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

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Message-ID: <20210312222447.rwnvmlp4i5jfmwyy@google.com>
Date:   Fri, 12 Mar 2021 14:24:47 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Prasad Sodagudi <psodagud@...cinc.com>,
        Nathan Chancellor <nathan@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] gcov: fix clang-11+ support

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@...glegroups.com> 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.