iverilog 10.2 internal error in bsc.misc/fwrite

[kenta2]

In the course of testing release 2025.07, the following error occurred in testsuite/bsc.misc/fwrite :

internal error: vpi_get_value(<format=2>) not implemented for __vpiVThrVec4Stack.
vvp: vpi_vthr_vector.cc:373: virtual void __vpiVThrVec4Stack::vpi_get_value(p_vpi_value): Assertion `0' failed.

The error occurred in the following files:

sysFWrite2.v-vcd.out
sysFWrite2.v.out
sysFWrite3.v-vcd.out
sysFWrite3.v.out
sysFWrites.v-vcd.out
sysFWrites.v.out

The error only occurred on Centos 7 and Rocky Linux 8, which are the only two platforms in which we test with iverilog 10.2 .

For Centos 7 (a clone of Redhat Enterprise Linux 7), we were testing with iverilog 10.2 because that is the version of iverilog packaged by Redhat. In Rocky Linux 8 (a clone of Redhat Enterprise Linux 8), Redhat stopped packaging iverilog entirely, so we built iverilog from source, choosing version 10.2 only because that is the version packaged in Centos 7.

iverilog 11.0 (built from source tag v11_0 of https://github.com/steveicarus/iverilog ) works for both platforms, so I have gone ahead with the release.

I recommend that we document that iverilog 11.0 or newer is required, and that iverilog 10.2 packaged with Centos 7 is known to fail.

Attached are the complete contents of testsuite/bsc.misc/fwrite for both failing platforms.

fwritebscfailure-centos7.tar.gz
fwritebscfailure-rocky8.tar.gz

[quark17]

Thanks for all the effort in testing and recording this!

Documenting a minimum version of iverilog would be good. In README.md, this can go next to the line about requirements for the testsuite. And we should also say something in testsuite/README.md, probably with an additional sentence after the apt command for iverilog (mentioning that the testsuite has guards on tests known to fail with certain versions of iverilog). I can try to draft a PR?

Note, though, that we can also mark the failure as expected in the testsuite .exp file. For example, there's this code in testsuite/bsc.verilog/tasks/tasks.exp:

# This fails with iverilog 10.1 and 10.2, but fixed in 10.3
# See https://github.com/steveicarus/iverilog/issues/198
#
set iverilog_bug {}
if { $verilog_compiler == "iverilog" &&
     ($verilog_compiler_version == "10.1" || $verilog_compiler_version == "10.2") } {
    # add 'iverilog' to the list of simulators to exempt
    set iverilog_bug $verilog_compiler
}
test_c_veri_bsv EvenMoreDisplay {} {} $iverilog_bug

The issue referred to in that comment is the same one you encountered. (And suggests that version 10.3 might be OK, but using 11 as a minimum is fine.)

For fwrite.exp, there's also the written file to check, that's separate from the test function:

test_veri_only_bsv FWrites
compare_file FWrites.dat.out

In the absence of a significant change like bundling that check into the test function or making the test function return a status code, I think we'd have to write something like this:

test_veri_only_bsv FWrites {} $iverilog_bug
if { [string equal $iverilog_bug ""] } {
    compare_file FWrites.dat.out
}