.
DPI Header Isolation
Verilator places the IEEE standard header files such as svdpi.h into a separate include
directory, vltstd (VeriLaTor STandarD). When compiling most applications
$VERILATOR_ROOT/include/vltstd would be in the include path along with the normal
$VERILATOR_ROOT/include. However, when compiling Verilated models into other simulators
which have their own svdpi.h and similar standard files with different contents, the
vltstd directory should not be included to prevent picking up incompatible definitions.
Public Functions
Instead of DPI exporting, there's also Verilator public functions, which are slightly
faster, but less compatible.
VERIFICATION PROCEDURAL INTERFACE (VPI)
Verilator supports a very limited subset of the VPI. This subset allows inspection,
examination, value change callbacks, and depositing of values to public signals only.
VPI is enabled with the verilator --vpi switch.
To access signals via the VPI, Verilator must be told exactly which signals are to be
accessed. This is done using the Verilator public pragmas documented below.
Verilator has an important difference from an event based simulator; signal values that
are changed by the VPI will not immediately propagate their values, instead the top level
header file's eval() method must be called. Normally this would be part of the normal
evaluation (i.e. the next clock edge), not as part of the value change. This makes the
performance of VPI routines extremely fast compared to event based simulators, but can
confuse some test-benches that expect immediate propagation.
Note the VPI by its specified implementation will always be much slower than accessing the
Verilator values by direct reference (structure->module->signame), as the VPI accessors
perform lookup in functions at simulation runtime requiring at best hundreds of
instructions, while the direct references are evaluated by the compiler and result in only
a couple of instructions.
For signal callbacks to work the main loop of the program must call
VerilatedVpi::callValueCbs().
VPI Example
In the below example, we have readme marked read-only, and writeme which if written from
outside the model will have the same semantics as if it changed on the specified clock
edge.
cat >our.v <<'EOF'
module our (input clk);
reg readme /*verilator public_flat_rd*/;
reg writeme /*verilator public_flat_rw @(posedge clk) */;
initial $finish;
endmodule
EOF
There are many online tutorials and books on the VPI, but an example that accesses the
above signal "readme" would be:
cat >sim_main.cpp <<'<<EOF'
#include "Vour.h"
#include "verilated.h"
#include "verilated_vpi.h" // Required to get definitions
vluint64_t main_time = 0; // See comments in first example
double sc_time_stamp() { return main_time; }
void read_and_check() {
vpiHandle vh1 = vpi_handle_by_name((PLI_BYTE8*)"TOP.our.readme", NULL);
if (!vh1) { vl_fatal(__FILE__, __LINE__, "sim_main", "No handle found"); }
const char* name = vpi_get_str(vpiName, vh1);
printf("Module name: %s\n", name); // Prints "readme"
s_vpi_value v;
v.format = vpiIntVal;
vpi_get_value(vh1, &v);
printf("Value of v: %d\n", v.value.integer); // Prints "readme"
}
int main(int argc, char** argv, char** env) {
Verilated::commandArgs(argc, argv);
Vour* top = new Vour;
Verilated::internalsDump(); // See scopes to help debug
while (!Verilated::gotFinish()) {
top->eval();
VerilatedVpi::callValueCbs(); // For signal callbacks
read_and_check();
}
delete top;
exit(0);
}
EOF
CROSS COMPILATION
Verilator supports cross-compiling Verilated code. This is generally used to run
Verilator on a Linux system and produce C++ code that is then compiled on Windows.
Cross compilation involves up to three different OSes. The build system is where you
configured and compiled Verilator, the host system where you run Verilator, and the target
system where you compile the Verilated code and run the simulation.
Currently, Verilator requires the build and host system type to be the same, though the
target system type may be different. To support this, ./configure and make Verilator on
the build system. Then, run Verilator on the host system. Finally, the output of
Verilator may be compiled on the different target system.
To support this, none of the files that Verilator produces will reference any configure
generated build-system specific files, such as config.h (which is renamed in Verilator to
config_build.h to reduce confusion.) The disadvantage of this approach is that
include/verilatedos.h must self-detect the requirements of the target system, rather than
using configure.
The target system may also require edits to the Makefiles, the simple Makefiles produced
by Verilator presume the target system is the same type as the build system.
CMake
Verilator can be run using CMake, which takes care of both running Verilator and compiling
the output. There is a CMake example in the examples/ directory. The following is a
minimal CMakeLists.txt that would build the code listed in "EXAMPLE C++ EXECUTION":
project(cmake_example)
find_package(verilator HINTS $ENV{VERILATOR_ROOT})
add_executable(Vour sim_main.cpp)
verilate(Vour SOURCES our.v)
find_package will automatically find an installed copy of Verilator, or use a local build
if VERILATOR_ROOT is set.
It is recommended to use CMake >= 3.12 and the Ninja generator, though other combinations
should work. To build with CMake, change to the folder containing CMakeLists.txt and run:
mkdir build
cd build
cmake -GNinja ..
ninja
Or to build with your system default generator:
mkdir build
cd build
cmake ..
cmake --build .
If you're building the example you should have an executable to run:
./Vour
The package sets the CMake variables verilator_FOUND, VERILATOR_ROOT and VERILATOR_BIN to
the appropriate values, and also creates a verilate() function. verilate() will
automatically create custom commands to run Verilator and add the generated C++ sources to
the target specified.
verilate(target SOURCES source ... [TOP_MODULE top] [PREFIX name]
[TRACE] [TRACE_FST] [SYSTEMC] [COVERAGE]
[INCLUDE_DIRS dir ...] [OPT_SLOW ...] [OPT_FAST ...]
[DIRECTORY dir] [VERILATOR_ARGS ...])
Lowercase and ... should be replaced with arguments, the uppercase parts delimit the
arguments and can be passed in any order, or left out entirely if optional.
verilate(target ...) can be called multiple times to add other verilog modules to an
executable or library target.
When generating Verilated SystemC sources, you should also include the SystemC include
directories and link to the SystemC libraries.
Verilator's CMake support provides a convenience function to automatically find and link
to the SystemC library. It can be used as:
verilator_link_systemc(target)
where target is the name of your target.
The search paths can be configured by setting some variables:
- The variables SYSTEMC_INCLUDE and SYSTEMC_LIBDIR to give a direct path to the SystemC
include an library path.
- SYSTEMC_ROOT to set the installation prefix of an installed SystemC
library.
- SYSTEMC to set the installation prefix of an installed SystemC library
(same as above).
- When using Accellera's SystemC with CMake support, a CMake target is available that will
easen above steps. This will only work if the SystemC installation can be found by CMake.
This can be configured by setting the CMAKE_PREFIX_PATH variable during CMake
configuration.
Don't forget to set the same C++ standard for the Verilated sources as the SystemC
library. This can be specified using the SYSTEMC_CXX_FLAGS environment variable.
target
Name of a target created by add_executable or add_library.
SOURCES
List of verilog files to Verilate. Must have at least one file.
PREFIX
Optional. Sets the Verilator output prefix. Defaults to the name of the first hdl
source with a "V" prepended. Must be unique in each call to verilate(), so this is
necessary if you build a module multiple times with different parameters. Must be a
valid C++ identifier, i.e. contains no whitespace and only characters A-Z, a-z, 0-9 or
_.
TOP_MODULE
Optional. Sets the name of the top module. Defaults to the name of the first file in
the SOURCES array.
TRACE
Optional. Enables VCD tracing if present, equivalent to "VERILATOR_ARGS --trace".
TRACE_FST
Optional. Enables FST tracing if present, equivalent to "VERILATOR_ARGS --trace-fst".
SYSTEMC
Optional. Enables SystemC mode, defaults to C++ if not specified.
COVERAGE
Optional. Enables coverage if present, equivalent to "VERILATOR_ARGS --coverage"
INCLUDE_DIRS
Optional. Sets directories that Verilator searches (same as -y).
OPT_SLOW
Optional. Set compiler flags for the slow path. You may want to reduce the
optimization level to improve compile times with large designs.
OPT_FAST
Optional. Set compiler flags for the fast path.
DIRECTORY
Optional. Set the verilator output directory. It is preferable to use the default,
which will avoid collisions with other files.
VERILATOR_ARGS
Optional. Extra arguments to Verilator. Do not specify --Mdir or --prefix here, use
DIRECTORY or PREFIX.
Cadence NC-SystemC Models
Similar to compiling Verilated designs with gcc, Verilated designs may be compiled inside
other simulators that support C++ or SystemC models. One such simulator is Cadence's NC-
SystemC, part of their Incisive Verification Suite. (Highly recommended.)
Using the example files above, the following command will build the model underneath NC:
cd obj_dir
ncsc_run \
sc_main.cpp \
Vour__ALLcls.cpp \
Vour__ALLsup.cpp \
verilated.cpp
For larger designs you'll want to automate this using makefiles, which pull the names of
the .cpp files to compile in from the make variables generated in obj_dir/Vour_classes.mk.
MULTITHREADING
Verilator experimentally supports multithreading.
With --no-threads, the default, the model is not thread safe, and any use of more than one
thread calling into one or even different Verilated models may result in unpredictable
behavior. This gives the highest single thread performance.
With --threads 1, the generated model is single threaded, however the support libraries
are multithread safe. This allows different instantiations of model(s) to potentially each
be run under a different thread. All threading is the responsibility of the user's C++
testbench.
With --threads N, where N is at least 2, the generated model will be designed to run in
parallel on N threads. The thread calling eval() provides one of those threads, and the
generated model will create and manage the other N-1 threads. It's the client's
responsibility not to oversubscribe the available CPU cores. Under CPU oversubscription,
the Verilated model should not livelock nor deadlock, however, you can expect performance
to be far worse than it would be with proper ratio of threads and CPU cores.
With --trace-fst-thread, tracing occurs in a separate thread from the main simulation
thread(s). This option is orthogonal to --threads.
The remainder of this section describe behavior with --threads 1 or --threads N (not
--no-threads).
VL_THREADED is defined when compiling a threaded Verilated module, causing the Verilated
support classes become threadsafe.
The thread used for constructing a model must be the same thread that calls eval() into
the model, this is called the "eval thread". The thread used to perform certain global
operations such as saving and tracing must be done by a "main thread". In most cases the
eval thread and main thread are the same thread (i.e. the user's top C++ testbench runs on
a single thread), but this is not required.
When running a multithreaded model, the default Linux task scheduler often works against
the model, by assuming threads are short lived, and thus often schedules threads using
multiple hyperthreads within the same physical core. For best performance use the
"numactl" program to (when the threading count fits) select unique physical cores on the
same socket. For example, if a model was Verilated with "--threads 4", we consult
egrep 'processor|physical id|core id' /proc/cpuinfo
To select cores 0, 1, 2, and 3 that are all located on the same socket (0) but different
physical cores. (Also useful is "numactl --hardware", or "lscpu" but those doesn't show
Hyperthreading cores.) Then we execute
numactl -m 0 -C 0,1,2,3 -- verilated_executable_name
This will limit memory to socket 0, and threads to cores 0, 1, 2, 3, (presumably on socket
0) optimizing performance. Of course this must be adjusted if you want another simulator
using e.g. socket 1, or if you Verilated with a different number of threads. To see what
CPUs are actually used, use --prof-threads.
Multithreaded Verilog and Library Support
$display/$stop/$finish are delayed until the end of an eval() call in order to maintain
ordering between threads. This may result in additional tasks completing after the $stop
or $finish.
If using --coverage, the coverage routines are fully thread safe.
If using --dpi, Verilator assumes pure DPI imports are thread safe, balancing
performance versus safety. See --threads-dpi.
If using --savable, the save/restore classes are not multithreaded and are must be
called only by the eval thread.
If using --sc, the SystemC kernel is not thread safe, therefore the eval thread and
main thread must be the same.
If using --trace, the tracing classes must be constructed and called from the main
thread.
