linux-kernel - Re: [PATCH] ARM: xor-neon: Replace __GNUC__ checks with CONFIG_CC_IS

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Date:   Thu, 30 May 2019 16:00:12 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Nicolas Pitre <nico@...xnic.net>,
        Stefan Agner <stefan@...er.ch>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: xor-neon: Replace __GNUC__ checks with CONFIG_CC_IS_GCC

On Tue, May 28, 2019 at 4:57 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> Currently, when compiling this code with clang, the following warning is
> emitted:
>
>     CC      arch/arm/lib/xor-neon.o
>   arch/arm/lib/xor-neon.c:33:2: warning: This code requires at least
>   version 4.6 of GCC [-W#warnings]
>
> This is because clang poses as GCC 4.2.1 with its __GNUC__ conditionals
> for glibc compatibility[1]:
>
> $ echo | clang -dM -E -x c /dev/null | grep GNUC | awk '{print $2" "$3}'
> __GNUC_MINOR__ 2
> __GNUC_PATCHLEVEL__ 1
> __GNUC_STDC_INLINE__ 1
> __GNUC__ 4
>
> As pointed out by Ard Biesheuvel and Arnd Bergmann in an earlier
> thread[2], the oldest version of GCC that is currently supported is gcc
> 4.6 after commit cafa0010cd51 ("Raise the minimum required gcc version
> to 4.6") so we do not need to check for anything older anymore.
>
> However, just removing the version check is not enough to silence clang
> because it does not recognize '#pragma GCC optimize':
>
>   arch/arm/lib/xor-neon.c:25:13: warning: unknown pragma ignored
>   [-Wunknown-pragmas]
>   #pragma GCC optimize "tree-vectorize"
>
> Looking into it further, -ftree-vectorize (which '#pragma GCC optimize
> "tree-vectorize"' enables) is an alias in clang for -fvectorize[3],
> which according to the documentation is on by default[4] (at least at
> -O2 or -Os).
>
> Just add the pragma when compiling with GCC so that clang does not
> unnecessarily warn.
>
> [1]: https://reviews.llvm.org/D51011#1206981
> [2]: https://lore.kernel.org/lkml/CAK8P3a3NjTCgFd2dQ9KbHP8DpXf6s-ULfeU6acAYC4SDi+2qvw@mail.gmail.com/
> [3]: https://github.com/llvm/llvm-project/blob/eafe8ef6f2b44ba/clang/include/clang/Driver/Options.td#L1729
> [4]: https://llvm.org/docs/Vectorizers.html#usage
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/496
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

LGTM, thanks Nathan.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/arm/lib/xor-neon.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index c691b901092f..d532bc072ee4 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -22,15 +22,8 @@ MODULE_LICENSE("GPL");
>   * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
>   * NEON instructions.
>   */
> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
> +#ifdef CONFIG_CC_IS_GCC
>  #pragma GCC optimize "tree-vectorize"
> -#else
> -/*
> - * While older versions of GCC do not generate incorrect code, they fail to
> - * recognize the parallel nature of these functions, and emit plain ARM code,
> - * which is known to be slower than the optimized ARM code in asm-arm/xor.h.
> - */
> -#warning This code requires at least version 4.6 of GCC
>  #endif
>
>  #pragma GCC diagnostic ignored "-Wunused-variable"
> --
> 2.22.0.rc1
>


-- 
Thanks,
~Nick Desaulniers