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From: "peter at cordes dot ca" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102494] Failure to optimize vector reduction
 properly especially when using OpenMP
Date: Mon, 25 Oct 2021 22:00:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102494

--- Comment #11 from Peter Cordes <peter at cordes dot ca> ---
Also, horizontal byte sums are generally best done with  VPSADBW against a =
zero
vector, even if that means some fiddling to flip to unsigned first and then
undo the bias.

simde_vaddlv_s8:
 vpxor    xmm0, xmm0, .LC0[rip]  # set1_epi8(0x80) flip to unsigned 0..255
range
 vpxor    xmm1, xmm1
 vpsadbw  xmm0, xmm0, xmm1       # horizontal byte sum within each 64-bit h=
alf
 vmovd    eax, xmm0              # we only wanted the low half anyway
 sub      eax, 8 * 128      # subtract the bias we added earlier by flipping
sign bits
 ret

This is so much shorter we'd still be ahead if we generated the vector cons=
tant
on the fly instead of loading it.  (3 instructions: vpcmpeqd same,same / vp=
absb
/ vpslld by 7.  Or pcmpeqd / psllw 8 / packsswb same,same to saturate to -1=
28)

If we had wanted a 128-bit (16 byte) vector sum, we'd need

  ...
  vpsadbw ...

  vpshufd  xmm1, xmm0, 0xfe     # shuffle upper 64 bits to the bottom
  vpaddd   xmm0, xmm0, xmm1
  vmovd    eax, xmm0
  sub      eax, 16 * 128

Works efficiently with only SSE2.  Actually with AVX2, we should unpack the=
 top
half with VUNPCKHQDQ to save a byte (no immediate operand), since we don't =
need
PSHUFD copy-and-shuffle.

Or movd / pextrw / scalar add but that's more uops: pextrw is 2 on its own.=