[llvm] [AArch64] Remove EXT instr before UZP when extracting elements from vector (PR #91328)

[David Green via llvm-commits]

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@@ -21448,6 +21448,29 @@ static SDValue performUzpCombine(SDNode *N, SelectionDAG &DAG,
   SDValue Op1 = N->getOperand(1);
   EVT ResVT = N->getValueType(0);
 
+  // uzp(extract_lo(x), extract_hi(x)) -> extract_lo(uzp x, x)
+  if (Op0.getOpcode() == ISD::EXTRACT_SUBVECTOR &&
+      Op1.getOpcode() == ISD::EXTRACT_SUBVECTOR &&
+      Op0.getOperand(0) == Op1.getOperand(0)) {
+
+    SDValue SourceVec = Op0.getOperand(0);
+    uint64_t ExtIdx0 = Op0.getConstantOperandVal(1);
+    uint64_t ExtIdx1 = Op1.getConstantOperandVal(1);
+    uint64_t NumElements = SourceVec.getValueType().getVectorMinNumElements();
+    if (ExtIdx0 == 0 && ExtIdx1 == NumElements / 2) {
+      EVT OpVT = Op0.getOperand(1).getValueType();
+      EVT WidenedResVT = ResVT.getDoubleNumVectorElementsVT(*DAG.getContext());
+      SDValue uzp2 =
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davemgreen wrote:

uzp2 -> Uzp2 (and maybe without the 2? UZP perhaps?)

Can the second SourceVec be Undef if we are only using the bottom half? Or does that produce worse code in places?

https://github.com/llvm/llvm-project/pull/91328