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

[Nathan Chancellor]

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@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>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Link: https://lkml.kernel.org/r/CAK8P3a0RJtbVi1JMsfik=jkHCNFv+DJn_FeDg-YLW+ueQW3tNg@xxxxxxxxxxxxxx
> ---
> Same version as in:
> https://lkml.kernel.org/r/20200526173312.GA30240@xxxxxxxxxx
> ---
>  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@xxxxxxxxx>
Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> # build