If using --vpi, since SystemVerilog VPI was not architected by IEEE to be
multithreaded, Verilator requires all VPI calls are only made from the main thread.
CONFIGURATION FILES
In addition to the command line, warnings and other features may be controlled by
configuration files, typically named with the .vlt extension. An example:
`verilator_config
lint_off -rule WIDTH
lint_off -rule CASEX -file "silly_vendor_code.v"
This disables WIDTH warnings globally, and CASEX for a specific file.
Configuration files are parsed after the normal Verilog preprocessing, so `ifdefs,
`defines, and comments may be used as if it were normal Verilog code.
Note that file or line-specific configuration only applies to files read after the
configuration file. It is therefore recommended to pass the configuration file to
Verilator as first file.
The grammar of configuration commands is as follows:
`verilator_config
Take remaining text and treat it as Verilator configuration commands.
coverage_on [-file "<filename>" [-lines <line> [ - <line> ]]]
coverage_off [-file "<filename>" [-lines <line> [ - <line> ]]]
Enable/disable coverage for the specified filename (or wildcard with '*' or '?', or
all files if omitted) and range of line numbers (or all lines if omitted). Often used
to ignore an entire module for coverage analysis purposes.
lint_on [-rule <message>] [-file "<filename>" [-lines <line> [ - <line>]]]
lint_off [-rule <message>] [-file "<filename>" [-lines <line> [ - <line>]]]
lint_off [-rule <message>] [-file "<filename>"] [-match "<string>"]
Enable/disables the specified lint warning, in the specified filename (or wildcard
with '*' or '?', or all files if omitted) and range of line numbers (or all lines if
omitted).
With lint_off using '*' will override any lint_on directives in the source, i.e. the
warning will still not be printed.
If the -rule is omitted, all lint warnings (see list in -Wno-lint) are
enabled/disabled. This will override all later lint warning enables for the specified
region.
If -match is set the linter warnings are matched against this (wildcard) string and
are waived in case they match iff rule and file (with wildcard) also match.
In previous versions -rule was named -msg. The latter is deprecated, but still works
with a deprecation info, it may be removed in future versions.
tracing_on [-file "<filename>" [-lines <line> [ - <line> ]]]
tracing_off [-file "<filename>" [-lines <line> [ - <line> ]]]
Enable/disable waveform tracing for all future signals declared in the specified
filename (or wildcard with '*' or '?', or all files if omitted) and range of line
numbers (or all lines if omitted).
For tracing_off, cells below any module in the files/ranges specified will also not be
traced.
clock_enable -module "<modulename>" -signal "<signame>"
Indicate the signal is used to gate a clock, and the user takes responsibility for
insuring there are no races related to it.
Same as /*verilator clock_enable*/, see "LANGUAGE EXTENSIONS" for more information and
an example.
clocker -module "<modulename>" [-task "<taskname>"] -signal "<signame>"
clocker -module "<modulename>" [-function "<funcname>"] -signal "<signame>"
no_clocker -module "<modulename>" [-task "<taskname>"] -signal "<signame>"
no_clocker -module "<modulename>" [-function "<funcname>"] -signal "<signame>"
Indicate the signal is used as clock or not. This information is used by Verilator to
mark the signal as clocker and propagate the clocker attribute automatically to
derived signals. See "--clk" for more information.
Same as /*verilator clocker*/, see "LANGUAGE EXTENSIONS" for more information.
coverage_block_off -module "<modulename>" -block "<blockname>"
coverage_block_off -file "<filename>" -line <lineno>
Specifies the entire begin/end block should be ignored for coverage analysis purposes.
Can either be specified as a named block or as a filename and line number.
Same as /*verilator coverage_block_off*/, see "LANGUAGE EXTENSIONS" for more
information.
full_case -file "<filename>" -lines <lineno>
parallel_case -file "<filename>" -lines <lineno>
Same as "//synopsys full_case" and "//synopsys parallel_case". When these synthesis
directives are discovered, Verilator will either formally prove the directive to be
true, or failing that, will insert the appropriate code to detect failing cases at
simulation runtime and print an "Assertion failed" error message.
inline -module "<modulename>"
Specifies the module may be inlined into any modules that use this module. This is
useful to speed up simulation runtime with some small loss of trace visibility and
modularity. Note signals under inlined submodules will be named
submodule__DOT__subsignal as C++ does not allow "." in signal names. When tracing
such signals the tracing routines will replace the __DOT__ with the period.
Same as /*verilator inline_module*/, see "LANGUAGE EXTENSIONS" for more information.
isolate_assignments -module "<modulename>" [-task "<taskname>"] -signal "<signame>"
isolate_assignments -module "<modulename>" [-function "<funcname>"] -signal "<signame>"
isolate_assignments -module "<modulename>" -function "<fname>"
Used to indicate the assignments to this signal in any blocks should be isolated into
new blocks. When there is a large combinatorial block that is resulting in a
UNOPTFLAT warning, attaching this to the signal causing a false loop may clear up the
problem.
Same as /* verilator isolate_assignments */, see "LANGUAGE EXTENSIONS" for more
information.
no_inline -module "<modulename>"
Specifies the module should not be inlined into any modules that use this module.
This is useful especially at the top level module to reduce the size of the interface
class, to aid compile time at a small performance loss.
Same as /*verilator no_inline_module*/, see "LANGUAGE EXTENSIONS" for more
information.
no_inline [-module "<modulename>"] -task "<taskname>"
no_inline [-module "<modulename>"] -function "<funcname>"
Specify the function or task should not be inlined into where it is used. This may
reduce the size of the final executable when a task is used a very large number of
times. For this flag to work, the task and tasks below it must be pure; they cannot
reference any variables outside the task itself.
Same as /*verilator no_inline_task*/, see "LANGUAGE EXTENSIONS" for more information.
sc_bv -module "<modulename>" [-task "<taskname>"] -signal "<signame>"
sc_bv -module "<modulename>" [-function "<funcname>"] -signal "<signame>"
Sets the port to be of sc_bv<width> type, instead of bool, vluint32_t or vluint64_t.
This may be useful if the port width is parameterized and different of such modules
interface a templated module (such as a transactor) or for other reasons. In general
you should avoid using this attribute when not necessary as with increasing usage of
sc_bv the performance decreases significantly.
Same as /*verilator sc_bv*/, see "LANGUAGE EXTENSIONS" for more information.
sformat [-module "<modulename>"] [-task "<taskname>"] -signal "<signame>"
sformat [-module "<modulename>"] [-function "<funcname>"] -signal "<signame>"
Final input of a function or task "input string" to indicate the function or task
should pass all remaining arguments through $sformatf. This allows creation of DPI
functions with $display like behavior. See the test_regress/t/t_dpi_display.v file
for an example.
Same as /*verilator sformat*/, see "LANGUAGE EXTENSIONS" for more information.
LANGUAGE STANDARD SUPPORT
Verilog 2001 (IEEE 1364-2001) Support
Verilator supports most Verilog 2001 language features. This includes signed numbers,
"always @*", generate statements, multidimensional arrays, localparam, and C-style
declarations inside port lists.
Verilog 2005 (IEEE 1364-2005) Support
Verilator supports most Verilog 2005 language features. This includes the `begin_keywords
and `end_keywords compiler directives, $clog2, and the uwire keyword.
SystemVerilog 2005 (IEEE 1800-2005) Support
Verilator supports ==? and !=? operators, ++ and -- in some contexts, $bits, $countones,
$error, $fatal, $info, $isunknown, $onehot, $onehot0, $unit, $warning, always_comb,
always_ff, always_latch, bit, byte, chandle, const, do-while, enum, export, final, import,
int, interface, logic, longint, modport, package, program, shortint, struct, time,
typedef, union, var, void, priority case/if, and unique case/if.
It also supports .name and .* interconnection.
Verilator partially supports concurrent assert and cover statements; see the enclosed
coverage tests for the syntax which is allowed.
SystemVerilog 2012 (IEEE 1800-2012) Support
Verilator implements a full SystemVerilog 2012 preprocessor, including function call-like
preprocessor defines, default define arguments, `__FILE__, `__LINE__ and `undefineall.
Verilator currently has some support for SystemVerilog synthesis constructs. As
SystemVerilog features enter common usage they are added; please file a bug if a feature
you need is missing.
SystemVerilog 2017 (IEEE 1800-2017) Support
Verilator supports the 2017 "for" loop constructs, and several minor cleanups made in
1800-2017.
Verilog AMS Support
Verilator implements a very small subset of Verilog AMS (Verilog Analog and Mixed-Signal
Extensions) with the subset corresponding to those VMS keywords with near equivalents in
the Verilog 2005 or SystemVerilog 2009 languages.
AMS parsing is enabled with "--language VAMS" or "--language 1800+VAMS".
At present Verilator implements ceil, exp, floor, ln, log, pow, sqrt, string, and wreal.
Synthesis Directive Assertion Support
With the --assert switch, Verilator reads any "//synopsys full_case" or "//synopsys
parallel_case" directives. The same applies to any "//ambit synthesis", "//cadence" or
"//pragma" directives of the same form.
When these synthesis directives are discovered, Verilator will either formally prove the
directive to be true, or failing that, will insert the appropriate code to detect failing
cases at simulation runtime and print an "Assertion failed" error message.
Verilator likewise also asserts any "unique" or "priority" SystemVerilog keywords on case
statement, as well as "unique" on if statements. However, "priority if" is currently
simply ignored.
LANGUAGE EXTENSIONS
The following additional constructs are the extensions Verilator supports on top of
standard Verilog code. Using these features outside of comments or `ifdef's may break
other tools.
`__FILE__
The __FILE__ define expands to the current filename as a string, like C++'s __FILE__.
This was incorporated into to the 1800-2009 standard (but supported by Verilator since
2006!)
`__LINE__
The __LINE__ define expands to the current filename as a string, like C++'s __LINE__.
This was incorporated into to the 1800-2009 standard (but supported by Verilator since
2006!)
`error string
This will report an error when encountered, like C++'s #error.
$c(string, ...);
The string will be embedded directly in the output C++ code at the point where the
surrounding Verilog code is compiled. It may either be a standalone statement (with a
trailing ; in the string), or a function that returns up to a 32-bit number (without a
trailing ;). This can be used to call C++ functions from your Verilog code.
String arguments will be put directly into the output C++ code. Expression arguments
will have the code to evaluate the expression inserted. Thus to call a C++ function,
$c("func(",a,")") will result in 'func(a)' in the output C++ code. For input
arguments, rather than hard-coding variable names in the string $c("func(a)"), instead
pass the variable as an expression $c("func(",a,")"). This will allow the call to
work inside Verilog functions where the variable is flattened out, and also enable
other optimizations.
If you will be reading or writing any Verilog variables inside the C++ functions, the
Verilog signals must be declared with /*verilator public*/.
You may also append an arbitrary number to $c, generally the width of the output.
[signal_32_bits = $c32("...");] This allows for compatibility with other simulators
which require a differently named PLI function name for each different output width.
$display, $write, $fdisplay, $fwrite, $sformat, $swrite
Format arguments may use C fprintf sizes after the % escape. Per the Verilog
standard, %x prints a number with the natural width, and %0x prints a number with
minimum width. Verilator extends this so %5x prints 5 digits per the C standard (it's
unspecified in Verilog).
`coverage_block_off
Specifies the entire begin/end block should be ignored for coverage analysis. Must be
inside a basic block, e.g. within a begin/end pair. Same as /* verilator
coverage_block_off */ and "coverage_block_off" in "CONFIGURATION FILES".
`systemc_header
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into the output .h file's header. Must be placed as a module item, e.g.
directly inside a module/endmodule pair. Despite the name of this macro, this also
works in pure C++ code.
`systemc_ctor
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into the C++ class constructor. Must be placed as a module item, e.g.
directly inside a module/endmodule pair. Despite the name of this macro, this also
works in pure C++ code.
