[llvm] [DAG] isSplatValue - only accept binop splats if they share all the same demanded undef elements (PR #135597)

[Nikita Popov via llvm-commits]

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@@ -3002,8 +3002,12 @@ bool SelectionDAG::isSplatValue(SDValue V, const APInt &DemandedElts,
     APInt UndefLHS, UndefRHS;
     SDValue LHS = V.getOperand(0);
     SDValue RHS = V.getOperand(1);
+    // Ensure the same demanded undef elts for both operands, otherwise we might
+    // fail to handle binop-specific undef handling.
+    // e.g. (and undef, 0) -> 0 etc.
     if (isSplatValue(LHS, DemandedElts, UndefLHS, Depth + 1) &&
-        isSplatValue(RHS, DemandedElts, UndefRHS, Depth + 1)) {
+        isSplatValue(RHS, DemandedElts, UndefRHS, Depth + 1) &&
+        (DemandedElts & UndefLHS) == (DemandedElts & UndefRHS)) {
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nikic wrote:

Hm, wouldn't it be sufficient to set `UndefElts = UndefLHS & UndefRHS`, but still have isSplatValue() return true?

https://github.com/llvm/llvm-project/pull/135597