NAME
    Verilator - Convert Verilog code to C++/SystemC

SYNOPSIS
        verilator --help
        verilator --version
        verilator --cc [options] [top_level.v]... [opt_c_files.cpp/c/cc/a/o/so]
        verilator --sc [options] [top_level.v]... [opt_c_files.cpp/c/cc/a/o/so]
        verilator --lint-only    [top_level.v]...

DESCRIPTION
    Verilator converts synthesizable (not behavioral) Verilog code, plus
    some Synthesis, SystemVerilog and a small subset of Verilog AMS and
    Sugar/PSL assertions, into C++, SystemC or SystemPerl code. It is not a
    complete simulator, just 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. For
    SystemPerl format, it outputs .sp files for the SystemPerl preprocessor,
    which greatly simplifies writing SystemC code and is available at
    <http://www.veripool.org>.

    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. See the detailed
    descriptions in the next sections for more information.

        {file.v}                    Verilog top level 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>
        --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
        --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 PSL/SVL user coverage
        --coverage-underscore       Enable coverage of _signals
         -D<var>[=<value>]          Set preprocessor define
        --debug                     Enable debugging
        --debug-check               Enable debugging assertions
        --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
        --dump-tree                 Enable dumping .tree files
        --dump-treei <level>        Enable dumping .tree files 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
        --gdb                       Run Verilator under GDB interactively
        --gdbbt                     Run Verilator under GDB for backtrace
        --help                      Display this help
         -I<dir>                    Directory to search for includes
        --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
         -LDLIBS <flags>            Linker library flags for makefile
        --language <lang>           Default language standard to parse
         +libext+<ext>+[ext]...     Extensions for finding modules
        --lint-only                 Lint, but do not make output
        --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-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 <bytes>      Split .cpp files into pieces
        --output-split-cfuncs <statements>   Split .cpp functions
        --output-split-ctrace <statements>   Split tracing functions
        --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
        --prefix <topname>          Name of top level class
        --profile-cfuncs            Name functions for profiling
        --private                   Debugging; see docs
        --psl                       Enable PSL parsing
        --public                    Debugging; see docs
        --report-unoptflat          Extra diagnostics for UNOPTFLAT
        --savable                   Enable model save-restore
        --sc                        Create SystemC output
        --sp                        Create SystemPerl output
        --stats                     Create statistics file
         -sv                        Enable SystemVerilog parsing
         +systemverilogext+<ext>    Synonym for +1800-2012ext+<ext>
        --top-module <topname>      Name of top level input module
        --trace                     Enable waveform creation
        --trace-depth <levels>      Depth of tracing
        --trace-max-array <depth>   Maximum bit width for tracing
        --trace-max-width <width>   Maximum array depth for tracing
        --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>
         -Werror-<message>          Convert warning to error
         -Wfuture-<message>         Disable unknown message warnings
         -Wno-<message>             Disable warning
         -Wno-lint                  Disable all lint warnings
         -Wno-style                 Disable all style warnings
         -Wno-fatal                 Disable fatal exit on warnings
        --x-assign <mode>           Initially assign Xs to this value
        --x-initial-edge            Enable initial X->0 and X->1 edge triggers
         -y <dir>                   Directory to search for modules

ARGUMENTS
    {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.
        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>". 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*
        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, includes enabling the --psl flag. (If psl is
        not desired, but other assertions are, use --assert --nopsl.)

        See also --x-assign and --x-initial-edge; setting "--x-assign
        unique" and/or "--x-initial-edge" may be desirable.

    --autoflush
        After every $display or $fdisplay, flush the output stream. This
        insures 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 NOPed, 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
        and the cmos and tran gate primitives. 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 flags to the generated makefiles. 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 and --sp.

    --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.

    --compiler *compiler-name*
        Enables tunings and work-arounds 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.pl.

        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 PSL which must be separately
        enabled with --psl.

        For example, the following PSL statement will add a coverage point,
        with the comment "DefaultClock":

           // psl default clock = posedge clk;
           // psl cover {cyc==9} report "DefaultClock,expect=1";

    -D*var*=*value*
        Defines the given preprocessor symbol. Same as +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, debugging messages, and intermediate form
        dump files.

    --debug-check
        Rarely needed. Enable internal debugging assertion checks, without
        changing debug verbosity. Enabled automatically when --debug
        specified.

    --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
        source file to the specified level. Higher levels produce more
        detailed messages (plain "--debug" is equivalent to "--debugi 4").

    --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" or "1800-2012".

