[llvm] [polly] [InstSimplify] Fix incorrect poison propagation when folding phi (PR #96631)

[via llvm-commits]

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@@ -5278,7 +5278,14 @@ static Value *simplifyPHINode(PHINode *PN, ArrayRef<Value *> IncomingValues,
     // If we have a PHI node like phi(X, undef, X), where X is defined by some
     // instruction, we cannot return X as the result of the PHI node unless it
     // dominates the PHI block.
-    return valueDominatesPHI(CommonValue, PN, Q.DT) ? CommonValue : nullptr;
+    if (!valueDominatesPHI(CommonValue, PN, Q.DT))
+      return nullptr;
+
+    // Make sure we do not replace an undef value with poison.
+    if (HasUndefInput &&
+        !isGuaranteedNotToBePoison(CommonValue, Q.AC, Q.CxtI, Q.DT))
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DianQK wrote:

Maybe this is just my own problem. For me, it's always difficult to understand the negation of may/must semantics.

https://github.com/llvm/llvm-project/pull/96631