[llvm] [DAG] optimize llvm.ucmp for 1-bit inputs to return subtraction of operands (PR #150058)

[Craig Topper via llvm-commits]

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@@ -10942,6 +10942,10 @@ SDValue TargetLowering::expandCMP(SDNode *Node, SelectionDAG &DAG) const {
   SDValue IsLT = DAG.getSetCC(dl, BoolVT, LHS, RHS, LTPredicate);
   SDValue IsGT = DAG.getSetCC(dl, BoolVT, LHS, RHS, GTPredicate);
 
+  if (isa<VTSDNode>(RHS->getOperand(1)) &&
+      cast<VTSDNode>(RHS->getOperand(1))->getVT().getScalarSizeInBits() == 1) {
+    return DAG.getNode(ISD::SUB, dl, VT, LHS, RHS);
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topperc wrote:

This code leaves the newly created `IsLT` and `IsGT` as dead nodes.

https://github.com/llvm/llvm-project/pull/150058