`systemc_dtor
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into the C++ class destructor. Must be placed as a module item, e.g.
directly inside a module/endmodule pair. Despite the name of this macro, this also
works in pure C++ code.
`systemc_interface
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into the C++ class interface. Must be placed as a module item, e.g. directly
inside a module/endmodule pair. Despite the name of this macro, this also works in
pure C++ code.
`systemc_imp_header
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into the header of all files for this C++ class implementation. Must be
placed as a module item, e.g. directly inside a module/endmodule pair. Despite the
name of this macro, this also works in pure C++ code.
`systemc_implementation
Take remaining text up to the next `verilog or `systemc_... mode switch and place it
verbatim into a single file of the C++ class implementation. Must be placed as a
module item, e.g. directly inside a module/endmodule pair. Despite the name of this
macro, this also works in pure C++ code.
If you will be reading or writing any Verilog variables in the C++ functions, the
Verilog signals must be declared with /*verilator public*/. See also the public task
feature; writing an accessor may result in cleaner code.
`SYSTEMVERILOG
The SYSTEMVERILOG, SV_COV_START and related standard defines are set by default when
--language is 1800-*.
`VERILATOR
`verilator
`verilator3
The VERILATOR, verilator and verilator3 defines are set by default so you may `ifdef
around compiler specific constructs.
`verilator_config
Take remaining text up to the next `verilog mode switch and treat it as Verilator
configuration commands.
`verilog
Switch back to processing Verilog code after a `systemc_... mode switch. The Verilog
code returns to the last language mode specified with `begin_keywords, or
SystemVerilog if none was specified.
/*verilator clock_enable*/
Used after a signal declaration to indicate the signal is used to gate a clock, and
the user takes responsibility for insuring there are no races related to it.
(Typically by adding a latch, and running static timing analysis.) For example:
reg enable_r /*verilator clock_enable*/;
wire gated_clk = clk & enable_r;
always_ff @ (posedge clk)
enable_r <= enable_early;
The clock_enable attribute will cause the clock gate to be ignored in the scheduling
algorithm, sometimes required for correct clock behavior, and always improving
performance. It's also a good idea to enable the IMPERFECTSCH warning, to ensure all
clock enables are properly recognized.
Same as "clock_enable" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator clocker*/
/*verilator no_clocker*/
Used after a signal declaration to indicate the signal is used as clock or not. This
information is used by Verilator to mark the signal as clocker and propagate the
clocker attribute automatically to derived signals. See "--clk" for more information.
Same as "clocker" and "no_clocker" in configuration files, see "CONFIGURATION FILES"
for more information.
/*verilator coverage_block_off*/
Specifies the entire begin/end block should be ignored for coverage analysis purposes.
Same as "coverage_block_off" in configuration files, see "CONFIGURATION FILES" for
more information.
/*verilator coverage_off*/
Specifies that following lines of code should have coverage disabled. Often used to
ignore an entire module for coverage analysis purposes.
/*verilator coverage_on*/
Specifies that following lines of code should have coverage re-enabled (if appropriate
--coverage flags are passed) after being disabled earlier with /*verilator
coverage_off*/.
/*verilator inline_module*/
Specifies the module the comment appears in may be inlined into any modules that use
this module. This is useful to speed up simulation runtime with some small loss of
trace visibility and modularity. Note signals under inlined submodules will be named
submodule__DOT__subsignal as C++ does not allow "." in signal names. When tracing
such signals the tracing routines will replace the __DOT__ with the period.
Same as "inline" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator isolate_assignments*/
Used after a signal declaration to indicate the assignments to this signal in any
blocks should be isolated into new blocks. When there is a large combinatorial block
that is resulting in a UNOPTFLAT warning, attaching this to the signal causing a false
loop may clear up the problem.
IE, with the following
reg splitme /* verilator isolate_assignments*/;
// Note the placement of the semicolon above
always @* begin
if (....) begin
splitme = ....;
other assignments
end
end
Verilator will internally split the block that assigns to "splitme" into two blocks:
It would then internally break it into (sort of):
// All assignments excluding those to splitme
always @* begin
if (....) begin
other assignments
end
end
// All assignments to splitme
always @* begin
if (....) begin
splitme = ....;
end
end
Same as "isolate_assignments" in configuration files, see "CONFIGURATION FILES" for
more information.
/*verilator lint_off msg*/
Disable the specified warning message for any warnings following the comment.
/*verilator lint_on msg*/
Re-enable the specified warning message for any warnings following the comment.
/*verilator lint_restore*/
After a /*verilator lint_save*/, pop the stack containing lint message state. Often
this is useful at the bottom of include files.
/*verilator lint_save*/
Push the current state of what lint messages are turned on or turned off to a stack.
Later meta-comments may then lint_on or lint_off specific messages, then return to the
earlier message state by using /*verilator lint_restore*/. For example:
// verilator lint_save
// verilator lint_off SOME_WARNING
... // code needing SOME_WARNING turned off
// verilator lint_restore
If SOME_WARNING was on before the lint_off, it will now be restored to on, and if it
was off before the lint_off it will remain off.
/*verilator no_inline_module*/
Specifies the module the comment appears in should not be inlined into any modules
that use this module. This is useful especially at the top level module to reduce the
size of the interface class, to aid compile time at a small performance loss.
Same as "no_inline" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator no_inline_task*/
Used in a function or task variable definition section to specify the function or task
should not be inlined into where it is used. This may reduce the size of the final
executable when a task is used a very large number of times. For this flag to work,
the task and tasks below it must be pure; they cannot reference any variables outside
the task itself.
Same as "no_inline" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public*/ (parameter)
Used after a parameter declaration to indicate the emitted C code should have the
parameter values visible. Due to C++ language restrictions, this may only be used on
64-bit or narrower integral enumerations.
parameter [2:0] PARAM /*verilator public*/ = 2'b0;
/*verilator public*/ (typedef enum)
Used after an enum typedef declaration to indicate the emitted C code should have the
enum values visible. Due to C++ language restrictions, this may only be used on 64-bit
or narrower integral enumerations.
typedef enum logic [2:0] { ZERO = 3'b0 } pub_t /*verilator public*/;
/*verilator public*/ (variable)
Used after an input, output, register, or wire declaration to indicate the signal
should be declared so that C code may read or write the value of the signal. This
will also declare this module public, otherwise use /*verilator public_flat*/.
Instead of using public variables, consider instead making a DPI or public function
that accesses the variable. This is nicer as it provides an obvious entry point that
is also compatible across simulators.
Same as "public" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public*/ (task/function)
Used inside the declaration section of a function or task declaration to indicate the
function or task should be made into a C++ function, public to outside callers.
Public tasks will be declared as a void C++ function, public functions will get the
appropriate non-void (bool, uint32_t, etc) return type. Any input arguments will
become C++ arguments to the function. Any output arguments will become C++ reference
arguments. Any local registers/integers will become function automatic variables on
the stack.
Wide variables over 64 bits cannot be function returns, to avoid exposing
complexities. However, wide variables can be input/outputs; they will be passed as
references to an array of 32-bit numbers.
Generally, only the values of stored state (flops) should be written, as the model
will NOT notice changes made to variables in these functions. (Same as when a signal
is declared public.)
You may want to use DPI exports instead, as it's compatible with other simulators.
Same as "public" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public_flat*/ (variable)
Used after an input, output, register, or wire declaration to indicate the signal
should be declared so that C code may read or write the value of the signal. This
will not declare this module public, which means the name of the signal or path to it
may change based upon the module inlining which takes place.
Same as "public_flat" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public_flat_rd*/ (variable)
Used after an input, output, register, or wire declaration to indicate the signal
should be declared public_flat (see above), but read-only.
Same as "public_flat_rd" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public_flat_rw @(<edge_list>) */ (variable)
Used after an input, output, register, or wire declaration to indicate the signal
should be declared public_flat_rd (see above), and also writable, where writes should
be considered to have the timing specified by the given sensitivity edge list. Set
for all variables, ports and wires using the --public-flat-rw switch.
Same as "public_flat_rw" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator public_module*/
Used after a module statement to indicate the module should not be inlined (unless
specifically requested) so that C code may access the module. Verilator automatically
sets this attribute when the module contains any public signals or `systemc_
directives. Also set for all modules when using the --public switch.
Same as "public" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator sc_clock*/
Deprecated. Used after an input declaration to indicate the signal should be declared
in SystemC as a sc_clock instead of a bool. This was needed in SystemC 1.1 and 1.2
only; versions 2.0 and later do not require clock pins to be sc_clocks and this is no
longer needed.
/*verilator sc_bv*/
Used after a port declaration. It sets the port to be of sc_bv<width> type, instead
of bool, vluint32_t or vluint64_t. This may be useful if the port width is
parameterized and different of such modules interface a templated module (such as a
transactor) or for other reasons. In general you should avoid using this attribute
when not necessary as with increasing usage of sc_bv the performance decreases
significantly.
Same as "sc_bv" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator sformat*/
Attached to the final input of a function or task "input string" to indicate the
function or task should pass all remaining arguments through $sformatf. This allows
creation of DPI functions with $display like behavior. See the
test_regress/t/t_dpi_display.v file for an example.
Same as "sformat" in configuration files, see "CONFIGURATION FILES" for more
information.
/*verilator tag <text...>*/
Attached after a variable or structure member to indicate opaque (to Verilator) text
that should be passed through to the XML output as a tag, for use by downstream
applications.
/*verilator tracing_off*/
Disable waveform tracing for all future signals that are declared in this module, or
cells below this module. Often this is placed just after a primitive's module
statement, so that the entire module and cells below it are not traced.
/*verilator tracing_on*/
Re-enable waveform tracing for all future signals or cells that are declared.
LANGUAGE LIMITATIONS
There are some limitations and lack of features relative to a commercial simulator, by
intent. User beware.
It is strongly recommended you use a lint tool before running this program. Verilator
isn't designed to easily uncover common mistakes that a lint program will find for you.
Synthesis Subset
Verilator supports only the Synthesis subset with a few minor additions such as $stop,
$finish and $display. That is, you cannot use hierarchical references, events or similar
features of the Verilog language. It also simulates as Synopsys's Design Compiler would;
namely a block of the form:
always @ (x) y = x & z;
This will recompute y when there is even a potential for change in x or a change in z,
that is when the flops computing x or z evaluate (which is what Design Compiler will
synthesize.) A compliant simulator would only calculate y if x changes. Use Verilog-
Mode's /*AS*/ or Verilog 2001's always @* to reduce missing activity items. Avoid putting
$displays in combo blocks, as they may print multiple times when not desired, even on
compliant simulators as event ordering is not specified.
Signal Naming
To avoid conflicts with C symbol naming, any character in a signal name that is not
alphanumeric nor a single underscore will be replaced by __0hh where hh is the hex code of
the character. To avoid conflicts with Verilator's internal symbols, any double underscore
are replaced with ___05F (5F is the hex code of an underscore.)
Bind
Verilator only supports "bind" to a target module name, not an instance path.
Dotted cross-hierarchy references
Verilator supports dotted references to variables, functions and tasks in different
modules. However, references into named blocks and function-local variables are not
supported. The portion before the dot must have a constant value; for example a[2].b is
acceptable, while a[x].b is not.
References into generated and arrayed instances use the instance names specified in the
Verilog standard; arrayed instances are named {cellName}[{instanceNumber}] in Verilog,
which becomes {cellname}__BRA__{instanceNumber}__KET__ inside the generated C++ code.
Verilator creates numbered "genblk" when a begin: name is not specified around a block
inside a generate statement. These numbers may differ between other simulators, but the
Verilog specification does not allow users to use these names, so it should not matter.
If you are having trouble determining where a dotted path goes wrong, note that Verilator
will print a list of known scopes to help your debugging.
Floating Point
Short floating point (shortreal) numbers are converted to real.