        Any language associated with a particular file extension (see the
        various +*lang*ext+ 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 +*lang*ext+ options should be used.

        If no language is specified, either by this flag or +*lang*ext+
        options, then the latest SystemVerilog language (IEEE 1800-2012) is
        used.

    +define+*var*+*value*
        Defines the given preprocessor symbol. Same as -D; +define is fairly
        standard across Verilog tools while -D is an alias for GCC
        compatibility.

    --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>
        Rarely needed. Enable writing .tree debug files with a specific
        dumping level, 0 disbles dumps and is equivelent 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 or warnings are encountered, exit.
        Defaults to 50.

    --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.

    --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.

    --help
        Displays this message and program version and exits.

    -I*dir*
        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 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. 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.)

    --language *value*
        A synonym for "--default-langauge", 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.

    --MMD
        Enable creation of .d dependency files, used for make dependency
        detection, similar to gcc -MMD option. On by default, use --no-MMD
        to disable.

    --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.

    --mod-prefix *topname*
        Specifies the name to prepend to all lower level classes. Defaults
        to the same as --prefix.

    --no-pins64
        Backward compatible alias for "--pins-bv 33".

    --no-skip-identical
        Rarely needed. Disables skipping execution of Verilator if all
        source files are identical, and all output files exist with newer
        dates.

    +notimingchecks
        Ignored for compatibility with other simulators.

    -O0 Disables optimization of the model.

    -O3 Enables slow optimizations. This may reduce simulation runtimes at
        the cost of compile time. This currently sets --inline-mult -1.

    -O*optimization-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 *bytes*
        Enables splitting the output .cpp/.sp 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
        slow routines will be placed into __Slow files. This accelerates
        compilation by as optimization can be disabled on the slow routines,
        and the remaining files can be compiled on parallel machines. Using
        --output-split should have only a trivial impact on performance.
        With GCC 3.3 on a 2GHz Opteron, --output-split 20000 will result in
        splitting into approximately one-minute-compile chunks.

    --output-split-cfuncs *statements*
        Enables splitting functions in the output .cpp/.sp 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/.sp files into
        multiple functions. Defaults to same setting as
        --output-split-cfuncs.

    --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". 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.

    --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.

    --profile-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.

    --private
        Opposite of --public. Is the default; this option exists for
        backwards compatibility.

    --psl
        Enable PSL parsing. Without this switch, PSL meta-comments are
        ignored. See the --assert flag to enable all assertions, and
        --coverage-user to enable functional coverage.

    --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*/.

    --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 analysing 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.

    --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. 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 and -sp.

    --sp
        Specifies SystemPerl output mode; see also --cc and -sc.

    --stats
        Creates a dump file with statistics on the design in
        {prefix}__stats.txt.

    -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 "+1800-2012ext+"*ext*.

    --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.

    --trace
        Adds waveform tracing code to the model. 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 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-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 runtime and trace file size.

    --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.

    --trace-underscore
        Enable tracing of signals that start with an underscore. Normally,
        these signals are not output during tracing. See also
        --coverage-underscore.

    -U*var*
        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.)

    -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

    -Wall
        Enable all warnings, including code style warnings that are normally
        disabled by default.

    -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.

    -Wno-lint
        Disable all lint related warning messages, and all style warnings.
        This is equivalent to "-Wno-ALWCOMBORDER -Wno-CASEINCOMPLETE
        -Wno-CASEOVERLAP -Wno-CASEX -Wno-CASEWITHX -Wno-CMPCONST
        -Wno-ENDLABEL -Wno-IMPLICIT -Wno-LITENDIAN -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-INCABSPATH -Wno-PINNOCONNECT -Wno-SYNCASYNCNET
        -Wno-UNDRIVEN -Wno-UNUSED -Wno-VARHIDDEN".

    -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.

    -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-CASEINCOMPLETE
        -Wwarn-CASEOVERLAP -Wwarn-CASEX -Wwarn-CASEWITHX -Wwarn-CMPCONST
        -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 set as though --x-assign
        unique had been 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
        → 0 triggering a "negedge", particularly in reset sequences. Using
        --x-initial-edge with Verilator will replicate this behaviour. It
        will also ensure that 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 modelled design that should be
        addressed.

    -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.

