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

[Matt Arsenault via llvm-commits]

================
@@ -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)
+      return;
+    // Either IncomingA does not have this value or IncomingA has the same
+    // value.
+    Mergeable.insert({BB, V});
+  }
+
+  unsigned PoolIndex;
+
+  if (FoundA || FoundB) {
+    PoolIndex = FoundA ? ItA->second : ItB->second;
+    BBValuesPool[PoolIndex].clear();
+    BBValuesPool[PoolIndex].append(Mergeable.begin(), Mergeable.end());
+  } else {
+    PoolIndex = BBValuesPool.size();
+    BBValuesPool.emplace_back(Mergeable.begin(), Mergeable.end());
+  }
+
+  // Skip insertion if Phi was already merged with other phi node.
+  if (!FoundA)
+    MergedPHIMap.insert({A, PoolIndex});
+
+  if (!FoundB)
+    MergedPHIMap.insert({B, PoolIndex});
+
+  return;
+}
+
 /// Add the real PHI value as soon as everything is set up
 void StructurizeCFG::setPhiValues() {
   SmallVector<PHINode *, 8> InsertedPhis;
   SSAUpdater Updater(&InsertedPhis);
+
+  // Find out phi nodes that have compatible incoming values (either they have
----------------
arsenm wrote:

```suggestion
  // Find phi nodes that have compatible incoming values (either they have
```

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