linux-kernel - Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC

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Message-ID: <CAKwvOdkNZ09kkzi+A8diaxViqSufxrHizuBu-7quG6an3Z8iDA@mail.gmail.com>
Date:   Tue, 19 Jan 2021 13:18:15 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Collabora Kernel ML <kernel@...labora.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

On Tue, Jan 19, 2021 at 5:17 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")
> and clarify that -ftree-vectorize now always needs enabling
> for GCC by directly testing the presence of CONFIG_CC_IS_GCC.
>
> Another reason to remove the warning is that Clang exposes
> itself as GCC < 4.6 so it triggers the warning about GCC
> which doesn't make much sense and misleads Clang users by
> telling them to update GCC.
>
> Because Clang is now supported by the kernel print a clear
> Clang-specific warning.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/496
> Link: https://github.com/ClangBuiltLinux/linux/issues/503
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

This is not the version of the patch I had reviewed; please drop my
reviewed-by tag when you change a patch significantly, as otherwise it
looks like I approved this patch.

Nacked-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 | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index b99dd8e1c93f..f9f3601cc2d1 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -14,20 +14,22 @@ MODULE_LICENSE("GPL");
>  #error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
>  #endif
>
> +/*
> + * TODO: Even though -ftree-vectorize is enabled by default in Clang, the
> + * compiler does not produce vectorized code due to its cost model.
> + * See: https://github.com/ClangBuiltLinux/linux/issues/503
> + */
> +#ifdef CONFIG_CC_IS_CLANG
> +#warning Clang does not vectorize code in this file.
> +#endif

Arnd, remind me again why it's a bug that the compiler's cost model
says it's faster to not produce a vectorized version of these loops?
I stand by my previous comment: https://bugs.llvm.org/show_bug.cgi?id=40976#c8

> +
>  /*
>   * Pull in the reference implementations while instructing GCC (through
>   * -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"
> --
> 2.30.0
>


-- 
Thanks,
~Nick Desaulniers