[llvm] [DomTreeUpdater] Move critical edge splitting code to updater (PR #115111)

[Jakub Kuderski via llvm-commits]

================
@@ -347,6 +389,131 @@ void GenericDomTreeUpdater<DerivedT, DomTreeT,
   PendPDTUpdateIndex -= dropIndex;
 }
 
+template <typename DerivedT, typename DomTreeT, typename PostDomTreeT>
+void GenericDomTreeUpdater<DerivedT, DomTreeT, PostDomTreeT>::
+    splitDTCriticalEdges(ArrayRef<CriticalEdge> Edges) {
+  // Bail out early if there is nothing to do.
+  if (Edges.empty())
+    return;
+
+  // Remember all the basic blocks that are inserted during
+  // edge splitting.
+  // Invariant: NewBBs == all the basic blocks contained in the NewBB
+  // field of all the elements of Edges.
+  // I.e., forall elt in Edges, it exists BB in NewBBs
+  // such as BB == elt.NewBB.
+  SmallSet<BasicBlockT *, 32> NewBBs;
+  for (auto &Edge : Edges)
+    NewBBs.insert(Edge.NewBB);
+  // For each element in Edges, remember whether or not element
+  // is the new immediate domminator of its successor. The mapping is done by
+  // index, i.e., the information for the ith element of Edges is
+  // the ith element of IsNewIDom.
+  SmallBitVector IsNewIDom(Edges.size(), true);
+  size_t Idx = 0;
+
+  // Collect all the dominance properties info, before invalidating
+  // the underlying DT.
+  for (const auto &Edge : Edges) {
+    // Update dominator information.
+    if (DT) {
+      BasicBlockT *Succ = Edge.ToBB;
+      auto *SuccDTNode = DT->getNode(Succ);
+
+      for (BasicBlockT *PredBB : predecessors(Succ)) {
+        if (PredBB == Edge.NewBB)
+          continue;
+        // If we are in this situation:
+        // FromBB1        FromBB2
+        //    +              +
+        //   + +            + +
+        //  +   +          +   +
+        // ...  Split1  Split2 ...
+        //           +   +
+        //            + +
+        //             +
+        //            Succ
+        // Instead of checking the domiance property with Split2, we check it
+        // with FromBB2 since Split2 is still unknown of the underlying DT
+        // structure.
+        if (NewBBs.count(PredBB)) {
----------------
kuhar wrote:

I think this should be supported?

```suggestion
        if (NewBBs.contains(PredBB)) {
```

https://github.com/llvm/llvm-project/pull/115111