Re: [x86 PATCH] Improve V[48]QI shifts on AVX512

["Roger Sayle" <roger@nextmovesoftware.com>]

Many thanks for the speedy review and correction/improvement.
It's interesting that you spotted the ternlog "spill"...
I have a patch that rewrites ternlog handling that's been
waiting for stage 1, that would also fix this mem operand
issue.  I hope to submit it for review this weekend.

Thanks again,
Roger

> From: Hongtao Liu <crazylht@gmail.com>
> On Fri, May 10, 2024 at 6:26 AM Roger Sayle <roger@nextmovesoftware.com>
> wrote:
> >
> >
> > The following one line patch improves the code generated for V8QI and
> > V4QI shifts when AV512BW and AVX512VL functionality is available.
> +      /* With AVX512 its cheaper to do vpmovsxbw/op/vpmovwb.  */
> +      && !(TARGET_AVX512BW && TARGET_AVX512VL && TARGET_SSE4_1)
>        && ix86_expand_vec_shift_qihi_constant (code, qdest, qop1, qop2)) I think
> TARGET_SSE4_1 is enough, it's always better w/ sse4.1 and above when not going
> into ix86_expand_vec_shift_qihi_constant.
> Others LGTM.
> >
> > For the testcase (from gcc.target/i386/vect-shiftv8qi.c):
> >
> > typedef signed char v8qi __attribute__ ((__vector_size__ (8))); v8qi
> > foo (v8qi x) { return x >> 5; }
> >
> > GCC with -O2 -march=cascadelake currently generates:
> >
> > foo:    movl    $67372036, %eax
> >         vpsraw  $5, %xmm0, %xmm2
> >         vpbroadcastd    %eax, %xmm1
> >         movl    $117901063, %eax
> >         vpbroadcastd    %eax, %xmm3
> >         vmovdqa %xmm1, %xmm0
> >         vmovdqa %xmm3, -24(%rsp)
> >         vpternlogd      $120, -24(%rsp), %xmm2, %xmm0
> It looks like a miss-optimization under AVX512, but it's a separate issue.
> >         vpsubb  %xmm1, %xmm0, %xmm0
> >         ret
> >
> > with this patch we now generate the much improved:
> >
> > foo:    vpmovsxbw       %xmm0, %xmm0
> >         vpsraw  $5, %xmm0, %xmm0
> >         vpmovwb %xmm0, %xmm0
> >         ret
> >
> > This patch also fixes the FAILs of gcc.target/i386/vect-shiftv[48]qi.c
> > when run with the additional -march=cascadelake flag, by splitting
> > these tests into two; one form testing code generation with -msse2
> > (and
> > -mno-avx512vl) as originally intended, and the other testing AVX512
> > code generation with an explicit -march=cascadelake.
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > and make -k check, both with and without --target_board=unix{-m32}
> > with no new failures.  Ok for mainline?
> >
> >
> > 2024-05-09  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386-expand.cc (ix86_expand_vecop_qihi_partial):
> >         Don't attempt ix86_expand_vec_shift_qihi_constant on AVX512.
> >
> > gcc/testsuite/ChangeLog
> >         * gcc.target/i386/vect-shiftv4qi.c: Specify -mno-avx512vl.
> >         * gcc.target/i386/vect-shiftv8qi.c: Likewise.
> >         * gcc.target/i386/vect-shiftv4qi-2.c: New test case.
> >         * gcc.target/i386/vect-shiftv8qi-2.c: Likewise.
> >
> >
> > Thanks in advance,
> > Roger
> > --
> >
> --
> BR,
> Hongtao