[llvm] [GlobalISel][AArch64] Combine unmerge(G_EXT v, undef) to unmerge(v). (PR #65263)

[Amara Emerson via llvm-commits]

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@@ -1067,6 +1067,53 @@ void applyVectorSextInReg(MachineInstr &MI, MachineRegisterInfo &MRI,
   Helper.lower(MI, 0, /* Unused hint type */ LLT());
 }
 
+/// Combine <N x t>, unused = unmerge(G_EXT <2*N x t> v, undef, N)
+///           => unused, <N x t> = unmerge v
+bool matchUnmergeExtToUnmerge(MachineInstr &MI, MachineRegisterInfo &MRI,
+                              Register &MatchInfo) {
+  assert(MI.getOpcode() == TargetOpcode::G_UNMERGE_VALUES);
+  if (MI.getNumDefs() != 2)
+    return false;
+  if (!MRI.use_nodbg_empty(MI.getOperand(1).getReg()))
+    return false;
+
+  LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
+  if (!DstTy.isVector())
+    return false;
+
+  MachineInstr *Ext = getDefIgnoringCopies(
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aemerson wrote:

getOpcodeDef() with the GUnmerge source reg helper here?

https://github.com/llvm/llvm-project/pull/65263