linux-kernel - Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar

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Date:   Wed, 27 May 2020 13:26:51 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Marco Elver <elver@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof
 compilation time

On Wed, May 27, 2020 at 12:33 PM Marco Elver <elver@...gle.com> wrote:
>
> If the compiler supports C11's _Generic, use it to speed up compilation
> times of __unqual_scalar_typeof(). GCC version 4.9 or later and
> all supported versions of Clang support the feature (the oldest
> supported compiler that doesn't support _Generic is GCC 4.8, for which
> we use the slower alternative).
>
> The non-_Generic variant relies on multiple expansions of
> __pick_integer_type -> __pick_scalar_type -> __builtin_choose_expr,
> which increases pre-processed code size, and can cause compile times to
> increase in files with numerous expansions of READ_ONCE(), or other
> users of __unqual_scalar_typeof().
>
> Summary of compile-time benchmarking done by Arnd Bergmann [1]:
>
>         <baseline normalized time>  clang-11   gcc-9
>         this patch                      0.78    0.91
>         ideal                           0.76    0.86
>
> [1] https://lkml.kernel.org/r/CAK8P3a3UYQeXhiufUevz=rwe09WM_vSTCd9W+KvJHJcOeQyWVA@mail.gmail.com
>
> Further compile-testing done with:
>         gcc 4.8, 4.9, 5.5, 6.4, 7.5, 8.4;
>         clang 9, 10.
>
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Marco Elver <elver@...gle.com>

This gives us back 80% of the performance drop on clang, and 50%
of the drop I saw with gcc, compared to current mainline.

Tested-by: Arnd Bergmann <arnd@...db.de>