Latches
Verilator is optimized for edge sensitive (flop based) designs. It will attempt to do the
correct thing for latches, but most performance optimizations will be disabled around the
latch.
Structures and Unions
Verilator only presently supports packed structs and packed unions. Rand and randc tags
on members are simply ignored. All structures and unions are represented as a single
vector, which means that generating one member of a structure from blocking, and another
from non-blocking assignments is unsupported.
Time
All delays (#) are ignored, as they are in synthesis.
Unknown states
Verilator is mostly a two state simulator, not a four state simulator. However, it has
two features which uncover most initialization bugs (including many that a four state
simulator will miss.)
Identity comparisons (=== or !==) are converted to standard ==/!== when neither side is a
constant. This may make the expression result differ from a four state simulator. An ===
comparison to X will always be false, so that Verilog code which checks for uninitialized
logic will not fire.
Assigning a variable to a X will actually assign the variable to a random value (see the
--x-assign switch and +verilator+rand+reset simulation runtime switch.) Thus if the value
is actually used, the random value should cause downstream errors. Integers also
randomize, even though the Verilog 2001 specification says they initialize to zero.
All variables, depending on --x-initial setting, are typically randomly initialized using
a function. By running several random simulation runs you can determine that reset is
working correctly. On the first run, the function initializes variables to zero. On the
second, have it initialize variables to one. On the third and following runs have it
initialize them randomly. If the results match, reset works. (Note this is what the
hardware will really do.) In practice, just setting all variables to one at startup finds
most problems (since typically control signals are active-high).
--x-assign applies to variables explicitly initialized or assigned to X. Uninitialized
clocks are initialized to zero, while all other state holding variables are initialized to
a random value. Event driven simulators will generally trigger an edge on a transition
from X to 1 ("posedge") or X to 0 ("negedge"). However, by default, since clocks are
initialized to zero, Verilator will not trigger an initial negedge. Some code
(particularly for reset) may rely on X->0 triggering an edge. The --x-initial-edge switch
enables this behavior. Comparing runs with and without this switch will find such
problems.
Tri/Inout
Verilator converts some simple tristate structures into two state. Pullup, pulldown,
bufif0, bufif1, notif0, notif1, pmos, nmos, tri0 and tri1 are also supported. Simple
comparisons with === 1'bz are also supported.
An assignment of the form:
inout driver;
wire driver = (enable) ? output_value : 1'bz;
Will be converted to
input driver; // Value being driven in from "external" drivers
output driver__en; // True if driven from this module
output driver__out; // Value being driven from this module
External logic will be needed to combine these signals with any external drivers.
Tristate drivers are not supported inside functions and tasks; an inout there will be
considered a two state variable that is read and written instead of a four state variable.
Functions & Tasks
All functions and tasks will be inlined (will not become functions in C.) The only
support provided is for simple statements in tasks (which may affect global variables).
Recursive functions and tasks are not supported. All inputs and outputs are automatic, as
if they had the Verilog 2001 "automatic" keyword prepended. (If you don't know what this
means, Verilator will do what you probably expect -- what C does. The default behavior of
Verilog is different.)
Generated Clocks
Verilator attempts to deal with generated and enabled clocks correctly, however some cases
cause problems in the scheduling algorithm which is optimized for performance. The safest
option is to have all clocks as primary inputs to the model, or wires directly attached to
primary inputs. For proper behavior clock enables may also need the /*verilator
clock_enable*/ attribute.
Ranges must be big-bit-endian
Bit ranges must be numbered with the MSB being numbered greater or the same as the LSB.
Little-bit-endian buses [0:15] are not supported as they aren't easily made compatible
with C++.
Gate Primitives
The 2-state gate primitives (and, buf, nand, nor, not, or, xnor, xor) are directly
converted to behavioral equivalents. The 3-state and MOS gate primitives are not
supported. Tables are not supported.
Specify blocks
All specify blocks and timing checks are ignored.
Array Initialization
When initializing a large array, you need to use non-delayed assignments. Verilator will
tell you when this needs to be fixed; see the BLKLOOPINIT error for more information.
Array Out of Bounds
Writing a memory element that is outside the bounds specified for the array may cause a
different memory element inside the array to be written instead. For power-of-2 sized
arrays, Verilator will give a width warning and the address. For non-power-of-2-sizes
arrays, index 0 will be written.
Reading a memory element that is outside the bounds specified for the array will give a
width warning and wrap around the power-of-2 size. For non-power-of-2 sizes, it will
return a unspecified constant of the appropriate width.
Assertions
Verilator is beginning to add support for assertions. Verilator currently only converts
assertions to simple "if (...) error" statements, and coverage statements to increment the
line counters described in the coverage section.
Verilator does not support SEREs yet. All assertion and coverage statements must be
simple expressions that complete in one cycle. (Arguably SEREs are much of the point, but
one must start somewhere.)
Encrypted Verilog
Open source simulators like Verilator are unable to use encrypted RTL (i.e. IEEE P1735).
Talk to your IP vendor about delivering IP blocks via Verilator's --protect-lib feature.
Language Keyword Limitations
This section describes specific limitations for each language keyword.
`__FILE__, `__LINE__, `begin_keywords, `begin_keywords, `begin_keywords, `begin_keywords,
`begin_keywords, `define, `else, `elsif, `end_keywords, `endif, `error, `ifdef, `ifndef,
`include, `line, `systemc_ctor, `systemc_dtor, `systemc_header, `systemc_imp_header,
`systemc_implementation, `systemc_interface, `timescale, `undef, `verilog
Fully supported.
always, always_comb, always_ff, always_latch, and, assign, begin, buf, byte, case, casex,
casez, default, defparam, do-while, else, end, endcase, endfunction, endgenerate,
endmodule, endspecify, endtask, final, for, function, generate, genvar, if, initial,
inout, input, int, integer, localparam, logic, longint, macromodule, module, nand,
negedge, nor, not, or, output, parameter, posedge, reg, scalared, shortint, signed,
supply0, supply1, task, time, tri, typedef, var, vectored, while, wire, xnor, xor
Generally supported.
++, -- operators
Increment/decrement can only be used as standalone statements or in for loops. They
cannot be used as side effect operators inside more complicate expressions ("a =
b++;").
'{} operator
Assignment patterns with order based, default, constant integer (array) or member
identifier (struct/union) keys are supported. Data type keys and keys which are
computed from a constant expression are not supported.
`uselib
Uselib, a vendor specific library specification method, is ignored along with anything
following it until the end of that line.
cast operator
Casting is supported only between simple scalar types, signed and unsigned, not arrays
nor structs.
chandle
Treated as a "longint"; does not yet warn about operations that are specified as
illegal on chandles.
disable
Disable statements may be used only if the block being disabled is a block the disable
statement itself is inside. This was commonly used to provide loop break and continue
functionality before SystemVerilog added the break and continue keywords.
inside
Inside expressions may not include unpacked array traversal or $ as an upper bound.
Case inside and case matches are also unsupported.
interface
Interfaces and modports, including with generated data types are supported. Generate
blocks around modports are not supported, nor are virtual interfaces nor unnamed
interfaces.
priority if, unique if
Priority and unique if's are treated as normal ifs and not asserted to be full nor
unique.
specify specparam
All specify blocks and timing checks are ignored.
string
String is supported only to the point that they can be assigned, concatenated,
compared, and passed to DPI imports. Standard method calls on strings are not
supported.
timeunit, timeprecision
All timing control statements are ignored.
uwire
Verilator does not perform warning checking on uwires, it treats the uwire keyword as
if it were the normal wire keyword.
$bits, $countones, $error, $fatal, $finish, $info, $isunknown, $onehot, $onehot0,
$readmemb, $readmemh, $signed, $stime, $stop, $time, $unsigned, $warning.
Generally supported.
$display, $write, $fdisplay, $fwrite, $swrite
$display and friends must have a constant format string as the first argument (as with
C's printf). The rare usage which lists variables standalone without a format is not
supported.
$displayb, $displayh, $displayo, $writeb, $writeh, $writeo, etc
The sized display functions are rarely used and so not supported. Replace them with a
$write with the appropriate format specifier.
$finish, $stop
The rarely used optional parameter to $finish and $stop is ignored.
$fopen, $fclose, $fdisplay, $feof, $fflush, $fgetc, $fgets, $fscanf, $fwrite
File descriptors passed to the file PLI calls must be file descriptors, not MCDs,
which includes the mode parameter to $fopen being mandatory.
$fscanf, $sscanf
The formats %r, %v, and %z are not supported.
$fullskew, $hold, $nochange, $period, $recovery, $recrem, $removal, $setup, $setuphold,
$skew, $timeskew, $width
All specify blocks and timing checks are ignored.
$random
$random does not support the optional argument to set the seed. Use the srand
function in C to accomplish this, and note there is only one random number generator
(not one per module).
$readmemb, $readmemh
Read memory commands should work properly. Note Verilator and the Verilog
specification does not include support for readmem to multi-dimensional arrays.
$test$plusargs, $value$plusargs
Supported, but the instantiating C++/SystemC testbench must call
Verilated::commandArgs(argc, argv);
to register the command line before calling $test$plusargs or $value$plusargs.
$timeformat
Not supported as Verilator needs to determine all formatting at compile time.
Generally you can just ifdef them out for no ill effect. Note also VL_TIME_MULTIPLIER
can be defined at compile time to move the decimal point when displaying all times,
model wide.
ERRORS AND WARNINGS
Warnings may be disabled in three ways. First, when the warning is printed it will
include a warning code. Simply surround the offending line with a warn_off/warn_on pair:
// verilator lint_off UNSIGNED
if (`DEF_THAT_IS_EQ_ZERO <= 3) $stop;
// verilator lint_on UNSIGNED
Second, warnings may be disabled using a configuration file with a lint_off command. This
is useful when a script is suppressing warnings and the Verilog source should not be
changed.
Warnings may also be globally disabled by invoking Verilator with the "-Wno-warning"
switch. This should be avoided, as it removes all checking across the designs, and
prevents other users from compiling your code without knowing the magic set of disables
needed to successfully compile your design.
List of all warnings:
ALWCOMBORDER
Warns that an always_comb block has a variable which is set after it is used. This
may cause simulation-synthesis mismatches, as not all commercial simulators allow this
ordering.
always_comb begin
a = b;
b = 1;
end
Ignoring this warning will only suppress the lint check, it will simulate correctly.
ASSIGNIN
Error that an assignment is being made to an input signal. This is almost certainly a
mistake, though technically legal.
input a;
assign a = 1'b1;
Ignoring this warning will only suppress the lint check, it will simulate correctly.
ASSIGNDLY
Warns that you have an assignment statement with a delayed time in front of it, for
example:
a <= #100 b;
assign #100 a = b;
Ignoring this warning may make Verilator simulations differ from other simulators,
however at one point this was a common style so disabled by default as a code style
warning.
BLKANDNBLK
BLKANDNBLK is an error that a variable comes from a mix of blocked and non-blocking
assignments. Generally, this is caused by a register driven by both combo logic and a
flop:
always @ (posedge clk) foo[0] <= ...
always @* foo[1] = ...
Simply use a different register for the flop:
always @ (posedge clk) foo_flopped[0] <= ...
always @* foo[0] = foo_flopped[0];
always @* foo[1] = ...
This is not illegal in SystemVerilog, but a violation of good coding practice.
Verilator reports this as an error, because ignoring this warning may make Verilator
simulations differ from other simulators.
It is generally safe to disable this error (with a "// verilator lint_off BLKANDNBLK"
metacomment or the -Wno-BLKANDNBLK option) when one of the assignments is inside a
public task, or when the blocked and non-blocking assignments have non-overlapping
bits and structure members.
BLKSEQ
This indicates that a blocking assignment (=) is used in a sequential block.
Generally non-blocking/delayed assignments (<=) are used in sequential blocks, to
avoid the possibility of simulator races. It can be reasonable to do this if the
generated signal is used ONLY later in the same block, however this style is generally
discouraged as it is error prone.
always @ (posedge clk) foo = ...