EXAMPLE C++ EXECUTION
    We'll compile this example into C++.

        mkdir test_our
        cd test_our

        cat <<EOF >our.v
          module our;
             initial begin $display("Hello World"); $finish; end
          endmodule
        EOF

        cat <<EOF >sim_main.cpp
          #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

    If you installed Verilator from sources, or a tarball, but not as part
    of your operating system (as an RPM), first you need to point to the
    kit:

        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

        cd obj_dir
        make -j -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

        cd ..
        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. See the test_c directory in the distribution for an example.

EXAMPLE SYSTEMC EXECUTION
    This is an example similar to the above, but using SystemPerl.

        mkdir test_our_sc
        cd test_our_sc

        cat <<EOF >our.v
          module our (clk);
             input clk;  // Clock is required to get initial activation
             always @ (posedge clk)
                begin $display("Hello World"); $finish; end
          endmodule
        EOF

        cat <<EOF >sc_main.cpp
          #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");   // SP_CELL (top, Vour);
              top->clk(clk);           // SP_PIN  (top, clk, clk);
              while (!Verilated::gotFinish()) { sc_start(1, SC_NS); }
              delete top; 
              exit(0);
          }
        EOF

    If you installed Verilator from sources, or a tarball, but not as part
    of your operating system (as an RPM), first you need to point to the
    kit:

        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 --sp our.v

    Then we convert the SystemPerl output to SystemC.

        cd obj_dir
        export SYSTEMPERL=/path/to/where/systemperl/kit/came/from
        $SYSTEMPERL/sp_preproc --preproc *.sp

    (You can also skip the above sp_preproc by getting pure SystemC from
    Verilator by replacing the verilator --sp flag in the previous step with
    -sc.)

    We then can compile it

        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
        # 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 test_sp directory in the distribution for an example.

BENCHMARKING & OPTIMIZATION
    For best performance, run Verilator with the "-O3 --x-assign=fast
    --noassert" flags. The -O3 flag will require longer compile times, and
    --x-assign=fast may increase the risk of reset bugs in trade for
    performance; see the above documentation for these flags.

    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 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" -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 affect. 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.

    You may uncover further tuning possibilities by profiling the Verilog
    code. Use Verilator's --profile-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:

        {prefix}.mk                         // Make include file for compiling
        {prefix}_classes.mk                 // Make include file with class names

    For -cc and -sc mode, it also creates:

        {prefix}.cpp                        // Top level C++ file
        {prefix}.h                          // Top level header
        {prefix}{each_verilog_module}.cpp   // Lower level internal C++ files
        {prefix}{each_verilog_module}.h     // Lower level internal header files

    For -sp mode, instead of .cpp and .h it creates:

        {prefix}.sp                         // Top level SystemC file
        {prefix}{each_verilog_module}.sp    // Lower level internal SC files

    In certain optimization modes, it also creates:

        {prefix}__Dpi.h                     // DPI import and export declarations
        {prefix}__Inlines.h                 // Inline support functions
        {prefix}__Slow.cpp                  // Constructors and infrequent routines
        {prefix}__Syms.cpp                  // Global symbol table C++
        {prefix}__Syms.h                    // Global symbol table header
        {prefix}__Trace.cpp                 // Wave file generation code (--trace)
        {prefix}__cdc.txt                   // Clock Domain Crossing checks (--cdc)
        {prefix}__stats.txt                 // Statistics (--stats)

    It also creates internal files that can be mostly ignored:

        {each_verilog_module}.vpp           // Post-processed verilog (--debug)
        {prefix}.flags_vbin                 // Verilator dependencies
        {prefix}.flags_vpp                  // Pre-processor dependencies
        {prefix}__verFiles.dat              // Timestamps for skip-identical
        {prefix}{misc}.d                    // Make dependencies (-MMD)
        {prefix}{misc}.dot                  // Debugging graph files (--debug)
        {prefix}{misc}.tree                 // Debugging files (--debug)

    After running Make, the C++ compiler should produce the following:

        {prefix}                            // Final executable (w/--exe argument)
        {prefix}__ALL.a                     // Library of all Verilated objects
        {prefix}{misc}.o                    // Intermediate objects

ENVIRONMENT
    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, it 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.

    SYSTEMPERL
        Specifies the directory containing the SystemPerl distribution kit.
        This is used to find the SystemPerl library and include files. If
        not specified, it will come from a default optionally specified at
        configure time (before Verilator was compiled). See also
        SYSTEMPERL_INCLUDE.

    SYSTEMPERL_INCLUDE
        Specifies the directory containing the Verilog-Perl include .cpp
        files, from the src/ directory of the SystemPerl kit. If not
        specified, it will be computed from the SYSTEMPERL environment
        variable if it is set, and if SYSTEMPERL is not set
        SYSTEMPERL_INCLUDE will come from a default optionally specified at
        configure time (before Verilator was compiled).

