lkml.org 
[lkml]   [2020]   [Nov]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization
On Tue, Nov 10, 2020 at 2:15 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> 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?

I suspect that loop-vectorize is a higher level pass that relies on
slp-vectorizer for the transform.

$ clang -O2 --target=arm-linux-gnueabi -S -o - foo.c -mfpu=neon -mllvm
-print-after-all
...
*** IR Dump After SLP Vectorizer ***
(bunch of <4 x i32> types)

If you add -Rpass-missed=slp-vectorizer, observe that the existing
warnings from -Rpass-missed=loop-vectorize disappear; I suspect
loop-vectorize will print a "remark" if passes it calls did not, but
returned some for of error code.

-Rpass=slp-vectorizer shows that it vectorizes two sequences of the
loop, and warns that some third portion (that's
non-immediately-obvious to me) was non beneficial.

--
Thanks,
~Nick Desaulniers

\
 
 \ /
  Last update: 2020-11-10 23:37    [W:0.196 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site