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

[Matt Arsenault via llvm-commits]

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@@ -4982,15 +4981,30 @@ void SelectionDAGLegalize::PromoteNode(SDNode *Node) {
     // Perform the larger operation. For CTPOP and CTTZ_ZERO_UNDEF, this is
     // already the correct result.
     Tmp1 = DAG.getNode(Node->getOpcode(), dl, NVT, Tmp1);
-    if (Node->getOpcode() == ISD::CTLZ ||
-        Node->getOpcode() == ISD::CTLZ_ZERO_UNDEF) {
+    if (Node->getOpcode() == ISD::CTLZ) {
       // Tmp1 = Tmp1 - (sizeinbits(NVT) - sizeinbits(Old VT))
       Tmp1 = DAG.getNode(ISD::SUB, dl, NVT, Tmp1,
                           DAG.getConstant(NVT.getSizeInBits() -
                                           OVT.getSizeInBits(), dl, NVT));
     }
     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));
+
+    // Tmp1 = Tmp1 << (sizeinbits(NVT) - sizeinbits(Old VT))
+    Tmp2 = DAG.getShiftAmountConstant(NVT.getSizeInBits() - OVT.getSizeInBits(),
+                                      Tmp1.getValueType(), dl);
+    Tmp1 = DAG.getNode(ISD::SHL, dl, NVT, Tmp1, Tmp2);
+
+    // Perform the larger operation
+    Tmp1 = DAG.getNode(Node->getOpcode(), dl, NVT, Tmp1);
+    Results.push_back(DAG.getNode(ISD::TRUNCATE, dl, OVT, Tmp1));
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arsenm wrote:

I think I never got around to fixing this in the DAG, but all of these bitcount operations in principle could have a different return type from the source operand. It would be a bit better to truncate to the result type, and use the source type as the old type throughout 

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