[llvm-bugs] [Bug 41433] New: Some 2-vector vector select identity shuffles may be better represented as moves

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41433

            Bug ID: 41433
           Summary: Some 2-vector vector select identity shuffles may be
                    better represented as moves
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Split off from https://bugs.llvm.org/show_bug.cgi?id=41429

https://godbolt.org/z/_n1ggH

void example(__m256i * __restrict__ dest, const __m256i * __restrict__ a) {
    (*dest)[2] = (*a)[2];
    (*dest)[3] = (*a)[3];
}

Here we do not ever touch the low half of dest, and replace the high part
of dest with high part of `a`.

The naive asm could be:

  vmovaps ymm0, ymmword ptr [rdi]
  vblendps ymm0, ymm0, ymmword ptr [rsi], 240 # ymm0 =
ymm0[0,1,2,3],mem[4,5,6,7]
  vmovaps ymmword ptr [rdi], ymm0
  vzeroupper

But we can also produce:

        vmovaps xmm0, xmmword ptr [rsi + 16]
        vmovaps xmmword ptr [rdi + 16], xmm0

I'm not quite sure what are the exact criteria when that is profitable to do
though.

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