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verilator

man page of verilator

verilator: Convert Verilog code to C++/SystemC

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 --sp [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 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 <//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 --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 --coverage Enable all coverage --coverage-line Enable line coverage --coverage-toggle Enable toggle coverage --coverage-user Enable PSL/SVL user coverage --coverage-underscore Enable covarge 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 +define+<var>+<value> Set preprocessor define --dump-tree Enable dumping .tree files -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 --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> 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 uint64_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 --output-split <bytes> Split .cpp files into pieces --output-split-cfuncs <statements> Split .ccp functions --pins-bv <bits> 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 --sc Create SystemC output --sp Create SystemPerl output --stats Create statistics file -sv Enable SystemVerilog parsing --top-module <topname> Name of top level input module --trace Enable waveform creation --trace-depth <levels> Depth of tracing --trace-max-array <bits> Maximum bit width for tracing --trace-max-width <depth> 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 -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 -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. --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; setting "--x-assign unique" may be desirable. --autoflush After every $display or $fdisplay, flush the output stream. This insures that messages will appear immediately but may reduce performance. 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. 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. --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"; -Dvar=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> =item --debugi-<srcfile> <level> Rarely needed - for developer use. Set internal debugging level globally or on the specified source file to the specified level. +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. This is enabled with --debug, so "--debug --no-dump-tree" may be useful if the dump files are large and not desired. -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 a 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. --gdbbt If --debug is specified, run Verilator underneath a GDB process and print a backtrace on exit. Without --debug or if GDB doesn't seem to work, this flag is ignored. --help Displays this message and program version and exits. -Idir Add the directory to the list of directories that should be searched for include directories or libraries. Same as +incdir and -y; +incdir and -y are fairly standard across Verilog tools while -I is an alias for GCC compatibility. --if-depth value Rarely needed. Set the depth at which the IFDEPTH warning will fire, defaults to 0 which disables this warning. +incdir+dir Add the directory to the list of directories that should be searched for include directories or libraries. Same as -I and -y; +incdir and -y are fairly standard across Verilog tools while -I is an alias for GCC compatibility. --inhibit-sim Rarely needed. Create a "inhibitSim(bool)" function to enable and disable evaluation. This allows a 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 Select the language to be used when first processing each Verilog file. The language value must be "1364-1995", "1364-2001", "1364-2001", "1364-2005", or "1800-2005". This should only be used for legacy code, as the preferable option is to edit the code to repair new keywords, or add appropriate 'begin_keywords. +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. -Ooptimization-letter Rarely needed. Enables or disables a specific optimizations, with the optimization selected based on the letter passed. A lowercase letter disables an optimization, an upper case letter enables it. This is intended for debugging use only; see the source code for version-dependent mappings of optimizations to -O letters. -o <executable> Specify the name for the final executable built if using --exe. Defaults to the --prefix if not specified. --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. --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/uint64_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. --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*/. --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. --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 bits 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 depth 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. -Uvar Undefines the given preprocessor symbol. --unroll-count loops Rarely needed. Specifies the maximum number of loop iterations that may be unrolled. See also BLKLOOPINIT warning. --unroll-stmts statements Rarely needed. Specifies the maximum number of statements in a loop for that loop to be unrolled. See also BLKLOOPINIT warning. --unused-regexp regexp Rarely needed. Specifies a simple regexp with * and ? that if a signal name matches will suppress the UNUSED warning. Defaults to "*unused*". Setting it to "" disables matching. -V Shows the verbose version, including configuration information compiled into Verilator. (Similar to perl -V.) -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. -Wall Enable all warnings, including code style warnings that are normally disabled by default. -Werror-message Convert the specified warning message into a 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-CASEINCOMPLETE -Wno-CASEOVERLAP -Wno-CASEX -Wno-CASEWITHX -Wno-CMPCONST -Wno-IMPLICIT -Wno-LITENDIAN -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-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-CASEINCOMPLETE -Wwarn-CASEOVERLAP -Wwarn-CASEX -Wwarn-CASEWITHX -Wwarn-CMPCONST -Wwarn-IMPLICIT -Wwarn-LITENDIAN -Wwarn-UNSIGNED -Wwarn-WIDTH". -Wwarn-style Enable all code style related warning messages. This is equivalent to "-Wwarn-DECLFILENAME -Wwarn-DEFPARAM -Wwarn-INCABSPATH -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. -y dir Add the directory to the list of directories that should be searched for include directories or libraries. Same as +incdir and -I; +incdir and +y are fairly standard across Verilog tools while -I is an alias for GCC compatibility.

