On Wed, May 27, 2020 at 12:33 PM Marco Elver <[email protected]> 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+kvjhjcoeqy...@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 <[email protected]> > Signed-off-by: Marco Elver <[email protected]>
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 <[email protected]>
