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Date: Wed, 20 Jan 2021 15:11:27 +0200 From: Adrian Ratiu <adrian.ratiu@...labora.com> To: Nick Desaulniers <ndesaulniers@...gle.com>, Adrian Ratiu <adrian.ratiu@...labora.com>, Arnd Bergmann <arnd@...db.de> Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>, Nathan Chancellor <natechancellor@...il.com>, Russell King <linux@...linux.org.uk>, Ard Biesheuvel <ardb@...nel.org>, Arvind Sankar <nivedita@...m.mit.edu>, clang-built-linux <clang-built-linux@...glegroups.com>, Collabora Kernel ML <kernel@...labora.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning On Tue, 19 Jan 2021, Nick Desaulniers <ndesaulniers@...gle.com> wrote: > On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu > <adrian.ratiu@...labora.com> wrote: >> >> From: Nathan Chancellor <natechancellor@...il.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@...gle.com> >> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com> > > This is not the version of the patch I had reviewed; please drop > my reviewed-by tag when you change a patch significantly, as > otherwise it looks like I approved this patch. > > Nacked-by: Nick Desaulniers <ndesaulniers@...gle.com> > Sorry for not removing the reviewed-by tags from the previous versions in this v4. I guess the only way forward with this is to actually make clang vectorization work. Also thanks for the patch suggestion in the other e-mail! >> Signed-off-by: Nathan Chancellor <natechancellor@...il.com> >> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com> >> --- >> 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 > > Arnd, remind me again why it's a bug that the compiler's cost model > says it's faster to not produce a vectorized version of these loops? > I stand by my previous comment: https://bugs.llvm.org/show_bug.cgi?id=40976#c8 > >> + >> /* >> * 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 >> > > > -- > Thanks, > ~Nick Desaulniers
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