[ChiselSim] Flag `--trace-threads` is deprecated since Verilator v5.048

[Hamster5295]

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

1. Install verilator v5.048 or later
2. Write a ChiselSim test with a CompilationSettings of

CompilationSettings.default.withTraceStyle(Some(CompilationSettings.TraceStyle(CompilationSettings.TraceKind.Fst(Some(2)))))

The test content doesn't matter
3. The test fails even if no expect is violated

What is the current behavior?

Currently the Test would fail with message below:

%Warning-DEPRECATED: Option '--trace-threads' is deprecated and has no effect.
                     ... For warning description see https://verilator.org/warn/DEPRECATED?v=5.048
                     ... Use "/* verilator lint_off DEPRECATED */" and lint_on around source to disable this message.
%Error: Exiting due to 1 warning(s)
make[1]: *** [Makefile:12: simulation] Error 1

What is the expected behavior?

The test should pass without any issue

Please tell us about your environment:

• version: 7.11.0
• OS: Linux <MyHostName> 6.6.114.1-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Mon Dec 1 20:46:23 UTC 2025 x86_64 GNU/Linux

Other Information

None

What is the use case for changing the behavior?

I suggest that ChiselSim should be able to support newer Verilator versions for rolling-released systems like Arch Linux to work properly

[Hamster5295]

chisel/svsim/src/main/scala/verilator/Backend.scala

Line 30 in 4bfd079

Seq("--trace-fst") ++ traceThreads.map(n => Seq("--trace-threads", n.toString)).toSeq.flatten

This issue seems to be related here, occuring only when providing an explicit thread count to TraceKind.Fst(Some(<count>)).

Maybe version checking is required before consturcting the argument seq.

[seldridge]

I'm fine to drop it.

It's a reasonable idea to do version checks here, though I worry about the complexity of tracking the minutia of different tools supported options.

What I would probably suggest is to drop the option and modify ChiselSim to store and print the version of Verilator that it was tested with if the Verilator call fails, similar to how things work with firtool.

[Hamster5295]

LGTM. We can change this behaviour after verilator v5.048 and later being provided by major Linux distributions.