[llvm] [GlobalISel] Fold G_SHUFFLE_VECTOR with a single element mask to G_EXTRACT_VECTOR_ELT (PR #65342)

[Jay Foad via llvm-commits]

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@@ -395,6 +395,39 @@ void CombinerHelper::applyCombineShuffleVector(MachineInstr &MI,
   replaceRegWith(MRI, DstReg, NewDstReg);
 }
 
+bool CombinerHelper::matchShuffleToExtract(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_SHUFFLE_VECTOR &&
+         "Invalid instruction kind");
+
+  ArrayRef<int> Mask = MI.getOperand(3).getShuffleMask();
+  return Mask.size() == 1 && Mask[0] >= 0;
+}
+
+void CombinerHelper::applyShuffleToExtract(MachineInstr &MI) {
+  Register DstReg = MI.getOperand(0).getReg();
+  Builder.setInsertPt(*MI.getParent(), MI);
+  Register NewDstReg = MRI.cloneVirtualRegister(DstReg);
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jayfoad wrote:

You should be able to build instructions that write to DstReg, instead of creating NewDstReg and then calling replaceRegWith. Unless I'm missing something.

https://github.com/llvm/llvm-project/pull/65342