Disabled by default as this is a code style warning; it will simulate correctly.
BLKLOOPINIT
This indicates that the initialization of an array needs to use non-delayed
assignments. This is done in the interest of speed; if delayed assignments were used,
the simulator would have to copy large arrays every cycle. (In smaller loops, loop
unrolling allows the delayed assignment to work, though it's a bit slower than a non-
delayed assignment.) Here's an example
always @ (posedge clk)
if (~reset_l) begin
for (i=0; i<`ARRAY_SIZE; i++) begin
array[i] = 0; // Non-delayed for verilator
end
This message is only seen on large or complicated loops because Verilator generally
unrolls small loops. You may want to try increasing --unroll-count (and occasionally
--unroll-stmts) which will raise the small loop bar to avoid this error.
BOUNDED
This indicates that bounded queues (e.g. "var name[$ : 3]") are unsupported.
Ignoring this warning may make Verilator simulations differ from other simulators.
BSSPACE
Warns that a backslash is followed by a space then a newline. Likely the intent was to
have a backslash directly followed by a newline (e.g. when making a `define) and
there's accidentally whitespace at the end of the line. If the space is not
accidental, suggest removing the backslash in the code as it serves no function.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
CASEINCOMPLETE
Warns that inside a case statement there is a stimulus pattern for which there is no
case item specified. This is bad style, if a case is impossible, it's better to have
a "default: $stop;" or just "default: ;" so that any design assumption violations will
be discovered in simulation.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
CASEOVERLAP
Warns that inside a case statement you have case values which are detected to be
overlapping. This is bad style, as moving the order of case values will cause
different behavior. Generally the values can be respecified to not overlap.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
CASEX
Warns that it is simply better style to use casez, and "?" in place of "x"'s. See
<http://www.sunburst-design.com/papers/CummingsSNUG1999Boston_FullParallelCase_rev1_1.pdf>
Ignoring this warning will only suppress the lint check, it will simulate correctly.
CASEWITHX
Warns that a case statement contains a constant with a "x". Verilator is two-state so
interpret such items as always false. Note a common error is to use a "X" in a case
or casez statement item; often what the user instead intended is to use a casez with
"?".
Ignoring this warning will only suppress the lint check, it will simulate correctly.
CDCRSTLOGIC
With --cdc only, warns that asynchronous flop reset terms come from other than primary
inputs or flopped outputs, creating the potential for reset glitches.
CLKDATA
Warns that clock signal is mixed used with/as data signal. The checking for this
warning is enabled only if user has explicitly marked some signal as clocker using
command line option or in-source meta comment (see "--clk").
The warning can be disabled without affecting the simulation result. But it is
recommended to check the warning as this may degrade the performance of the Verilated
model.
CMPCONST
Warns that you are comparing a value in a way that will always be constant. For
example "X > 1" will always be true when X is a single bit wide.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
COLONPLUS
Warns that a :+ is seen. Likely the intent was to use +: to select a range of bits. If
the intent was a range that is explicitly positive, suggest adding a space, e.g. use
": +".
Ignoring this warning will only suppress the lint check, it will simulate correctly.
COMBDLY
Warns that you have a delayed assignment inside of a combinatorial block. Using
delayed assignments in this way is considered bad form, and may lead to the simulator
not matching synthesis. If this message is suppressed, Verilator, like synthesis,
will convert this to a non-delayed assignment, which may result in logic races or
other nasties. See
<http://www.sunburst-design.com/papers/CummingsSNUG2000SJ_NBA_rev1_2.pdf>
Ignoring this warning may make Verilator simulations differ from other simulators.
CONTASSREG
Error that a continuous assignment is setting a reg. According to IEEE Verilog, but
not SystemVerilog, a wire must be used as the target of continuous assignments.
This error is only reported when "--language 1364-1995", "--language 1364-2001", or
"--language 1364-2005" is used.
Ignoring this error will only suppress the lint check, it will simulate correctly.
DECLFILENAME
Warns that a module or other declaration's name doesn't match the filename with path
and extension stripped that it is declared in. The filename a
modules/interfaces/programs is declared in should match the name of the module etc. so
that -y directory searching will work. This warning is printed for only the first
mismatching module in any given file, and -v library files are ignored.
Disabled by default as this is a code style warning; it will simulate correctly.
DEFPARAM
Warns that the "defparam" statement was deprecated in Verilog 2001 and all designs
should now be using the #(...) format to specify parameters.
Disabled by default as this is a code style warning; it will simulate correctly.
DETECTARRAY
Error when Verilator tries to deal with a combinatorial loop that could not be
flattened, and which involves a datatype which Verilator cannot handle, such as an
unpacked struct or a large unpacked array. This typically occurs when -Wno-UNOPTFLAT
has been used to override an UNOPTFLAT warning (see below).
The solution is to break the loop, as described for UNOPTFLAT.
DIDNOTCONVERGE
Error at simulation runtime when model did not properly settle.
Verilator sometimes has to evaluate combinatorial logic multiple times, usually around
code where a UNOPTFLAT warning was issued, but disabled. For example:
always_comb b = ~a;
always_comb a = b
This code will toggle forever, and thus to prevent an infinite loop, the executable
will give the didn't converge error.
To debug this, first is to review any UNOPTFLAT warnings that were ignored. Though
typically it is safe to ignore UNOPTFLAT (at a performance cost), at the time of
issuing a UNOPTFLAT Verilator didn't know if they would eventually converge and
assumed the would.
Next, run Verilator with --prof-cfuncs. Run make on the generated files with
"CPP_FLAGS=-DVL_DEBUG", to allow enabling simulation runtime debug messages. Rerun
the test. Now just before the convergence error you should see additional output
similar to this:
CHANGE: filename.v:1: b
CHANGE: filename.v:2: a
This means that signal b and signal a keep changing, inspect the code that modifies
these signals. Note if many signals are getting printed then most likely all of them
are oscillating. It may also be that e.g. "a" may be oscillating, then "a" feeds
signal "c" which then is also reported as oscillating.
Finally, rare, more difficult cases can be debugged like a "C" program; either enter
GDB and use its tracing facilities, or edit the generated C++ code to add appropriate
prints to see what is going on.
ENDLABEL
Warns that a label attached to a "end"-something statement does not match the label
attached to the block start.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
GENCLK
Warns that the specified signal is generated, but is also being used as a clock.
Verilator needs to evaluate sequential logic multiple times in this situation. In
somewhat contrived cases having any generated clock can reduce performance by almost a
factor of two. For fastest results, generate ALL clocks outside in C++/SystemC and
make them primary inputs to your Verilog model. (However once need to you have even
one, don't sweat additional ones.)
Ignoring this warning may make Verilator simulations differ from other simulators.
IFDEPTH
Warns that if/if else statements have exceeded the depth specified with --if-depth, as
they are likely to result in slow priority encoders. Unique and priority if
statements are ignored. Solutions include changing the code to a case statement, or a
SystemVerilog 'unique if' or 'priority if'.
Disabled by default as this is a code style warning; it will simulate correctly.
IGNOREDRETURN
Warns that a non-void function is being called as a task, and hence the return value
is being ignored.
This warning is required by IEEE. The portable way to suppress this warning (in
SystemVerilog) is to use a void cast, e.g.
void'(function_being_called_as_task());
Ignoring this warning will only suppress the lint check, it will simulate correctly.
IMPERFECTSCH
Warns that the scheduling of the model is not absolutely perfect, and some manual code
edits may result in faster performance. This warning defaults to off, is not part of
-Wall, and must be turned on explicitly before the top module statement is processed.
IMPLICIT
Warns that a wire is being implicitly declared (it is a single bit wide output from a
sub-module.) While legal in Verilog, implicit declarations only work for single bit
wide signals (not buses), do not allow using a signal before it is implicitly declared
by a cell, and can lead to dangling nets. A better option is the /*AUTOWIRE*/ feature
of Verilog-Mode for Emacs, available from <https://www.veripool.org/verilog-mode>
Ignoring this warning will only suppress the lint check, it will simulate correctly.
IMPORTSTAR
Warns that an "import package::*" statement is in $unit scope. This causes the
imported symbols to pollute the global namespace, defeating much of the purpose of
having a package. Generally "import ::*" should only be used inside a lower scope such
as a package or module.
Disabled by default as this is a code style warning; it will simulate correctly.
IMPURE
Warns that a task or function that has been marked with /*verilator no_inline_task*/
references variables that are not local to the task. Verilator cannot schedule these
variables correctly.
Ignoring this warning may make Verilator simulations differ from other simulators.
INCABSPATH
Warns that an `include filename specifies an absolute path. This means the code will
not work on any other system with a different file system layout. Instead of using
absolute paths, relative paths (preferably without any directory specified whatever)
should be used, and +incdir used on the command line to specify the top include source
directories.
Disabled by default as this is a code style warning; it will simulate correctly.
INFINITELOOP
Warns that a while or for statement has a condition that is always true. and thus
result in an infinite loop if the statement ever executes.
This might be unintended behavior if the loop body contains statements that in other
statements that would make time pass, which Verilator is ignoring due to e.g. STMTDLY
warnings being disabled.
Ignoring this warning will only suppress the lint check, it will simulate correctly
(i.e. hang due to the infinite loop).
INITIALDLY
Warns that you have a delayed assignment inside of an initial or final block. If this
message is suppressed, Verilator will convert this to a non-delayed assignment. See
also the COMBDLY warning.
Ignoring this warning may make Verilator simulations differ from other simulators.
INSECURE
Warns that the combination of options selected may be defeating the attempt to
protect/obscure identifiers or hide information in the model. Correct the options
provided, or inspect the output code to see if the information exposed is acceptable.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
LITENDIAN
Warns that a packed vector is declared with little endian bit numbering (i.e. [0:7]).
Big endian bit numbering is now the overwhelming standard, and little numbering is now
thus often due to simple oversight instead of intent.
Also warns that a cell is declared with little endian range (i.e. [0:7] or [7]) and is
connected to a N-wide signal. Based on IEEE the bits will likely be backwards from
what you expect (i.e. cell [0] will connect to signal bit [N-1] not bit [0]).
Ignoring this warning will only suppress the lint check, it will simulate correctly.
MODDUP
Warns that a module has multiple definitions. Generally this indicates a coding
error, or a mistake in a library file and it's good practice to have one module per
file (and only put each file once on the command line) to avoid these issues. For
some gate level netlists duplicates are sometimes unavoidable, and MODDUP should be
disabled.
Ignoring this warning will cause the more recent module definition to be discarded.
MULTIDRIVEN
Warns that the specified signal comes from multiple always blocks. This is often
unsupported by synthesis tools, and is considered bad style. It will also cause
longer simulation runtimes due to reduced optimizations.
Ignoring this warning will only slow simulations, it will simulate correctly.
MULTITOP
Warns that there are multiple top level modules, that is modules not instantiated by
any other module, and both modules were put on the command line (not in a library).
Three likely cases:
1. A single module is intended to be the top. This warning then occurs because some
low level cell is being read in, but is not really needed as part of the design. The
best solution for this situation is to ensure that only the top module is put on the
command line without any flags, and all remaining library files are read in as
libraries with -v, or are automatically resolved by having filenames that match the
module names.
2. A single module is intended to be the top, the name of it is known, and all other
modules should be ignored if not part of the design. The best solution is to use the
--top-module option to specify the top module's name. All other modules that are not
part of the design will be for the most part ignored (they must be clean in syntax and
their contents will be removed as part of the Verilog module elaboration process.)
3. Multiple modules are intended to be design tops, e.g. when linting a library file.
As multiple modules are desired, disable the MULTITOP warning. All input/outputs will
go uniquely to each module, with any conflicting and identical signal names being
uniquified by adding a prefix based on the top module name followed by __02E (a
Verilator-encoded ASCII ".'). This renaming is done even if the two modules' signals
seem identical, e.g. multiple modules with a "clk" input.
