Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-01-19 21:20:17
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On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu [off-list ref] wrote:
From: Nathan Chancellor <redacted>
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>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 [off-list ref]
quoted hunk ↗ jump to hunk
Signed-off-by: Nathan Chancellor <redacted> Signed-off-by: Adrian Ratiu <redacted> --- 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel