[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);
+
+  for (auto &[BB, V] : IncomingB) {
+    if (UnionSet.contains(BB) && UnionSet[BB] != V)
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ssahasra wrote:

There are three lookups on the map here, worst case being when the union already contains the same value, but we insert it anyway. Should be possible to replace all that with a single `find()`.

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