PINCONNECTEMPTY
Warns that a cell instantiation has a pin which is connected to .pin_name(), e.g. not
another signal, but with an explicit mention of the pin. It may be desirable to
disable PINCONNECTEMPTY, as this indicates intention to have a no-connect.
Disabled by default as this is a code style warning; it will simulate correctly.
PINMISSING
Warns that a module has a pin which is not mentioned in a cell instantiation. If a
pin is not missing it should still be specified on the cell declaration with a empty
connection, using "(.pin_name())".
Ignoring this warning will only suppress the lint check, it will simulate correctly.
PINNOCONNECT
Warns that a cell instantiation has a pin which is not connected to another signal.
Disabled by default as this is a code style warning; it will simulate correctly.
PROCASSWIRE
Error that a procedural assignment is setting a wire. According to IEEE, a var/reg
must be used as the target of procedural assignments.
REALCVT
Warns that a real number is being implicitly rounded to an integer, with possible loss
of precision.
REDEFMACRO
Warns that you have redefined the same macro with a different value, for example:
`define MACRO def1
//...
`define MACRO otherdef
The best solution is to use a different name for the second macro. If this is not
possible, add a undef to indicate the code is overriding the value:
`define MACRO def1
//...
`undef MACRO
`define MACRO otherdef
SELRANGE
Warns that a selection index will go out of bounds:
wire vec[6:0];
initial out = vec[7]; // There is no 7
Verilator will assume zero for this value, instead of X. Note that in some cases this
warning may be false, when a condition upstream or downstream of the access means the
access out of bounds will never execute or be used.
wire vec[6:0];
initial begin
seven = 7;
...
if (seven != 7) out = vec[seven]; // Never will use vec[7]
SHORTREAL
Warns that Verilator does not support "shortreal" and they will be automatically
promoted to "real". The recommendation is to replace any "shortreal" in the code with
"real", as "shortreal" is not widely supported across industry tools.
Ignoring this warning may make Verilator simulations differ from other simulators, if
the increased precision of real affects your model or DPI calls.
STMTDLY
Warns that you have a statement with a delayed time in front of it, for example:
#100 $finish;
Ignoring this warning may make Verilator simulations differ from other simulators.
SYMRSVDWORD
Warning that a symbol matches a C++ reserved word and using this as a symbol name
would result in odd C compiler errors. You may disable this warning, but the symbol
will be renamed by Verilator to avoid the conflict.
SYNCASYNCNET
Warns that the specified net is used in at least two different always statements with
posedge/negedges (i.e. a flop). One usage has the signal in the sensitivity list and
body, probably as an async reset, and the other usage has the signal only in the body,
probably as a sync reset. Mixing sync and async resets is usually a mistake. The
warning may be disabled with a lint_off pragma around the net, or either flopped
block.
Disabled by default as this is a code style warning; it will simulate correctly.
TASKNSVAR
Error when a call to a task or function has a output from that task tied to a non-
simple signal. Instead connect the task output to a temporary signal of the
appropriate width, and use that signal to set the appropriate expression as the next
statement. For example:
task foo; output sig; ... endtask
always @* begin
foo(bus_we_select_from[2]); // Will get TASKNSVAR error
end
Change this to:
reg foo_temp_out;
always @* begin
foo(foo_temp_out);
bus_we_select_from[2] = foo_temp_out;
end
Verilator doesn't do this conversion for you, as some more complicated cases would
result in simulator mismatches.
TICKCOUNT
Warns that the number of ticks to delay a $past variable is greater than 10. At
present Verilator effectively creates a flop for each delayed signals, and as such any
large counts may lead to large design size increases.
Ignoring this warning will only slow simulations, it will simulate correctly.
UNDRIVEN
Warns that the specified signal is never sourced. Verilator is fairly liberal in the
usage calculations; making a signal public, or loading only a single array element
marks the entire signal as driven.
Disabled by default as this is a code style warning; it will simulate correctly.
UNOPT
Warns that due to some construct, optimization of the specified signal or block is
disabled. The construct should be cleaned up to improve simulation runtime.
A less obvious case of this is when a module instantiates two submodules. Inside
submodule A, signal I is input and signal O is output. Likewise in submodule B,
signal O is an input and I is an output. A loop exists and a UNOPT warning will
result if AI & AO both come from and go to combinatorial blocks in both submodules,
even if they are unrelated always blocks. This affects performance because Verilator
would have to evaluate each submodule multiple times to stabilize the signals crossing
between the modules.
Ignoring this warning will only slow simulations, it will simulate correctly.
UNOPTFLAT
Warns that due to some construct, optimization of the specified signal is disabled.
The signal specified includes a complete scope to the signal; it may be only one
particular usage of a multiply instantiated block. The construct should be cleaned up
to improve simulation runtime; two times better performance may be possible by fixing
these warnings.
Unlike the UNOPT warning, this occurs after netlist flattening, and indicates a more
basic problem, as the less obvious case described under UNOPT does not apply.
Often UNOPTFLAT is caused by logic that isn't truly circular as viewed by synthesis
which analyzes interconnection per-bit, but is circular to simulation which analyzes
per-bus:
wire [2:0] x = {x[1:0], shift_in};
This statement needs to be evaluated multiple times, as a change in "shift_in"
requires "x" to be computed 3 times before it becomes stable. This is because a
change in "x" requires "x" itself to change value, which causes the warning.
For significantly better performance, split this into 2 separate signals:
wire [2:0] xout = {x[1:0], shift_in};
and change all receiving logic to instead receive "xout". Alternatively, change it to
wire [2:0] x = {xin[1:0], shift_in};
and change all driving logic to instead drive "xin".
With this change this assignment needs to be evaluated only once. These sort of
changes may also speed up your traditional event driven simulator, as it will result
in fewer events per cycle.
The most complicated UNOPTFLAT path we've seen was due to low bits of a bus being
generated from an always statement that consumed high bits of the same bus processed
by another series of always blocks. The fix is the same; split it into two separate
signals generated from each block.
The UNOPTFLAT warning may also be due to clock enables, identified from the reported
path going through a clock gating cell. To fix these, use the clock_enable meta
comment described above.
The UNOPTFLAT warning may also occur where outputs from a block of logic are
independent, but occur in the same always block. To fix this, use the
isolate_assignments meta comment described above.
To assist in resolving UNOPTFLAT, the option "--report-unoptflat" can be used, which
will provide suggestions for variables that can be split up, and a graph of all the
nodes connected in the loop. See the Arguments section for more details.
Ignoring this warning will only slow simulations, it will simulate correctly.
UNOPTTHREADS
Warns that the thread scheduler was unable to partition the design to fill the
requested number of threads.
One workaround is to request fewer threads with "--threads".
Another possible workaround is to allow more MTasks in the simulation runtime, by
increasing the value of --threads-max-mtasks. More MTasks will result in more
communication and synchronization overhead at simulation runtime; the scheduler
attempts to minimize the number of MTasks for this reason.
Ignoring this warning will only slow simulations, it will simulate correctly.
UNPACKED
Warns that unpacked structs and unions are not supported.
Ignoring this warning will make Verilator treat the structure as packed, which may
make Verilator simulations differ from other simulators.
UNSIGNED
Warns that you are comparing a unsigned value in a way that implies it is signed, for
example "X < 0" will always be true when X is unsigned.
Ignoring this warning will only suppress the lint check, it will simulate correctly.
UNUSED
Warns that the specified signal is never used/consumed. Verilator is fairly liberal
in the usage calculations; making a signal public, a signal matching --unused-regexp
("*unused*") or accessing only a single array element marks the entire signal as used.
Disabled by default as this is a code style warning; it will simulate correctly.
A recommended style for unused nets is to put at the bottom of a file code similar to
the following:
wire _unused_ok = &{1'b0,
sig_not_used_a,
sig_not_used_yet_b, // To be fixed
1'b0};
The reduction AND and constant zeros mean the net will always be zero, so won't use
simulation runtime. The redundant leading and trailing zeros avoid syntax errors if
there are no signals between them. The magic name "unused" (-unused-regexp) is
recognized by Verilator and suppresses warnings; if using other lint tools, either
teach to tool to ignore signals with "unused" in the name, or put the appropriate
lint_off around the wire. Having unused signals in one place makes it easy to find
what is unused, and reduces the number of lint_off pragmas, reducing bugs.
USERINFO, USERWARN, USERERROR, USERFATAL
A SystemVerilog elaboration-time assertion print was executed.
VARHIDDEN
Warns that a task, function, or begin/end block is declaring a variable by the same
name as a variable in the upper level module or begin/end block (thus hiding the upper
variable from being able to be used.) Rename the variable to avoid confusion when
reading the code.
Disabled by default as this is a code style warning; it will simulate correctly.
WIDTH
Warns that based on width rules of Verilog, two operands have different widths.
Verilator generally can intuit the common usages of widths, and you shouldn't need to
disable this message like you do with most lint programs. Generally other than simple
mistakes, you have two solutions:
If it's a constant 0 that's 32 bits or less, simply leave it unwidthed. Verilator
considers zero to be any width needed.
Concatenate leading zeros when doing arithmetic. In the statement
wire [5:0] plus_one = from[5:0] + 6'd1 + carry[0];
The best fix, which clarifies intent and will also make all tools happy is:
wire [5:0] plus_one = from[5:0] + 6'd1 + {5'd0, carry[0]};
Ignoring this warning will only suppress the lint check, it will simulate correctly.
WIDTHCONCAT
Warns that based on width rules of Verilog, a concatenate or replication has an
indeterminate width. In most cases this violates the Verilog rule that widths inside
concatenates and replicates must be sized, and should be fixed in the code.
wire [63:0] concat = {1, 2};
An example where this is technically legal (though still bad form) is:
parameter PAR = 1;
wire [63:0] concat = {PAR, PAR};
The correct fix is to either size the 1 ("32'h1"), or add the width to the parameter
definition ("parameter [31:0]"), or add the width to the parameter usage
("{PAR[31:0],PAR[31:0]}".
The following describes the less obvious errors:
Internal Error
This error should never occur first, though may occur if earlier warnings or error
messages have corrupted the program. If there are no other warnings or errors, submit
a bug report.
Unsupported: ....
This error indicates that you are using a Verilog language construct that is not yet
supported in Verilator. See the Limitations chapter.
DEPRECATIONS
The following deprecated items are scheduled for future removal:
Pre-C++11 compiler support
Verilator supports pre-C++11 compilers for non-threaded models when configured with
--enable-prec11. This flag will be removed and C++11 compilers will be required for
both compiling Verilator and compiling Verilated models no sooner than September 2020.
SystemC 2.1 and earlier support
Support for SystemC versions 2.1 and earlier and the related sc_clock variable
attribute will be removed no sooner than July 2020.
Configuration File -msg
The -msg argument to lint_off has been replaced with -rule. -msg is planned for
removal no sooner than January 2021.
XML locations
The XML "fl" attribute has been replaced with "loc". "fl" is planned for removal no
sooner than January 2021.
FAQ/FREQUENTLY ASKED QUESTIONS
Can I contribute?
Please contribute! Just file an issue asking for a merge, or ask on the forums if
looking for something to help on. For more information see our contributor agreement.
How commonly is Verilator used?
Verilator is used by many of the largest silicon design companies, and all the way
down to college projects. Verilator is a "big 4" simulator, meaning one of the 4 main
SystemVerilog simulators available, namely the commercial products Synopsys' VCS (tm),
Mentor's ModelSim (tm), Cadence Incisive/NC-Verilog/NC-Sim, and the open-source
Verilator. The three commercial choices are commonly collectively called the "big 3"
simulators.
Does Verilator run under Windows?
Yes, using Cygwin. Verilated output also compiles under Microsoft Visual C++, but
this is not tested every release.
Can you provide binaries?
