[llvm] [InstSimplify] Fix Inconsistent PHI Simplification (PR #113037)

[via llvm-commits]

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@@ -5309,20 +5357,18 @@ static Value *simplifyPHINode(PHINode *PN, ArrayRef<Value *> IncomingValues,
       continue;
     }
     if (Q.isUndefValue(Incoming)) {
-      // Remember that we saw an undef value, but otherwise ignore them.
+      // Remember that we saw an undef value.
       HasUndefInput = true;
-      continue;
     }
-    if (CommonValue && Incoming != CommonValue)
+    CommonValue = getCommonPHIValue(CommonValue, Incoming, Q);
+    if (!CommonValue)
       return nullptr; // Not the same, bail out.
-    CommonValue = Incoming;
   }
 
-  // If CommonValue is null then all of the incoming values were either undef,
-  // poison or equal to the phi node itself.
+  // If CommonValue is null then all of the incoming values were either poison
+  // or equal to the phi node itself.
   if (!CommonValue)
-    return HasUndefInput ? UndefValue::get(PN->getType())
-                         : PoisonValue::get(PN->getType());
+    return PoisonValue::get(PN->getType());
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goldsteinn wrote:

Is this not turning `undef` into `poison` in some cases?

https://github.com/llvm/llvm-project/pull/113037