[llvm] [SelectionDAG] Expand CTTZ_ELTS[_ZERO_POISON] and handle legalization (PR #188691)

[Luke Lau via llvm-commits]

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@@ -1221,6 +1225,18 @@ SDValue DAGTypeLegalizer::ScalarizeVecOp_VECTOR_FIND_LAST_ACTIVE(SDNode *N) {
   return DAG.getConstant(0, SDLoc(N), VT);
 }
 
+SDValue DAGTypeLegalizer::ScalarizeVecOp_CTTZ_ELTS(SDNode *N) {
+  // The number of trailing zero elements is 1 if the element is 0, and 0
+  // otherwise.
+  if (N->getOpcode() == ISD::CTTZ_ELTS_ZERO_POISON)
+    return DAG.getConstant(0, SDLoc(N), N->getValueType(0));
+  SDValue Op = GetScalarizedVector(N->getOperand(0));
+  SDValue SetCC =
+      DAG.getSetCC(SDLoc(N), MVT::i1, Op,
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lukel97 wrote:

I think that would mean that the contents aren't guaranteed to have only the first bit set when true, i.e. dependent on TLI.getBooleanContents. We need only the first bit to be set since we rely on the zext below to produce one. FWIW `DAGTypeLegalizer::ScalarizeVecOp_VSETCC` seems to create a `MVT::i1` SETCC too. 

https://github.com/llvm/llvm-project/pull/188691