    VCS_HOME
        If set, specifies the directory containing the Synopsys VCS
        distribution. When set, a 'make test' in the Verilator distribution
        will also run VCS baseline regression tests.

    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_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 test_c directory in the kit for an example.

    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 "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.

    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 (IE 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 it's 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 runtime requiring at best hundreds of instructions, while
    the direct references are evaluated by the compiler and result in only a
    couple of instructions.

  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.

        module t;
           reg readme   /*verilator public_flat_rd*/;
           reg writeme  /*verilator public_flat_rw @(posedge clk) */;
        endmodule

    There are many online tutorials and books on the VPI, but an example
    that accesses the above would be:

    void read_and_check() { vpiHandle vh1 =
    vpi_handle_by_name((PLI_BYTE8*)"t.readme", NULL); if (!vh1) { error... }
    const char* name = vpi_get_str(vpiName, vh1); printf("Module name:
    %s\n"); // 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"
    }

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.

  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.

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 -msg WIDTH
      lint_off -msg 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.

    The grammar of configuration commands is as follows:

    `verilator_config
        Take remaining text up the the next `verilog mode switch and treat
        it as Verilator configuration commands.

    coverage_off [-file "<filename>" [-lines <line> [ - <line> ]]]
        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_off [-msg <message>] [-file "<filename>" [-lines <line> [ -
    <line>]]]
        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).

        If the -msg is omitted, all lint warnings are disabled. This will
        override all later lint warning enables for the specified region.

    tracing_off [-file "<filename>" [-lines <line> [ - <line> ]]]
        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).

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.

  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 equivelents 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.

  Sugar/PSL Support
    Most future work is being directed towards improving SystemVerilog
    assertions instead of PSL. If you are using these PSL features, please
    contact the author as they may be depreciated in future versions.

    With the --assert switch, Verilator enables support of the Property
    Specification Language (PSL), specifically the simple PSL subset without
    time-branching primitives. Verilator currently only converts PSL
    assertions to simple "if (...) error" statements, and coverage
    statements to increment the line counters described in the coverage
    section.

    Verilator implements these keywords: assert, assume (same as assert),
    default (for clocking), countones, cover, isunknown, onehot, onehot0,
    report, and true.

    Verilator implements these operators: -> (logical if).

    Verilator does not support SEREs yet. All assertion and coverage
    statements must be simple expressions that complete in one cycle. PSL
    vmode/vprop/vunits are not supported. PSL statements must be in the
    module they reference, at the module level where you would put an
    initial... statement.

    Verilator only supports (posedge CLK) or (negedge CLK), where CLK is the
    name of a one bit signal. You may not use arbitrary expressions as
    assertion clocks.

  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 runtime and print an
    "Assertion failed" error message.

    Verilator likewise also asserts any "unique" or "priority" SystemVerilog
    keywords on case statements. However, "unique if" and "priority if" are
    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.

    _(*expr*)
        A underline followed by an expression in parenthesis returns a
        Verilog expression. This is different from normal parenthesis in
        special contexts, such as PSL expressions, and can be used to embed
        bit concatenation ({}) inside of PSL statements.

    $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. Same as /* verilator coverage_block_off */.

    `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.
        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. 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. 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. 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. 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. 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 the 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 were 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 insure all clock enables are
        properly recognized.

    /*verilator coverage_block_off*/
        Specifies the entire begin/end block should be ignored for coverage
        analysis purposes.

    /*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
        time 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. SystemPerl when tracing such signals will replace the __DOT__
        with the period.

    /*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

    /*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_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.

    /*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.

    /*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.

    /*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.

    /*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.

    /*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.

    /*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.

    /*verilator sc_clock*/
        Rarely needed. 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 parametrized 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
        increases significantly.

    /*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.

    /*verilator tracing_off*/
        Disable waveform tracing for all future signals that are declared in
        this module. Often this is placed just after a primitive's module
        statement, so that the entire module is not traced.

    /*verilator tracing_on*/
        Re-enable waveform tracing for all future signals 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.

  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
    Floating Point (real) numbers are supported.

  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.) 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 are 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.

    Note. --x-assign applies to variables explicitly initialized or assigned
    to X. Unititialized 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 (particulary for reset) may rely on X->0
    triggering an edge. Verilator provides a switch (see --x-initial-edge)
    to enable 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 busses [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.)

  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.

