On Wed, Jan 20, 2021 at 03:09:53PM -0800, Nick Desaulniers wrote: > On Tue, Jan 19, 2021 at 1:35 PM Arnd Bergmann <a...@kernel.org> wrote: > > > > On Tue, Jan 19, 2021 at 10:18 PM 'Nick Desaulniers' via Clang Built > > Linux <clang-built-li...@googlegroups.com> wrote: > > > > > > On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu <adrian.ra...@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?
The gcc version is at least useful when the arguments _don't_ overlap, which is presumably most/all the time. There's no point to building this file if it's not going to produce a vectorized version at all. The warning seems unnecessary, and it's not really a compiler bug either -- until we agree on a way to get clang to produce a vectorized version, the best thing would be for the neon version to not be built at all with clang. Is that too messy to achieve? > > 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>. We can't directly do the patch below since there are other users of the asm-generic/xor.h implementations than just the neon file. If it's too much work to check and add __restrict everywhere, I think we'd either need to copy the code into the xor-neon file, or maybe do some ifdeffery so __restrict is only used for the neon version. > > 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" > -#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" > #include <asm-generic/xor.h> > diff --git a/include/asm-generic/xor.h b/include/asm-generic/xor.h > index b62a2a56a4d4..69df62095c33 100644 > --- a/include/asm-generic/xor.h > +++ b/include/asm-generic/xor.h > @@ -8,7 +8,7 @@ > #include <linux/prefetch.h> > > static void > -xor_8regs_2(unsigned long bytes, unsigned long *p1, unsigned long *p2) > +xor_8regs_2(unsigned long bytes, unsigned long * __restrict p1, > unsigned long * __restrict p2) > { > long lines = bytes / (sizeof (long)) / 8; > > > -- > Thanks, > ~Nick Desaulniers