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

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Message-ID: <87o8hjg3cw.fsf@collabora.com>
Date:   Wed, 20 Jan 2021 15:11:27 +0200
From:   Adrian Ratiu <adrian.ratiu@...labora.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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, 19 Jan 2021, Nick Desaulniers <ndesaulniers@...gle.com> 
wrote:
> 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> 
> 

Sorry for not removing the reviewed-by tags from the previous 
versions in this v4. I guess the only way forward with this is to 
actually make clang vectorization work. Also thanks for the patch 
suggestion in the other e-mail!

>> 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