[llvm] [CodeGen] Add widening support for ISD::CTTZ_ELTS (PR #205841)

[Luke Lau via llvm-commits]

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@@ -8723,6 +8727,20 @@ SDValue DAGTypeLegalizer::WidenVecOp_VSELECT(SDNode *N) {
   return DAG.getExtractSubvector(DL, VT, Select, 0);
 }
 
+SDValue DAGTypeLegalizer::WidenVecOp_CttzElements(SDNode *N) {
+  SDLoc DL(N);
+  SDValue Source = N->getOperand(0);
+  EVT WideVT = GetWidenedVector(Source).getValueType();
+
+  // Pad the widened portion with all-ones so the extra lanes appear as
+  // active (non-zero) elements and do not contribute trailing zeros.
+  SDValue AllOnes = DAG.getAllOnesConstant(DL, WideVT);
+  SDValue WideSource = DAG.getInsertSubvector(DL, AllOnes, Source, 0);
+
+  return DAG.getNode(N->getOpcode(), DL, N->getValueType(0), WideSource,
+                     N->getFlags());
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lukel97 wrote:

For CTTZ_ELTS_ZERO_POISON I don't think we need to pad the widened lanes as zero, it should be fine if they're undefined. But we can do that as a follow up.

https://github.com/llvm/llvm-project/pull/205841