[llvm] [SimplifyCFG] Skip phi scans when folded block shares no predecessors (PR #225647)

[Ramkumar Ramachandra via llvm-commits]

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@@ -963,6 +961,25 @@ static void gatherIncomingValuesToPhi(PHINode *PN,
   }
 }
 
+/// If the incoming values of a phi at \p UndefOps are a mix of undef and
+/// poison, replace the poison values with undef.
+///
+/// \param PN The phi whose incoming values are replaced.
+/// \param UndefOps The indices of incoming values that are undef or poison.
+static void replacePoisonWithUndefIfMixed(PHINode *PN,
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artagnon wrote:

So, the reason I'm suggesting this is because the patch as-is has degenerate compile-time blow up in certain cases, as I suspected: https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/1525. I think the ultimate fix is actually just stripping the undef handling?

https://github.com/llvm/llvm-project/pull/225647