Verilator is available as a RPM for Debian/Ubuntu, SuSE, Fedora, and other
distributions; this is done by porters and may slightly lag the primary distribution.
If there isn't a binary build for your distribution, how about you set one up? Please
contact the authors for assistance.
Note people sometimes request binaries when they are having problems with their C++
compiler. Alas, binaries won't help this, as in the end a fully working C++ compiler
is required to compile the output of Verilator.
How can it be faster than (name-the-commercial-simulator)?
Generally, the implied part is of the question is "... with all of the manpower they
can put into developing it."
Most commercial simulators have to be Verilog compliant, meaning event driven. This
prevents them from being able to reorder blocks and make netlist-style optimizations,
which are where most of the gains come from.
Non-compliance shouldn't be scary. Your synthesis program isn't compliant, so your
simulator shouldn't have to be -- and Verilator is closer to the synthesis
interpretation, so this is a good thing for getting working silicon.
Will Verilator output remain under my own license?
Yes, it's just like using GCC on your programs; this is why Verilator uses the "GNU
*Lesser* Public License Version 3" instead of the more typical "GNU Public License".
See the licenses for details, but in brief, if you change Verilator itself or the
header files Verilator includes, you must make the source code available under the GNU
Lesser Public License. However, Verilator output (the Verilated code) only "include"s
the licensed files, and so you are NOT required to release any output from Verilator.
You also have the option of using the Perl Artistic License, which again does not
require you release your Verilog or generated code, and also allows you to modify
Verilator for internal use without distributing the modified version. But please
contribute back to the community!
One limit is that you cannot under either license release a commercial Verilog
simulation product incorporating Verilator without making the source code available.
As is standard with Open Source, contributions back to Verilator will be placed under
the Verilator copyright and LGPL/Artistic license. Small test cases will be released
into the public domain so they can be used anywhere, and large tests under the
LGPL/Artistic, unless requested otherwise.
Why is running Verilator (to create a model) so slow?
Verilator needs more memory than the resulting simulator will require, as Verilator
creates internally all of the state of the resulting generated simulator in order to
optimize it. If it takes more than a minute or so (and you're not using --debug since
debug is disk bound), see if your machine is paging; most likely you need to run it on
a machine with more memory. Verilator is a full 64-bit application and may use more
than 4GB, but about 1GB is the maximum typically needed, and very large commercial
designs have topped 16GB.
How do I generate waveforms (traces) in C++?
See the next question for tracing in SystemC mode.
Add the --trace switch to Verilator, and in your top level C code, call
Verilated::traceEverOn(true). Then create a VerilatedVcdC object, and in your main
loop call "trace_object->dump(time)" every time step, and finally call
"trace_object->close()". For an example, see below and the
examples/make_tracing_c/sim_main.cpp file of the distribution.
You also need to compile verilated_vcd_c.cpp and add it to your link, preferably by
adding the dependencies in $(VK_GLOBAL_OBJS) to your Makefile's link rule. This is
done for you if using the Verilator --exe flag.
Note you can also call ->trace on multiple Verilated objects with the same trace file
if you want all data to land in the same output file.
#include "verilated_vcd_c.h"
...
int main(int argc, char** argv, char** env) {
...
Verilated::traceEverOn(true);
VerilatedVcdC* tfp = new VerilatedVcdC;
topp->trace(tfp, 99); // Trace 99 levels of hierarchy
tfp->open("obj_dir/t_trace_ena_cc/simx.vcd");
...
while (sc_time_stamp() < sim_time && !Verilated::gotFinish()) {
main_time += #;
tfp->dump(main_time);
}
tfp->close();
}
How do I generate waveforms (traces) in SystemC?
Add the --trace switch to Verilator, and in your top level C sc_main code, include
verilated_vcd_sc.h. Then call Verilated::traceEverOn(true). Then create a
VerilatedVcdSc object as you would create a normal SystemC trace file. For an
example, see the call to VerilatedVcdSc in the examples/make_tracing_sc/sc_main.cpp
file of the distribution, and below.
Alternatively you may use the C++ trace mechanism described in the previous question,
however the timescale and timeprecision will not inherited from your SystemC settings.
You also need to compile verilated_vcd_sc.cpp and verilated_vcd_c.cpp and add them to
your link, preferably by adding the dependencies in $(VK_GLOBAL_OBJS) to your
Makefile's link rule. This is done for you if using the Verilator --exe flag.
Note you can also call ->trace on multiple Verilated objects with the same trace file
if you want all data to land in the same output file.
#include "verilated_vcd_sc.h"
...
int main(int argc, char** argv, char** env) {
...
Verilated::traceEverOn(true);
VerilatedVcdSc* tfp = new VerilatedVcdSc;
topp->trace(tfp, 99); // Trace 99 levels of hierarchy
tfp->open("obj_dir/t_trace_ena_cc/simx.vcd");
...
sc_start(1);
...
tfp->close();
}
How do I generate FST waveforms (traces) in C++?
FST a format by GTKWave. This version provides a basic FST support. To dump FST
format, add the --trace-fst switch to Verilator and change the include path in the
testbench to:
#include "verilated_fst_c.h"
VerilatedFstC* tfp = new VerilatedFstC;
Note that currently supporting both FST and VCD in a single simulation is impossible,
but such requirement could be rare.
How do I generate FST waveforms (traces) in SystemC?
The FST library from GTKWave does not currently support SystemC; use VCD format
instead.
How do I view waveforms (traces)?
Verilator makes standard VCD (Value Change Dump) and FST files. VCD files are
viewable with the public domain GTKWave (recommended) or Dinotrace (legacy) programs,
or any of the many commercial offerings; FST is supported by GTKWave only.
How do I reduce the size of large waveform (trace) files?
First, instead of calling VerilatedVcdC->open at the beginning of time, delay calling
it until the time stamp where you want to tracing to begin. Likewise you can also
call VerilatedVcdC->open before the end of time (perhaps a short period after you
detect a verification error.)
Next, add /*verilator tracing_off*/ to any very low level modules you never want to
trace (such as perhaps library cells). Finally, use the --trace-depth option to limit
the depth of tracing, for example --trace-depth 1 to see only the top level signals.
Also be sure you write your trace files to a local solid-state disk, instead of to a
network disk. Network disks are generally far slower.
How do I do coverage analysis?
Verilator supports both block (line) coverage and user inserted functional coverage.
First, run verilator with the --coverage option. If you're using your own makefile,
compile the model with the GCC flag -DVM_COVERAGE (if using Verilator's, it will do
this for you.)
At the end of your test, call VerilatedCov::write passing the name of the coverage
data file (typically "logs/coverage.dat").
Run each of your tests in different directories. Each test will create a
logs/coverage.dat file.
After running all of your tests, verilator_coverage is executed. Verilator_coverage
reads the logs/coverage.dat file(s), and creates an annotated source code listing
showing code coverage details.
For an example, after running 'make test' in the Verilator distribution, see the
examples/make_tracing_c/logs directory. Grep for lines starting with '%' to see what
lines Verilator believes need more coverage.
Where is the translate_off command? (How do I ignore a construct?)
Translate on/off pragmas are generally a bad idea, as it's easy to have mismatched
pairs, and you can't see what another tool sees by just preprocessing the code.
Instead, use the preprocessor; Verilator defines the "VERILATOR" define for you, so
just wrap the code in an ifndef region:
`ifndef VERILATOR
Something_Verilator_Dislikes;
`endif
Most synthesis tools similarly define SYNTHESIS for you.
Why do I get "unexpected `do'" or "unexpected `bit'" errors?
Do, bit, ref, return, and other words are now SystemVerilog keywords. You should
change your code to not use them to ensure it works with newer tools. Alternatively,
surround them by the Verilog 2005/SystemVerilog begin_keywords pragma to indicate
Verilog 2001 code.
`begin_keywords "1364-2001"
integer bit; initial bit = 1;
`end_keywords
If you want the whole file to be parsed as Verilog 2001, just create a file with
`begin_keywords "1364-2001"
and add it before other Verilog files on the command line. (Note this will also
change the default for --prefix, so if you're not using --prefix, you will now need
to.)
How do I prevent my assertions from firing during reset?
Call Verilated::assertOn(false) before you first call the model, then turn it back on
after reset. It defaults to true. When false, all assertions controlled by --assert
are disabled.
Why do I get "undefined reference to `sc_time_stamp()'"?
In C++ (non SystemC) code you need to define this function so that the simulator knows
the current time. See the "CONNECTING TO C++" examples.
Why do I get "undefined reference to `VL_RAND_RESET_I' or `Verilated::...'"?
You need to link your compiled Verilated code against the verilated.cpp file found in
the include directory of the Verilator kit. This is one target in the
$(VK_GLOBAL_OBJS) make variable, which should be part of your Makefile's link rule.
If you use --exe, this is done for you.
Is the PLI supported?
Only somewhat. More specifically, the common PLI-ish calls $display, $finish, $stop,
$time, $write are converted to C++ equivalents. You can also use the "import DPI"
SystemVerilog feature to call C code (see the chapter above). There is also limited
VPI access to public signals.
If you want something more complex, since Verilator emits standard C++ code, you can
simply write your own C++ routines that can access and modify signal values without
needing any PLI interface code, and call it with $c("{any_c++_statement}").
How do I make a Verilog module that contain a C++ object?
You need to add the object to the structure that Verilator creates, then use $c to
call a method inside your object. The test_regress/t/t_extend_class files show an
example of how to do this.
How do I get faster build times?
When running make pass the make variable VM_PARALLEL_BUILDS=1 so that builds occur in
parallel.
Use a recent compiler. Newer compilers tend do be faster, with the now relatively old
GCC 3.0 to 3.3 being horrible.
Compile in parallel on many machines and use caching; see the web for the ccache,
distcc and icecream packages. ccache will skip GCC runs between identical source
builds, even across different users. You can use the OBJCACHE environment variable to
use these CC wrappers. Also see the --output-split option.
To reduce the compile time of classes that use a Verilated module (e.g. a top CPP
file) you may wish to add /*verilator no_inline_module*/ to your top level module.
This will decrease the amount of code in the model's Verilated class, improving
compile times of any instantiating top level C++ code, at a relatively small cost of
execution performance.
Why do so many files need to recompile when I add a signal?
Adding a new signal requires the symbol table to be recompiled. Verilator uses one
large symbol table, as that results in 2-3 less assembly instructions for each signal
access. This makes the execution time 10-15% faster, but can result in more
compilations when something changes.
How do I access functions/tasks in C?
Use the SystemVerilog Direct Programming Interface. You write a Verilog function or
task with input/outputs that match what you want to call in with C. Then mark that
function as an external function. See the DPI chapter in the manual.
How do I access signals in C?
The best thing is to make a SystemVerilog "export DPI task" or function that accesses
that signal, as described in the DPI chapter in the manual and DPI tutorials on the
web. This will allow Verilator to better optimize the model and should be portable
across simulators.
If you really want raw access to the signals, declare the signals you will be
accessing with a /*verilator public*/ comment before the closing semicolon. Then
scope into the C++ class to read the value of the signal, as you would any other
member variable.
Signals are the smallest of 8-bit chars, 16-bit shorts, 32-bit longs, or 64-bit long
longs that fits the width of the signal. Generally, you can use just uint32_t's for 1
to 32 bits, or vluint64_t for 1 to 64 bits, and the compiler will properly up-convert
smaller entities.
Signals wider than 64 bits are stored as an array of 32-bit uint32_t's. Thus to read
bits 31:0, access signal[0], and for bits 63:32, access signal[1]. Unused bits (for
example bit numbers 65-96 of a 65-bit vector) will always be zero. if you change the
value you must make sure to pack zeros in the unused bits or core-dumps may result.
(Because Verilator strips array bound checks where it believes them to be
unnecessary.)
