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SubjectRe: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof compilation time
On Wed, May 27, 2020 at 12:32:36PM +0200, 'Marco Elver' via Clang Built Linux 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@arndb.de>
> Signed-off-by: Marco Elver <elver@google.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will@kernel.org>
> Link: https://lkml.kernel.org/r/CAK8P3a0RJtbVi1JMsfik=jkHCNFv+DJn_FeDg-YLW+ueQW3tNg@mail.gmail.com
> ---
> Same version as in:
> https://lkml.kernel.org/r/20200526173312.GA30240@google.com
> ---
> include/linux/compiler_types.h | 22 +++++++++++++++++++++-
> 1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 5faf68eae204..a529fa263906 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -245,7 +245,9 @@ struct ftrace_likely_data {
> /*
> * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
> * non-scalar types unchanged.
> - *
> + */
> +#if defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900
> +/*
> * We build this out of a couple of helper macros in a vain attempt to
> * help you keep your lunch down while reading it.
> */
> @@ -267,6 +269,24 @@ struct ftrace_likely_data {
> __pick_integer_type(x, int, \
> __pick_integer_type(x, long, \
> __pick_integer_type(x, long long, x))))))
> +#else
> +/*
> + * If supported, prefer C11 _Generic for better compile-times. As above, 'char'
> + * is not type-compatible with 'signed char', and we define a separate case.
> + */
> +#define __scalar_type_to_expr_cases(type) \
> + type: (type)0, unsigned type: (unsigned type)0
> +
> +#define __unqual_scalar_typeof(x) typeof( \
> + _Generic((x), \
> + __scalar_type_to_expr_cases(char), \
> + signed char: (signed char)0, \
> + __scalar_type_to_expr_cases(short), \
> + __scalar_type_to_expr_cases(int), \
> + __scalar_type_to_expr_cases(long), \
> + __scalar_type_to_expr_cases(long long), \
> + default: (x)))
> +#endif
>
> /* Is this type a native word size -- useful for atomic operations */
> #define __native_word(t) \
> --
> 2.27.0.rc0.183.gde8f92d652-goog
>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build

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