[PATCH] D104156: [DAGCombine][X86][ARM] EXTRACT_SUBVECTOR(VECTOR_SHUFFLE(?,?,Mask)) -> VECTOR_SHUFFLE(EXTRACT_SUBVECTOR(?, ?), EXTRACT_SUBVECTOR(?, ?), Mask')

Mon Dec 6 13:04:51 PST 2021

lebedev.ri added inline comments.

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Comment at: llvm/test/CodeGen/X86/vector-fshr-rot-128.ll:402
+; AVX2-NEXT:    vpshufb {{.*#+}} ymm0 = ymm0[0,1,4,5,8,9,12,13,8,9,12,13,12,13,14,15,16,17,20,21,24,25,28,29,24,25,28,29,28,29,30,31]
+; AVX2-NEXT:    vextracti128 $1, %ymm0, %xmm1
+; AVX2-NEXT:    vpunpcklqdq {{.*#+}} xmm0 = xmm0[0],xmm1[0]
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The problem is that cheap!=free, so we could be turning a cheap & free extraction (from 0'th subvec)
into a cheap non-free extraction (from non-0'th subvec), or even into two of these.
Or i'm wrong and this explicit extraction is actually better?
Because if it's not, we'll need 'is free subvec extract' hook, i believe?

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104156/new/

https://reviews.llvm.org/D104156