In the SYSTEMC example above, if you had in our.v:
input clk /*verilator public*/;
// Note the placement of the semicolon above
From the sc_main.cpp file, you'd then:
#include "Vour.h"
#include "Vour_our.h"
cout << "clock is " << top->our->clk << endl;
In this example, clk is a bool you can read or set as any other variable. The value
of normal signals may be set, though clocks shouldn't be changed by your code or
you'll get strange results.
Should a module be in Verilog or SystemC?
Sometimes there is a block that just interconnects cells, and have a choice as to if
you write it in Verilog or SystemC. Everything else being equal, best performance is
when Verilator sees all of the design. So, look at the hierarchy of your design,
labeling cells as to if they are SystemC or Verilog. Then:
A module with only SystemC cells below must be SystemC.
A module with a mix of Verilog and SystemC cells below must be SystemC. (As Verilator
cannot connect to lower-level SystemC cells.)
A module with only Verilog cells below can be either, but for best performance should
be Verilog. (The exception is if you have a design that is instantiated many times;
in this case Verilating one of the lower modules and instantiating that Verilated
cells multiple times into a SystemC module *may* be faster.)
BUGS
First, check the coding limitations section.
Next, try the --debug switch. This will enable additional internal assertions, and may
help identify the problem.
Finally, reduce your code to the smallest possible routine that exhibits the bug. Even
better, create a test in the test_regress/t directory, as follows:
cd test_regress
cp -p t/t_EXAMPLE.pl t/t_BUG.pl
cp -p t/t_EXAMPLE.v t/t_BUG.v
There are many hits on how to write a good test in the driver.pl documentation which can
be seen by running:
cd $VERILATOR_ROOT # Need the original distribution kit
test_regress/driver.pl --help
Edit t/t_BUG.pl to suit your example; you can do anything you want in the Verilog code
there; just make sure it retains the single clk input and no outputs. Now, the following
should fail:
cd $VERILATOR_ROOT # Need the original distribution kit
cd test_regress
t/t_BUG.pl # Run on Verilator
t/t_BUG.pl --debug # Run on Verilator, passing --debug to Verilator
t/t_BUG.pl --vcs # Run on a commercial simulator
t/t_BUG.pl --nc|--iv|--ghdl # Likewise on other simulators
The test driver accepts a number of options, many of which mirror the main Verilator
option. For example the previous test could have been run with debugging enabled. The
full set of test options can be seen by running driver.pl --help as shown above.
Finally, report the bug using the bug tracker at <https://verilator.org/issues>. The bug
will become publicly visible; if this is unacceptable, mail the bug report to
"wsnyder@wsnyder.org".
HISTORY
Verilator was conceived in 1994 by Paul Wasson at the Core Logic Group at Digital
Equipment Corporation. The Verilog code that was converted to C was then merged with a C
based CPU model of the Alpha processor and simulated in a C based environment called CCLI.
In 1995 Verilator started being used also for Multimedia and Network Processor development
inside Digital. Duane Galbi took over active development of Verilator, and added several
performance enhancements. CCLI was still being used as the shell.
In 1998, through the efforts of existing DECies, mainly Duane Galbi, Digital graciously
agreed to release the source code. (Subject to the code not being resold, which is
compatible with the GNU Public License.)
In 2001, Wilson Snyder took the kit, and added a SystemC mode, and called it Verilator2.
This was the first packaged public release.
In 2002, Wilson Snyder created Verilator 3.000 by rewriting Verilator from scratch in C++.
This added many optimizations, yielding about a 2-5x performance gain.
In 2009, major SystemVerilog and DPI language support was added.
In 2018, Verilator 4.000 was released with multithreaded support.
Currently, various language features and performance enhancements are added as the need
arises. Verilator is now about 3x faster than in 2002, and is faster than many popular
commercial simulators.
AUTHORS
When possible, please instead report bugs to <https://verilator.org/issues>.
Wilson Snyder <wsnyder@wsnyder.org>
Major concepts by Paul Wasson, Duane Galbi, John Coiner and Jie Xu.
CONTRIBUTORS
Many people have provided ideas and other assistance with Verilator.
The major corporate sponsors of Verilator, by providing significant contributions of time
or funds include include Atmel Corporation, Cavium Inc., Compaq Corporation, Digital
Equipment Corporation, Embecosm Ltd., Hicamp Systems, Intel Corporation, Mindspeed
Technologies Inc., MicroTune Inc., picoChip Designs Ltd., Sun Microsystems Inc., Nauticus
Networks Inc., and SiCortex Inc.
The people who have contributed major functionality are Byron Bradley, Jeremy Bennett,
Lane Brooks, John Coiner, Duane Galbi, Todd Strader, Paul Wasson, Jie Xu, and Wilson
Snyder. Major testers included Jeff Dutton, Jonathon Donaldson, Ralf Karge, David Hewson,
Iztok Jeras, Wim Michiels, Alex Solomatnikov, Sebastien Van Cauwenberghe, Gene Weber, and
Clifford Wolf.
Some of the people who have provided ideas and feedback for Verilator include: David
Addison, Tariq B. Ahmad, Nikana Anastasiadis, Hans Van Antwerpen, Vasu Arasanipalai, Jens
Arm, Sharad Bagri, Matthew Ballance, Andrew Bardsley, Matthew Barr, Geoff Barrett, Julius
Baxter, Jeremy Bennett, Michael Berman, Victor Besyakov, David Binderman, Piotr Binkowski,
Johan Bjork, David Black, Tymoteusz Blazejczyk, Daniel Bone, Gregg Bouchard, Christopher
Boumenot, Nick Bowler, Byron Bradley, Bryan Brady, Maarten De Braekeleer, Charlie Brej, J
Briquet, Lane Brooks, John Brownlee, Jeff Bush, Lawrence Butcher, Tony Bybell, Ted
Campbell, Chris Candler, Lauren Carlson, Donal Casey, Sebastien Van Cauwenberghe, Alex
Chadwick, Terry Chen, Yi-Chung Chen, Enzo Chi, Robert A. Clark, Allan Cochrane, John
Coiner, Gianfranco Costamagna, George Cuan, Joe DErrico, Lukasz Dalek, Laurens van Dam,
Gunter Dannoritzer, Ashutosh Das, Bernard Deadman, John Demme, Mike Denio, John Deroo,
Philip Derrick, John Dickol, Ruben Diez, Danny Ding, Jacko Dirks, Ivan Djordjevic,
Jonathon Donaldson, Leendert van Doorn, Sebastian Dressler, Alex Duller, Jeff Dutton,
Tomas Dzetkulic, Usuario Eda, Charles Eddleston, Chandan Egbert, Joe Eiler, Ahmed El-
Mahmoudy, Trevor Elbourne, Robert Farrell, Eugen Fekete, Fabrizio Ferrandi, Udi
Finkelstein, Brian Flachs, Andrea Foletto, Bob Fredieu, Duane Galbi, Benjamin Gartner,
Christian Gelinek, Peter Gerst, Glen Gibb, Shankar Giri, Dan Gisselquist, Sam Gladstone,
Amir Gonnen, Chitlesh Goorah, Sergi Granell, Al Grant, Xuan Guo, Driss Hafdi, Neil
Hamilton, Oyvind Harboe, Jannis Harder, Junji Hashimoto, Thomas Hawkins, Mitch Hayenga,
Robert Henry, David Hewson, Jamey Hicks, Joel Holdsworth, Andrew Holme, Hiroki Honda, Alex
Hornung, David Horton, Jae Hossell, Alan Hunter, James Hutchinson, Jamie Iles, Ben
Jackson, Shareef Jalloq, Krzysztof Jankowski, HyungKi Jeong, Iztok Jeras, James Johnson,
Christophe Joly, Franck Jullien, James Jung, Mike Kagen, Arthur Kahlich, Kaalia Kahn, Guy-
Armand Kamendje, Vasu Kandadi, Kanad Kanhere, Patricio Kaplan, Pieter Kapsenberg, Ralf
Karge, Dan Katz, Sol Katzman, Jonathan Kimmitt, Olof Kindgren, Kevin Kiningham, Dan
Kirkham, Sobhan Klnv, Gernot Koch, Soon Koh, Steve Kolecki, Brett Koonce, Will Korteland,
Wojciech Koszek, Varun Koyyalagunta, David Kravitz, Roland Kruse, Sergey Kvachonok, Ed
Lander, Steve Lang, Stephane Laurent, Walter Lavino, Christian Leber, Larry Lee, Igor
Lesik, John Li, Eivind Liland, Yu Sheng Lin, Charlie Lind, Andrew Ling, Paul Liu, Derek
Lockhart, Jake Longo, Arthur Low, Stefan Ludwig, Dan Lussier, Fred Ma, Duraid Madina, Affe
Mao, Julien Margetts, Mark Marshall, Alfonso Martinez, Yves Mathieu, Patrick Maupin, Jason
McMullan, Elliot Mednick, Wim Michiels, Miodrag Milanovic, Wai Sum Mong, Peter Monsson,
Sean Moore, Dennis Muhlestein, John Murphy, Matt Myers, Richard Myers, Dimitris Nalbantis,
Peter Nelson, Bob Newgard, Cong Van Nguyen, Paul Nitza, Yossi Nivin, Pete Nixon, Lisa
Noack, Mark Nodine, Andreas Olofsson, Aleksander Osman, James Pallister, Brad Parker, Dan
Petrisko, Maciej Piechotka, David Pierce, Dominic Plunkett, David Poole, Mike Popoloski,
Roman Popov, Rich Porter, Niranjan Prabhu, Usha Priyadharshini, Mark Jackson Pulver,
Prateek Puri, Marshal Qiao, Danilo Ramos, Chris Randall, Anton Rapp, Josh Redford, Odd
Magne Reitan, Frederic Requin, Frederick Requin, Alberto Del Rio, Eric Rippey, Oleg
Rodionov, Paul Rolfe, Arjen Roodselaar, Tobias Rosenkranz, Jan Egil Ruud, Denis Rystsov,
John Sanguinetti, Galen Seitz, Salman Sheikh, Hao Shi, Mike Shinkarovsky, Rafael
Shirakawa, Jeffrey Short, Anderson Ignacio Da Silva, Rodney Sinclair, Steven Slatter,
Brian Small, Garrett Smith, Wilson Snyder, Stan Sokorac, Alex Solomatnikov, Wei Song, Art
Stamness, John Stevenson, Patrick Stewart, Rob Stoddard, Todd Strader, John Stroebel, Sven
Stucki, Howard Su, Emerson Suguimoto, Gene Sullivan, Renga Sundararajan, Rupert Swarbrick,
Yutetsu Takatsukasa, Peter Tengstrand, Wesley Terpstra, Rui Terra, Stefan Thiede, Gary
Thomas, Ian Thompson, Kevin Thompson, Mike Thyer, Hans Tichelaar, Viktor Tomov, Steve
Tong, Michael Tresidder, Neil Turton, Bogdan Vukobratovic, Holger Waechtler, Philipp
Wagner, Stefan Wallentowitz, Shawn Wang, Paul Wasson, Greg Waters, Thomas Watts, Eugene
Weber, David Welch, Thomas J Whatson, Leon Wildman, Gerald Williams, Trevor Williams, Jeff
Winston, Joshua Wise, Clifford Wolf, Johan Wouters, Junyi Xi, Ding Xiaoliang, Jie Xu,
Mandy Xu, Luke Yang, and Amir Yazdanbakhsh.
Thanks to them, and all those we've missed including above, or wished to remain anonymous.
DISTRIBUTION
The latest version is available from <https://verilator.org>.
Copyright 2003-2020 by Wilson Snyder. Verilator is free software; you can redistribute it
and/or modify the Verilator internals under the terms of either the GNU Lesser General
Public License Version 3 or the Perl Artistic License Version 2.0.
SEE ALSO
verilator_coverage, verilator_gantt, verilator_profcfunc, make,
"verilator --help" which is the source for this document,
and internals.txt in the distribution.