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

[Matt Arsenault via llvm-commits]

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@@ -710,10 +717,104 @@ void StructurizeCFG::findUndefBlocks(
   }
 }
 
+// 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), let them share the merged incoming values.
+void StructurizeCFG::mergeIfCompatible(PHINode *A, PHINode *B) {
+
+  auto ItA = MergedPHIMap.find(A);
+  auto ItB = MergedPHIMap.find(B);
+  bool FoundA = ItA != MergedPHIMap.end();
+  bool FoundB = ItB != MergedPHIMap.end();
+
+  // Skip phis that were both already merged with others.
+  if (FoundA && FoundB)
+    return;
+
+  const auto &IncomingA =
+      FoundA ? BBValuesPool[ItA->second] : DeletedPhis[A->getParent()][A];
+  const auto &IncomingB =
+      FoundB ? BBValuesPool[ItB->second] : DeletedPhis[B->getParent()][B];
+
+  DenseMap<BasicBlock *, Value *> Mergeable(IncomingA.begin(), IncomingA.end());
+  for (auto &[BB, V] : IncomingB) {
+    if (Mergeable.contains(BB) && Mergeable[BB] != V)
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arsenm wrote:

This is the same lookup twice, use find? 

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