    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 passed to DPI
        imports.

    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
        Only integer formats are supported; %e, %f, %m, %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_MULTIPLER 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 two 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

    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 good coding practice anyways.

        It is also possible to disable this error when one of the
        assignments is inside a public task.

        Ignoring this warning may make Verilator simulations differ from
        other simulators.

    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.

    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_FullPa
        rallelCase_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.

    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.

    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.

    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 ocurrs when -Wno-UNOPTFLAT has been
        used to override an UNOPTFLAT warning (see below).

        The solution is to break the loop, as described for UNOPTFLAT.

    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.

    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, 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
        <http://www.veripool.org/>

        Ignoring this warning will only suppress the lint check, 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 +include used on the command line to specify the top
        include source directory.

        Disabled by default as this is a code style warning; it will
        simulate correctly.

    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.

    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.

        Ignoring this warning will only suppress the lint check, it will
        simulate correctly.

    MODDUP
        Error 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 to avoid these issues. For
        some gate level netlists duplicates are unavoidable, and this error
        may be disabled.

    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 runtimes due to reduced
        optimizations.

        Ignoring this warning will only slow simulations, it will simulate
        correctly.

    MULTITOP
        Error that there are multiple top level modules, that is modules not
        instantiated by any other module. Verilator only supports a single
        top level, if you need more, create a module that wraps all of the
        top modules.

        Often this error is because some low level cell is being read in,
        but is not really needed. The best solution is to insure that each
        module is in a unique file by the same name. Otherwise, make sure
        all library files are read in as libraries with -v, instead of
        automatically with -y.

    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.

    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]

    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
        Error 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 error message as you would disable warnings, 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.

    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 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
        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.

    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 sinked. 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 time. 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.

    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.

    Verilated model didn't converge
        Verilator sometimes has to evaluate combinatorial logic multiple
        times, usually around code where a UNOPTFLAT warning was issued, but
        disabled. For example:

           always @ (a)  b=~a;
           always @ (b)  a=b

        will toggle forever and thus the executable will give the didn't
        converge error to prevent an infinite loop.

        To debug this, run Verilator with --profile-cfuncs. Run make on the
        generated files with "OPT=-DVL_DEBUG". Then call Verilated::debug(1)
        in your main.cpp.

        This will cause each change in a variable to print a message. Near
        the bottom you'll see the code and variable that causes the problem.
        For the program above:

                CHANGE: filename.v:1: b
                CHANGE: filename.v:2: a

FAQ/FREQUENTLY ASKED QUESTIONS
    Does it run under Windows?
        Yes, using Cygwin. Verilated output should also compile under
        Microsoft Visual C++ Version 7 or newer, but this is not tested by
        the author.

    Can you provide binaries?
        Verilator is available as a RPM for SuSE, Fedora, and perhaps other
        systems; 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-simulator)?
        Generally, the implied part of the question is "... with all of
        their manpower they can put into 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 copyright?
        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, large tests under the LGPL/Artistic,
        unless requested otherwise.

    Why is Verilation so slow?
        Verilator needs more memory than the resulting simulator will
        require, as Verilator creates internally all of the state of the
        resulting 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.

    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 test_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.

        Note also older versions of Verilator used the SystemPerl package
        and SpTraceVcdC class. This still works, but is depreciated as it
        requires strong coupling between the Verilator and SystemPerl
        versions.

            #include "verilated_vcd_c.h"
            ...
            int main(int argc, char **argv, char **env) {
                ...
                Verilated::traceEverOn(true);
                VerilatedVcdC* tfp = new VerilatedVcdC;
                topp->trace (tfp, 99);
                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 test_sp/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);
                tfp->open ("obj_dir/t_trace_ena_cc/simx.vcd");
                ...
                sc_start(1);
                ...
                tfp->close();
            }

    How do I view waveforms (traces)?
        Verilator makes standard VCD (Value Change Dump) files. They are
        viewable with the public domain Dinotrace or GtkWave programs, or
        any of the many commercial offerings.

    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 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. Both require the SystemPerl package to be
        installed but do not require use of the SystemPerl output mode.

        First, run verilator with the --coverage option. If you're using
        your own makefile, compile the model with the GCC flag
        -DSP_COVERAGE_ENABLE (if using Verilator's, it will do this for
        you.)

        Run your tests in different directories. Each test will create a
        logs/coverage.pl file.

        After running all of your tests, the vcoverage utility (from the
        SystemPerl package) is executed. Vcoverage reads the
        logs/coverage.pl 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 test_sp/logs/coverage_source 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

    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 insure 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.

