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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, Nathan Chancellor <natechancellor@gmail.com> 
wrote:
> On Tue, Jan 19, 2021 at 03:17:23PM +0200, Adrian Ratiu 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>
>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>> Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
>
> The commit message looks like it is written by me but I never
> added a Clang specific warning. I appreciate wanting to give me
> credit but when you change things about my original commit
> message, please make it clear that you did the edits, something
> like:
>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> [adrian: Add clang specific warning] Signed-off-by: Adrian Ratiu
> <adrian.ratiu@collabora.com>
>

Thanks for the suggestion. Makes sense. I contemplated adding
another patch by me on top but thought it was too much
churn. Sorry if my edits were unclear.

>> ---
>> 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
>
> I really do not like this. With the GCC specific warning, the
> user could just upgrade their GCC. With this warning, it is
> basically telling them don't use clang, in which case, it would
> just be better to disable this code altogether. I would rather
> see:
>
> 1. Just don't build this file with clang altogether, which I
> believe was
> v1's 2/2 patch.
>
> OR
>
> 2. Use the pragma:
>
> #pragma clang loop vectorize(enable)
>
> as Nick suggests in v1's 2/2 patch.
>
> Alternatively, __restrict__ sounds like it might be beneficial
> for both GCC and clang:
>
> https://lore.kernel.org/lkml/20201112215033.GA438824@rani.riverdale.lan/
>

Option 1 from v1 got clearly NACKed by Nick a while back so the
only option gonig forward is to also fix clang vectorization
together with these changes so the warning becomes unnecessary.

>> /*
>> * 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
>>

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