Messages in this thread | | | From | Adrian Ratiu <> | Subject | Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning | Date | Wed, 20 Jan 2021 15:18:17 +0200 |
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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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