Messages in this thread | | | From | Nick Desaulniers <> | Date | Wed, 20 Jan 2021 15:15:40 -0800 | Subject | Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning |
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On Wed, Jan 20, 2021 at 3:09 PM Nick Desaulniers <ndesaulniers@google.com> wrote: > > On Tue, Jan 19, 2021 at 1:35 PM Arnd Bergmann <arnd@kernel.org> wrote: > > > > On Tue, Jan 19, 2021 at 10:18 PM 'Nick Desaulniers' via Clang Built > > Linux <clang-built-linux@googlegroups.com> wrote: > > > > > > On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu <adrian.ratiu@collabora.com> wrote: > > > > 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 > > > > The point is that without vectorizing the code, there is no point in building > > both the default xor code and a "neon" version that has to save/restore > > the neon registers but doesn't actually use them. > > Doesn't that already happen today with GCC when the pointer arguments > are overlapping? The loop is "versioned" and may not actually use the > NEON registers at all at runtime for such arguments. > https://godbolt.org/z/q48q8v See also: > https://bugs.llvm.org/show_bug.cgi?id=40976#c11. Or am I missing > something? > > So I'm thinking if we extend out this pattern to the rest of the > functions, we can actually avoid calls to > kernel_neon_begin()/kernel_neon_end() for cases in which pointers > would be too close to use the vectorized loop version; meaning for GCC > this would be an optimization (don't save neon registers when you're > not going to use them). I would probably consider moving > include/asm-generic/xor.h somewhere under arch/arm/ > perhaps...err...something for the other users of <asm-generic/xor.h>. > > diff --git a/arch/arm/include/asm/xor.h b/arch/arm/include/asm/xor.h > index aefddec79286..abd748d317e8 100644 > --- a/arch/arm/include/asm/xor.h > +++ b/arch/arm/include/asm/xor.h > @@ -148,12 +148,12 @@ extern struct xor_block_template const > xor_block_neon_inner; > static void > xor_neon_2(unsigned long bytes, unsigned long *p1, unsigned long *p2) > { > - if (in_interrupt()) { > - xor_arm4regs_2(bytes, p1, p2); > - } else { > + if (!in_interrupt() && abs((uintptr_t)p1, (uintptr_t)p2) >= 8) { > kernel_neon_begin(); > xor_block_neon_inner.do_2(bytes, p1, p2); > kernel_neon_end(); > + } else { > + xor_arm4regs_2(bytes, p1, p2); > } > } > diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c > index b99dd8e1c93f..0e8e474c0523 100644 > --- a/arch/arm/lib/xor-neon.c > +++ b/arch/arm/lib/xor-neon.c > @@ -14,22 +14,6 @@ MODULE_LICENSE("GPL"); > #error You should compile this file with '-march=armv7-a > -mfloat-abi=softfp -mfpu=neon' > #endif > > -/* > - * 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) > -#pragma GCC optimize "tree-vectorize"
Err...we need to keep this but use the -f flag with __restrict for GCC to vectorize: https://godbolt.org/z/9acnEv
-- Thanks, ~Nick Desaulniers
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