[llvm] StructurizeCFG: Optimize phi insertion during ssa reconstruction (PR #101301)

[Sameer Sahasrabuddhe via llvm-commits]

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@@ -710,10 +714,114 @@ void StructurizeCFG::findUndefBlocks(
   }
 }
 
+// Return true if two PHI nodes have compatible incoming values (for each
+// incoming block, either they have the same incoming value or only one PHI
+// node has a incoming value). And return the union of the incoming values
+// through \p Merged.
+bool StructurizeCFG::isCompatible(const BBValueVector &IncomingA,
+                                  const BBValueVector &IncomingB,
+                                  BBValueVector &Merged) {
+  DenseMap<BasicBlock *, Value *> UnionSet;
+  for (auto &V : IncomingA)
+    UnionSet.insert(V);
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ssahasra wrote:

Does it work if you initialize the variable with begin/end iterators from `IncomingA`? Or use `insert()` that takes those same iterators?

https://github.com/llvm/llvm-project/pull/101301