[llvm] [LegalizeDAG] Optimize CodeGen for `ISD::CTLZ_ZERO_UNDEF` (PR #83039)

[Craig Topper via llvm-commits]

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@@ -5003,6 +5002,22 @@ void SelectionDAGLegalize::PromoteNode(SDNode *Node) {
     }
     Results.push_back(DAG.getNode(ISD::TRUNCATE, dl, OVT, Tmp1));
     break;
+  case ISD::CTLZ_ZERO_UNDEF:
+    // We know that the argument is unlikely to be zero, hence we can take a
+    // different approach as compared to ISD::CTLZ
+
+    // Zero Extend the argument
+    Tmp1 = DAG.getNode(ISD::ZERO_EXTEND, dl, NVT, Node->getOperand(0));
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topperc wrote:

This can be ANY_EXTEND

https://github.com/llvm/llvm-project/pull/83039