Cocotb: Verilator: Offer option to use FST Tracer

[markusdd]

Hi,

very short proposal: Verilator includes FST-Wavedump support for some time now, enabling it would only require an option variabe and instantiating a VerilatedFstC instead of the vcd tracer.

FSTs are much more space efficient compared to vcd's .

[cmarqu]

The main location that needs a change is probably this:
https://github.com/cocotb/cocotb/blob/66d1cad02c003d3e010993d186439394114041cb/cocotb/share/lib/verilator/verilator.cpp#L37-L43

[markusdd]

Yes. For FST, it should something like this (without the switch yet):

#if VM_TRACE
    Verilated::traceEverOn(true);

    std::unique_ptr<VerilatedFstC> tfp(new VerilatedFstC);
    top->trace(tfp.get(), 99);
    tfp->open("dump.fst");
#endif

[bluecmd]

+1! Another really cool feature is the support for enum values.

I tested the above patch and it worked (just needed an extra include as well, and needed to install libz-dev).

[bluecmd]

@cmarqu Would you be open for a first time code contribution for this? It doesn't look that hard and it would benefit me so I can give it a shot.

[themperek]

One could reuse VM_TRACE and add recognistion for VM_TRACE=fst or VM_TRACE=dump.fst ....

[cmarqu]

@bluecmd Yes, sure!

[markusdd]

Also, the makefile would need to change as well:

╰─ grep -ir Vcd .                                                                                                                                                                                           ─╯
./share/makefiles/simulators/Makefile.verilator:    @rm -f dump.vcd

[markusdd]

Closing here as my PR was just accepted.