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

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Message-ID: <CAKwvOd=8WM9KVoPxy7x9kQR5cEz1M+hJ4Pbz8kU+5ipMSzSJ+A@mail.gmail.com>
Date:   Fri, 12 Mar 2021 14:05:02 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Fangrui Song <maskray@...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 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.

> 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.
-- 
Thanks,
~Nick Desaulniers