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

[Manish Kausik H via llvm-commits]

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@@ -5115,6 +5114,22 @@ 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
+
+    // Any Extend the argument
+    Tmp1 = DAG.getNode(ISD::ANY_EXTEND, dl, NVT, Node->getOperand(0));
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Nirhar wrote:

I agree, but I saw that Tmp1, Tmp2 etc are explicitly used for temporary nodes in other cases in this switch statement, which I guess comes from the limitation that we aren't allowed to declare new variables inside a case. Hence I decided to follow the same convention.

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