linux-kernel - Re: [PATCH 1/2] arm: lib: xor-neon: remove unnecessary GCC

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Date:   Fri, 6 Nov 2020 15:46:36 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Collabora kernel ML <kernel@...labora.com>
Subject: Re: [PATCH 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

On Fri, Nov 6, 2020 at 6:14 AM Adrian Ratiu <adrian.ratiu@...labora.com> wrote:
>
> From: Nathan Chancellor <natechancellor@...il.com>
>
> Drop warning because kernel now requires GCC >= v4.9 after
> commit 6ec4476ac825 ("Raise gcc version requirement to 4.9").
>
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.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 b99dd8e1c93f..e1e76186ec23 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -19,15 +19,8 @@ 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

I think we still need the #else path, otherwise we don't warn about
clang being broken here.

If it is intentional that we now silently build this code with clang
without it working as intended, that should be mentioned in the
changelog.

      Arnd