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SubjectRe: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
Date
On Tue, 19 Jan 2021, Nick Desaulniers <ndesaulniers@google.com> 
wrote:
> 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>
>

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@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

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