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"); $finish; end endmodule EOF cat <<EOF >sim_main.cpp #include "Vour.h" #include "verilated.h" int main(int argc, char **argv, char **env) { Verilated::commandArgs(argc, argv); Vour* top = new Vour; while (!Verilated::gotFinish()) { top->eval(); } exit(0); } EOF If you installed Verilator from sources, or a tarball, but not as part of your operating system (as an RPM), first you need to point to the kit: export VERILATOR_ROOT=/path/to/where/verilator/was/installed export PATH=$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"); $finish; end endmodule EOF cat <<EOF >sc_main.cpp #include "Vour.h" int sc_main(int argc, char **argv) { Verilated::commandArgs(argc, argv); sc_clock clk ("clk",10, 0.5, 3, true); Vour* top; top = new Vour("top"); // SP_CELL (top, Vour); top->clk(clk); // SP_PIN (top, clk, clk); while (!Verilated::gotFinish()) { sc_start(1, SC_NS); } exit(0); } EOF If you installed Verilator from sources, or a tarball, but not as part of your operating system (as an RPM), first you need to point to the kit: export VERILATOR_ROOT=/path/to/where/verilator/was/installed export PATH=$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 $SYSTEMPERL/sp_preproc --preproc *.sp (You can also skip the above sp_preproc by getting pure SystemC from Verilator by replacing the verilator --sp flag in the previous step with -sc.) We then can compile it make -j -f Vour.mk Vour__ALL.a make -j -f Vour.mk ../sc_main.o verilated.o And link with SystemC. Note your path to the libraries may vary, depending on the operating system. export SYSTEMC=/path/to/where/systemc/was/built/or/installed g++ -L$SYSTEMC/lib-linux ../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 in lib/verilated.mk. 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". 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. 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. OBJCACHE_JOBS Optionally lists Make flags that specifies parallel make jobs. For example a 4 CPU system may use "-j 6" to request 6 parallel compiles when making. SYSTEMC Required for SystemC output mode. If set, specifies the directory containing the SystemC distribution. This is used to find the SystemC include files. If not specified, it will come from a default optionally specified at configure time (before Verilator was compiled). SYSTEMC_ARCH 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. 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_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 unsigned int main_time = 0; // Current simulation time double sc_time_stamp () { // Called by $time in Verilog return main_time; } 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) } 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 uint64_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); 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 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. 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, do-while, enum, export, final, import, int, logic, longint, package, program, shortint, time, typedef, 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 2009 (IEEE 1800-2009) Support Verilator implements a full SystemVerilog 2009 preprocessor, including function call-like preprocessor defines, default define arguments, '__FILE__, '__LINE__ and 'undefineall. Verilator currently has some support for SystemVerilog 2009 synthesis constructs. As SystemVerilog features enter common usage they are added; please file a bug if a feature you need is missing. 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 "//cadence" or "// ambit synthesis" 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 a 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 a accessor may result in cleaner code. '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.) This will cause the clock gate to be ignored in the scheduling algorithm, 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 a 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 a 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 a 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 a 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 a 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 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. 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 numbers are not synthesizable, and so not 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. Time All delays (#) are ignored, as they are in synthesis. Two State Verilator is 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.) First, 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. 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. All variables are 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. Tri/Inout Verilator converts some simple tristate structures into two state. An assignment of the form: inout driver; wire driver = (enable) ? output_value : 1'bz; Will be converted to input driver__in; // Value being driven in from "external" drivers output driver__en; // True if driven from this module output driver__enout; // Value being driven from this module Pullup, pulldown, bufif0, bufif1, notif0, notif1 are also supported. External logic will be needed to combine these signals with any external drivers. Tristate drivers are not supported inside functions and tasks; a 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 clocks correctly, however new cases may turn up bugs in the scheduling algorithm. The safest option is to have all clocks as primary inputs to the model, or wires directly attached to primary inputs. 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++;"). 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. 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. Verilator will convert the integer used to hold the file descriptor into a internal FILE*. To prevent core dumps due to mis-use, and because integers are 32 bits while FILE*s may be 64 bits, the descriptor must be stored in a reg [63:0] rather than an integer. The define 'verilator_file_descriptor in verilated.v can be used to hide this difference. $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. $realtime Treated as $time. $test$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: 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 //www.sunburst-design.com/papers/CummingsSNUG1999Boston_FullParallelCase_rev1_1.pdf <//www.sunburst- design.com/papers/CummingsSNUG1999Boston_FullParallelCase_rev1_1.pdf> Ignoring this warning will only suppress the lint check, it will simulate correctly. CASEWITHX Warns that a case statement contains a constant with a "x". Verilator is two-state so interpret such items as always false. Note a common error is to use a "X" in a case or casez statement item; often what the user instead intended is to use a casez with "?". Ignoring this warning will only suppress the lint check, it will simulate correctly. CDCRSTLOGIC With --cdc only, warns that asynchronous flop reset terms come from other than primary inputs or flopped outputs, creating the potential for reset glitches. 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 //www.sunburst-design.com/papers/CummingsSNUG2000SJ_NBA_rev1_2.pdf <//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. 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 <//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. LITENDIAN Warns that a 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. 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 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 a 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. Ignoring this warning will only slow simulations, it will simulate correctly. 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 a 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. 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 currently require SystemPerl output mode and the SystemPerl package. 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 a 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, and the Make::Cache package available from <//www.veripool.org/>. Make::Cache 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 uint64_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 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 test_regress t/t_BUG.pl Finally, report the bug using the bug tracker at <//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 Compaq Corporation, Digital Equipment Corporation, 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, Lane Brooks, Duane Galbi, Paul Wasson, and Wilson Snyder. Major testers include Jeff Dutton, Ralf Karge, David Hewson, Wim Michiels, and Gene Weber. Some of the people who have provided ideas and feedback for Verilator include: David Addison, Hans Van Antwerpen, 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, John Deroo, John Dickol, 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, Mike Kagen, Guy-Armand Kamendje, Vasu Kandadi, Patricio Kaplan, Ralf Karge, Dan Katz, Sol Katzman, Jonathan Kimmitt, Gernot Koch, Soon Koh, Steve Kolecki, Steve Lang, Stephane Laurent, Christian Leber, Charlie Lind, 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, 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, Holger Waechtler, Stefan Wallentowitz, Shawn Wang, Greg Waters, Eugene Weber, David Welch, Leon Wildman, Gerald Williams, Trevor Williams, Jeff Winston, Joshua Wise, Johan Wouters, and Ding Xiaoliang. Thanks all.

DISTRIBUTION

The latest version is available from <//www.veripool.org/>. Copyright 2003-2011 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 <//www.veripool.org/>. Wilson Snyder <wsnyder@wsnyder.org> Major concepts by Paul Wasson and Duane Galbi.
SEE ALSO
verilator_profcfunc, systemperl, vcoverage, make And internals.txt in the distribution. VERILATOR(1)
 
 
 

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