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

[Manish Kausik H via llvm-commits]

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@@ -642,21 +642,43 @@ SDValue DAGTypeLegalizer::PromoteIntRes_CTLZ(SDNode *N) {
     }
   }
 
-  // Zero extend to the promoted type and do the count there.
-  SDValue Op = ZExtPromotedInteger(N->getOperand(0));
+  unsigned CtlzOpcode = N->getOpcode();
+  if (CtlzOpcode == ISD::CTLZ) {
+    // Zero extend to the promoted type and do the count there.
+    SDValue Op = ZExtPromotedInteger(N->getOperand(0));
+
+    // Subtract off the extra leading bits in the bigger type.
+    SDValue ExtractLeadingBits = DAG.getConstant(
+        NVT.getScalarSizeInBits() - OVT.getScalarSizeInBits(), dl, NVT);
+    if (!N->isVPOpcode())
+      return DAG.getNode(ISD::SUB, dl, NVT,
+                         DAG.getNode(N->getOpcode(), dl, NVT, Op),
+                         ExtractLeadingBits);
+    SDValue Mask = N->getOperand(1);
+    SDValue EVL = N->getOperand(2);
+    return DAG.getNode(ISD::VP_SUB, dl, NVT,
+                       DAG.getNode(N->getOpcode(), dl, NVT, Op, Mask, EVL),
+                       ExtractLeadingBits, Mask, EVL);
+  } else if (CtlzOpcode == ISD::CTLZ_ZERO_UNDEF) {
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Nirhar wrote:

Hope the code formatting now is correct

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