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SubjectRe: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
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On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@kernel.org> wrote:
> On Fri, 13 Nov 2020 at 12:05, Adrian Ratiu
> <adrian.ratiu@collabora.com> wrote:
>>
>> Hi Ard,
>>
>> On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@kernel.org> wrote:
>> > On Thu, 12 Nov 2020 at 22:23, 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").
>> >>
>> >> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
>> >> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>> >> Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
>> >
>> > Again, this does not do what it says on the tin.
>> >
>> > If you want to disable the pragma for Clang, call that out in
>> > the commit log, and don't hide it under a GCC version change.
>>
>> I am not doing anything for Clang in this series.
>>
>> The option to auto-vectorize in Clang is enabled by default but
>> doesn't work for some reason (likely to do with how it computes
>> the cost model, so maybe not even a bug at all) and if we
>> enable it explicitely (eg via a Clang specific pragma) we get
>> some warnings we currently do not understand, so I am not
>> changing the Clang behaviour at the recommendation of Nick.
>>
>> So this is only for GCC as the "tin" says :) We can fix clang
>> separately as the Clang bug has always been present and is
>> unrelated.
>>
>
> But you are adding the IS_GCC check here, no? Is that
> equivalent? IOW, does Clang today identify as GCC <= 4.6?
>

I see what you mean now. Thanks.

Clang identifies as GCC <= 4.6 yes, so the code is not strictly
speaking equivalent. The warning to upgrade GCC doesn't make sense
for Clang but I should mention removing it in the commit message
as well.

>> >
>> > Without the pragma, the generated code is the same as the
>> > generic code, so it makes no sense to build xor-neon.ko at all,
>> > right?
>> >
>>
>> Yes that is correct and that is the reason why in v1 I opted to
>> not build xor-neon.ko for Clang anymore, but that got NACKed, so
>> here I'm fixing the low hanging fruit: the very obvious & clear
>> GCC problems.
>>
>>
>
> Fair enough.
>
>> >> ---
>> >> arch/arm/lib/xor-neon.c | 9 +--------
>> >> 1 file changed, 1 insertion(+), 8 deletions(-)
>> >>
>> >> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
>> >> index b99dd8e1c93f..e1e76186ec23 100644
>> >> --- a/arch/arm/lib/xor-neon.c
>> >> +++ b/arch/arm/lib/xor-neon.c
>> >> @@ -19,15 +19,8 @@ MODULE_LICENSE("GPL");
>> >> * -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"
>> >> --
>> >> 2.29.2
>> >>

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