Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof compilation time

[Arnd Bergmann]

On Wed, May 27, 2020 at 12:33 PM Marco Elver <elver@xxxxxxxxxx> 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@xxxxxxxxxxxxxx
>
> 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@xxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

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@xxxxxxxx>