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SubjectRe: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization
On Tue, Nov 10, 2020 at 01:41:17PM -0800, Nick Desaulniers wrote:
> On Mon, Nov 9, 2020 at 11:51 AM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
> >
> > On Fri, 06 Nov 2020, Nick Desaulniers <ndesaulniers@google.com>
> > wrote:
> > > +#pragma clang loop vectorize(enable)
> > > do {
> > > p1[0] ^= p2[0] ^ p3[0] ^ p4[0] ^ p5[0]; p1[1] ^=
> > > p2[1] ^ p3[1] ^ p4[1] ^ p5[1];
> > > ``` seems to generate the vectorized code.
> > >
> > > Why don't we find a way to make those pragma's more toolchain
> > > portable, rather than open coding them like I have above rather
> > > than this series?
> >
> > Hi again Nick,
> >
> > How did you verify the above pragmas generate correct vectorized
> > code? Have you tested this specific use case?
>
> I read the disassembly before and after my suggested use of pragmas;
> look for vld/vstr. You can also add -Rpass-missed=loop-vectorize to
> CFLAGS_xor-neon.o in arch/arm/lib/Makefile and rebuild
> arch/arm/lib/xor-neon.o with CONFIG_BTRFS enabled.
>

https://godbolt.org/z/1oo9M6

With the __restrict__ keywords added, clang seems to vectorize the loop,
but still reports that vectorization wasn't beneficial -- any idea
what's going on?

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