From: Nick Desaulniers <ndesaulniers@google.com>
To: Adrian Ratiu <adrian.ratiu@collabora.com>, Arnd Bergmann <arnd@arndb.de>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
Nathan Chancellor <natechancellor@gmail.com>,
Russell King <linux@armlinux.org.uk>,
Ard Biesheuvel <ardb@kernel.org>,
Arvind Sankar <nivedita@alum.mit.edu>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Collabora Kernel ML <kernel@collabora.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
Date: Tue, 19 Jan 2021 13:18:15 -0800 [thread overview]
Message-ID: <CAKwvOdkNZ09kkzi+A8diaxViqSufxrHizuBu-7quG6an3Z8iDA@mail.gmail.com> (raw)
In-Reply-To: <20210119131724.308884-2-adrian.ratiu@collabora.com>
On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
>
> From: Nathan Chancellor <natechancellor@gmail.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@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.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@google.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.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
next prev parent reply other threads:[~2021-01-19 21:20 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-19 13:17 [PATCH v4 0/2] xor-neon: Remove GCC warn & pragmas Adrian Ratiu
2021-01-19 13:17 ` [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning Adrian Ratiu
2021-01-19 21:18 ` Nick Desaulniers [this message]
2021-01-19 21:34 ` Arnd Bergmann
2021-01-19 22:04 ` Nick Desaulniers
2021-01-19 23:10 ` Nick Desaulniers
2021-01-20 23:09 ` Nick Desaulniers
2021-01-20 23:15 ` Nick Desaulniers
2021-01-21 4:13 ` Arvind Sankar
2021-01-21 4:24 ` Nick Desaulniers
2021-01-21 8:46 ` Ard Biesheuvel
2021-01-20 13:11 ` Adrian Ratiu
2021-01-19 21:54 ` Nathan Chancellor
2021-01-20 13:18 ` Adrian Ratiu
2021-01-19 22:04 ` David Laight
2021-01-19 22:05 ` Nick Desaulniers
2021-01-19 13:17 ` [PATCH v4 2/2] arm: lib: xor-neon: move pragma options to makefile Adrian Ratiu
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