[llvm] Deduplication of cyclic PHI nodes (PR #86662)

[via llvm-commits]

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@@ -1406,15 +1446,16 @@ EliminateDuplicatePHINodesSetBasedImpl(BasicBlock *BB,
       return PN == getEmptyKey() || PN == getTombstoneKey();
     }
 
-    // WARNING: this logic must be kept in sync with
-    //          Instruction::isIdenticalToWhenDefined()!
     static unsigned getHashValueImpl(PHINode *PN) {
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annamthomas wrote:

Why do you want to do this deduplication in the set based implementation: we always do the naive implementation as long as the number of phis in a block is less than 32. 

In this way, you don't need to change the hash implementation here. 
I'm a bit wary of removing this "WARNING" above and not having any change in the `isIdenticalToWhenDefined` counterpart. 





https://github.com/llvm/llvm-project/pull/86662