Re: [PATCH] gcov: fix clang-11+ support

[Fangrui Song]

On 2021-03-12, Nick Desaulniers wrote:
> On Fri, Mar 12, 2021 at 12:25 PM 'Fangrui Song' via Clang Built Linux
> <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > function_name can be unconditionally deleted. It is not used by llvm-cov
> > gcov.  You'll need to delete a few assignments to gcov_info_free but you
> > can then unify the gcov_fn_info_dup and gcov_info_free implementations.
> >
> > LG. On big-endian systems, clang < 11 emitted .gcno/.gcda files do not
> > work with llvm-cov gcov < 11.  To fix it and make .gcno/.gcda work with
> > gcc gcov I chose to break compatibility (and make all the breaking
> > changes like deleting some CC1 options) in a short window. At that time
> > I was not aware that there is the kernel implementation. Later on I was
> > CCed on a few https://github.com/ClangBuiltLinux/linux/ gcov issues but
> > I forgot to mention the interface change.
>
> These are all good suggestions. Since in v2 I'll drop support for
> clang < 11, I will skip additional patches to disable GCOV when using
> older clang for BE, and the function_name cleanup.

Only llvm_gcda_start_file & llvm_gcda_emit_function need version
dispatch. In that case (since there will just be two #if in the file) we don't even need

  depends on CC_IS_GCC || CLANG_VERSION >= 110000

> > Now in clang 11 onward, clang --coverage defaults to the gcov 4.8
> > compatible format. You can specify the CC1 option (internal option,
> > subject to change) -coverage-version to make it compatible with other
> > versions' gcov.
> >
> > -Xclang -coverage-version='407*' => 4.7
> > -Xclang -coverage-version='704*' => 7.4
> > -Xclang -coverage-version='B02*' => 10.2 (('B'-'A')*10 = 10)
>
> How come LLVM doesn't default to 10.2 format, if it can optionally
> produce it?  We might be able to reuse more code in the kernel between
> the two impelementations, though I expect the symbols the runtime is
> expected to provide will still differ. Seeing the `B` in `B02*` is
> also curious.
>
> Thanks for the review, will include your tag in v2.

4.8 has the widest range of compiler support. gcov 4.8~7.* use the same format.

clang instrumentation does not support the column field (useless in my opinion) introduced in gcov 9, so it just writes zeros.