[llvm] DAG: Fix vector bin op scalarize defining a partially undef vector (PR #122459)

[Simon Pilgrim via llvm-commits]

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@@ -27525,8 +27525,12 @@ static SDValue scalarizeBinOpOfSplats(SDNode *N, SelectionDAG &DAG,
   // If all lanes but 1 are undefined, no need to splat the scalar result.
   // TODO: Keep track of undefs and use that info in the general case.
   if (N0.getOpcode() == ISD::BUILD_VECTOR && N0.getOpcode() == N1.getOpcode() &&
-      count_if(N0->ops(), [](SDValue V) { return !V.isUndef(); }) == 1 &&
-      count_if(N1->ops(), [](SDValue V) { return !V.isUndef(); }) == 1) {
+      // This is assuming if either input is undef, the result will fold out.
+      //
+      // TODO: Do we need to check if the opcode/operand propagates undef?
+      // Should we ignore operation identity values?
+      ((count_if(N0->ops(), [](SDValue V) { return !V.isUndef(); }) == 1) ||
+       (count_if(N1->ops(), [](SDValue V) { return !V.isUndef(); }) == 1))) {
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RKSimon wrote:

The opcode specific under handling does worry me- how well does this work if we completely scalarize all elements and leave getNode() to handle the undefs / constant handling etc? Similar to what getKnownUndefForVectorBinop does in TargetLowering

https://github.com/llvm/llvm-project/pull/122459