linux-kernel - Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression

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Date:   Tue, 26 May 2020 23:36:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Will Deacon <will@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.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>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement
 expression

On Tue, May 26, 2020 at 07:33:12PM +0200, Marco Elver wrote:
> 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) \
> 

Yeah, this shaves around 5% off of my kernel builds. The _Atomic hack is
every so slightly faster on GCC but apparently doesn't work on clang --
also, it's disguisting :-)

Ack!