linux-kernel - Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN

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Message-ID: <20210225192941.GA2026@MSI.localdomain>
Date:   Thu, 25 Feb 2021 12:29:41 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Randy Dunlap <rdunlap@...radead.org>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] linux/compiler-clang.h: define HAVE_BUILTIN_BSWAP*

On Thu, Feb 25, 2021 at 05:45:09PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Separating compiler-clang.h from compiler-gcc.h inadventently dropped the
> definitions of the three HAVE_BUILTIN_BSWAP macros, which requires falling
> back to the open-coded version and hoping that the compiler detects it.
> 
> Since all versions of clang support the __builtin_bswap interfaces,
> add back the flags and have the headers pick these up automatically.
> 
> This results in a 4% improvement of compilation speed for arm defconfig.
> 
> Fixes: 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h mutually exclusive")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  include/linux/compiler-clang.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 6478bff6fcc2..bbfa9ff6a2ec 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -33,6 +33,16 @@
>  #define __no_sanitize_thread
>  #endif
>  
> +/*
> + * sparse (__CHECKER__) pretends to be gcc, but can't do constant
> + * folding in __builtin_bswap*() (yet), so don't set these for it.
> + */
> +#if defined(CONFIG_ARCH_USE_BUILTIN_BSWAP) && !defined(__CHECKER__)
> +#define __HAVE_BUILTIN_BSWAP32__
> +#define __HAVE_BUILTIN_BSWAP64__
> +#define __HAVE_BUILTIN_BSWAP16__
> +#endif /* CONFIG_ARCH_USE_BUILTIN_BSWAP && !__CHECKER__ */
> +
>  #if __has_feature(undefined_behavior_sanitizer)
>  /* GCC does not have __SANITIZE_UNDEFINED__ */
>  #define __no_sanitize_undefined \
> -- 
> 2.29.2
>