Avoid double branch / conditional move for promoted cttz

[dotdash]

Summary

Instead of handling the zero case for that promoted value and then
adjusting the result for the original type, we can set the appropriate
bit in the promoted value.

[• koriakin]

Looks good, but there's room for a minor improvement still - see inline comment.

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

4051

Another trick that can be added here: operation can now be safely changed to CTTZ_UNDEF.

[xbolva00]

Tests?

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

void SelectionDAGLegalize::PromoteNode(SDNode *Node) {
   switch (Node->getOpcode()) {
   case ISD::CTTZ:
   case ISD::CTTZ_ZERO_UNDEF:
   case ISD::CTLZ:
   case ISD::CTLZ_ZERO_UNDEF:
   case ISD::CTPOP:
     // Zero extend the argument.
     Tmp1 = DAG.getNode(ISD::ZERO_EXTEND, dl, NVT, Node->getOperand(0));
+
+    if (Node->getOpcode() == ISD::CTTZ) {
+      Tmp1 = DAG.getNode(ISD::OR, dl, NVT, Tmp1, DAG.getConstant(1 << OVT.getSizeInBits(), NVT));
+    }
+
     // 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::CTTZ) {
-      // FIXME: This should set a bit in the zero extended value instead.
-      Tmp2 = DAG.getSetCC(dl, getSetCCResultType(NVT),
-                          Tmp1, DAG.getConstant(NVT.getSizeInBits(), NVT),
-                          ISD::SETEQ);
-      Tmp1 = DAG.getSelect(dl, NVT, Tmp2,
-                           DAG.getConstant(OVT.getSizeInBits(), NVT), Tmp1);
-    } else if (Node->getOpcode() == ISD::CTLZ ||
+
+    if (Node->getOpcode() == ISD::CTLZ ||
                Node->getOpcode() == ISD::CTLZ_ZERO_UNDEF) {
       // Tmp1 = Tmp1 - (sizeinbits(NVT) - sizeinbits(Old VT))
       Tmp1 = DAG.getNode(ISD::SUB, dl, NVT, Tmp1,
                           DAG.getConstant(NVT.getSizeInBits() -
                                           OVT.getSizeInBits(), NVT));
     }
     Results.push_back(DAG.getNode(ISD::TRUNCATE, dl, OVT, Tmp1));
     break;