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

[Matt Arsenault via llvm-commits]

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@@ -710,39 +716,133 @@ 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 an incoming value), let them share the merged incoming values. The
+// merge process is guided by the equivalence information from \p PhiClasses.
+// The function will possibly update the incoming values of leader phi in
+// DeletedPhis.
+void StructurizeCFG::mergeIfCompatible(
+    EquivalenceClasses<PHINode *> &PhiClasses, PHINode *A, PHINode *B) {
+  auto ItA = PhiClasses.findLeader(PhiClasses.insert(A));
+  auto ItB = PhiClasses.findLeader(PhiClasses.insert(B));
+  // They are already in the same class, no work needed.
+  if (ItA == ItB)
+    return;
+
+  PHINode *LeaderA = *ItA;
+  PHINode *LeaderB = *ItB;
+  BBValueVector &IncomingA = DeletedPhis[LeaderA->getParent()][LeaderA];
+  BBValueVector &IncomingB = DeletedPhis[LeaderB->getParent()][LeaderB];
+
+  DenseMap<BasicBlock *, Value *> Mergeable(IncomingA.begin(), IncomingA.end());
+  for (auto [BB, V] : IncomingB) {
+    auto BBIt = Mergeable.find(BB);
+    if (BBIt != Mergeable.end() && BBIt->second != V)
+      return;
+    // Either IncomingA does not have this value or IncomingA has the same
+    // value.
+    Mergeable.insert({BB, V});
+  }
+
+  // Update the incoming value of leaderA.
+  IncomingA.assign(Mergeable.begin(), Mergeable.end());
+  PhiClasses.unionSets(ItA, ItB);
+
+  return;
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arsenm wrote:

Don't need return 

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