On Fri, 13 Nov 2020, Ard Biesheuvel <[email protected]> wrote:
On Fri, 13 Nov 2020 at 12:05, Adrian Ratiu
<[email protected]> wrote:
Hi Ard,
On Fri, 13 Nov 2020, Ard Biesheuvel <[email protected]> wrote:
> On Thu, 12 Nov 2020 at 22:23, Adrian Ratiu
> <[email protected]> wrote:
>>
>> From: Nathan Chancellor <[email protected]>
>>
>> Drop warning because kernel now requires GCC >= v4.9 after
>> commit 6ec4476ac825 ("Raise gcc version requirement to
>> 4.9").
>>
>> Reported-by: Nick Desaulniers <[email protected]>
>> Signed-off-by: Nathan Chancellor <[email protected]>
>> Signed-off-by: Adrian Ratiu <[email protected]>
>
> 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
>>
