Messages in this thread | | | From | Adrian Ratiu <> | Subject | Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning | Date | Fri, 13 Nov 2020 13:59:48 +0200 |
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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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