    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?
        Between GCC 3.0 to 3.3, each compiled progressively slower, thus if
        you can use GCC 2.95, or GCC 3.4 you'll have faster builds. Two ways
        to cheat are to compile on parallel machines and avoid compilations
        altogether. See the --output-split option, and 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.

    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->v->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 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
    <http://www.veripool.org/verilator>. 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 Verilator3 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.

    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.

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 Cavium Networks, Compaq
    Corporation, Digital Equipment Corporation, Embecosm Ltd., Hicamp
    Systems, Intel Corporation, Mindspeed Technologies Inc., MicroTune Inc.,
    picoChip Designs Ltd., Sun Microsystems, Nauticus Networks, and SiCortex
    Inc.

    The people who have contributed major functionality are Byron Bradley,
    Jeremy Bennett, Lane Brooks, Duane Galbi, Paul Wasson, and Wilson
    Snyder. Major testers include Jeff Dutton, Ralf Karge, David Hewson, Wim
    Michiels, Alex Solomatnikov, Sebastien Van Cauwenberghe and Gene Weber.

    Some of the people who have provided ideas and feedback for Verilator
    include: David Addison, Vasu Arasanipalai, Jens Arm, J Baxter, Jeremy
    Bennett, David Black, Gregg Bouchard, Christopher Boumenot, Nick Bowler,
    Byron Bradley, Bryan Brady, Lane Brooks, John Brownlee, Lawrence
    Butcher, Chris Candler, Lauren Carlson, Donal Casey, Terry Chen, Robert
    A. Clark, Allan Cochrane, Gunter Dannoritzer, Ashutosh Das, Bernard
    Deadman, Mike Denio, John Deroo, John Dickol, Ruben Diez, Danny Ding,
    Ivan Djordjevic, Alex Duller, Jeff Dutton, Chandan Egbert, Joe Eiler,
    Ahmed El-Mahmoudy, Robert Farrell, Eugen Fekete, Andrea Foletto, Bob
    Fredieu, Shankar Giri, Sam Gladstone, Amir Gonnen, Chitlesh Goorah, Neil
    Hamilton, Thomas Hawkins, David Hewson, Jae Hossell, Ben Jackson, Iztok
    Jeras, Christophe Joly, Mike Kagen, Guy-Armand Kamendje, Vasu Kandadi,
    Patricio Kaplan, Ralf Karge, Dan Katz, Sol Katzman, Jonathan Kimmitt,
    Gernot Koch, Soon Koh, Steve Kolecki, David Kravitz, Steve Lang,
    Stephane Laurent, Walter Lavino, Christian Leber, John Li, Charlie Lind,
    Andrew Ling, Paul Liu, Dan Lussier, Fred Ma, Duraid Madina, Mark
    Marshall, Jason McMullan, Wim Michiels, Dennis Muhlestein, John Murphy,
    Richard Myers, Dimitris Nalbantis, Paul Nitza, Pete Nixon, Lisa Noack,
    Mark Nodine, Andreas Olofsson, Brad Parker, Dominic Plunkett, Niranjan
    Prabhu, Usha Priyadharshini, Alberto Del Rio, Oleg Rodionov, John
    Sanguinetti, Salman Sheikh, Mike Shinkarovsky, Rafael Shirakawa, Jeffrey
    Short, Rodney Sinclair, Brian Small, Alex Solomatnikov, Art Stamness,
    John Stroebel, Emerson Suguimoto, Gene Sullivan, Renga Sundararajan,
    Stefan Thiede, Gary Thomas, Steve Tong, Hans Van Antwerpen, Holger
    Waechtler, Stefan Wallentowitz, Shawn Wang, Greg Waters, Thomas Watts,
    Eugene Weber, David Welch, Leon Wildman, Gerald Williams, Trevor
    Williams, Jeff Winston, Joshua Wise, Johan Wouters, and Ding Xiaoliang.

    Thanks to them, and all those we've missed including above.

DISTRIBUTION
    The latest version is available from <http://www.veripool.org/>.

    Copyright 2003-2013 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.

AUTHORS
    When possible, please instead report bugs to <http://www.veripool.org/>.

    Wilson Snyder <wsnyder@wsnyder.org>

    Major concepts by Paul Wasson and Duane Galbi.

SEE ALSO
    verilator_profcfunc, systemperl, vcoverage, make,

    "verilator --help" which is the source for this document,

    and internals.txt in the distribution.

