[Dominators] Implement incremental insertions

[kuhar]

Summary

This patch introduces incremental edge insertions based on the Depth Based Search algorithm.

Insertions should work for both dominators and postdominators.

[kuhar]

include/llvm/Support/GenericDomTreeConstruction.h

171

Extra semicolon.

[• dberlin]

include/llvm/Support/GenericDomTree.h

451

s/edge insertion/CFG edge insertion/

455

Will it really work if you do it before the actual cfg change exists?
(IE without that edge, will you still be able to find the right successors, predecessors?)
If so, great!

[kuhar]

include/llvm/Support/GenericDomTree.h

455

Yes, but only for insertions, not for deletions.

The proof goes like this:
There are 2 cases - insertion from (x -> y) with incoming edges to reachable (a) nodes and unreachable (b) nodes.
a) To notice that the about-to-be inserted edge (x -> y) is missing, the path P mentioned in lemma 2.5 would have to go to back to x. So when we visit x we iterate it's successors to find affected nodes. y is not there, since there is no edge (yet) from x to y, but it doesn't matter, as we only visit each node once and the algorithm starts by visiting y.
b) In the case when y is unreachable, the algorithm reruns SemiNCA on a subtree starting at y. SemiNCA only processed unreachable nodes and y will be processed first. Then, it performs reachable insertions on the connecting edges it found during the DFS walk. (x -> y) cannot be a connecting edge, as y was unreachable.

So there's no way for the algorithm to notice that there's no edge from x to y.

When it comes to deletions, there are cases when the algorithm needs to rebuild the whole tree -- so the deletion has to happen before, or it'll reconstruct the exact same tree.

include/llvm/IR/Dominators.h

 namespace DomTreeBuilder {
 using BBDomTree = DominatorTreeBase<BasicBlock, false>;
 using BBPostDomTree = DominatorTreeBase<BasicBlock, true>;
 
 extern template void Calculate<BBDomTree, Function>(BBDomTree &DT, Function &F);
 extern template void Calculate<BBPostDomTree, Function>(BBPostDomTree &DT,
                                                         Function &F);
 
+extern template void InsertEdge<BBDomTree>(BBDomTree &DT, BasicBlock *From,
+                                           BasicBlock *To);
+extern template void InsertEdge<BBPostDomTree>(BBPostDomTree &DT,
+                                               BasicBlock *From,
+                                               BasicBlock *To);
+
 extern template bool Verify<BBDomTree>(const BBDomTree &DT);
 extern template bool Verify<BBPostDomTree>(const BBPostDomTree &DT);
 }  // namespace DomTreeBuilder
 
 using DomTreeNode = DomTreeNodeBase<BasicBlock>;
 
 class BasicBlockEdge {
   const BasicBlock *Start;

include/llvm/Support/GenericDomTree.h

 #include <utility>
 #include <vector>
 
 namespace llvm {
 
 template <typename NodeT, bool IsPostDom>
 class DominatorTreeBase;
 
+namespace DomTreeBuilder {
+template <class DomTreeT>
+struct SemiNCAInfo;
+}  // namespace DomTreeBuilder
+
 /// \brief Base class for the actual dominator tree node.
 template <class NodeT> class DomTreeNodeBase {
   friend struct PostDominatorTree;
   friend class DominatorTreeBase<NodeT, false>;
   friend class DominatorTreeBase<NodeT, true>;
+  friend struct DomTreeBuilder::SemiNCAInfo<DominatorTreeBase<NodeT, false>>;
+  friend struct DomTreeBuilder::SemiNCAInfo<DominatorTreeBase<NodeT, true>>;
 
   NodeT *TheBB;
   DomTreeNodeBase *IDom;
   unsigned Level;
   std::vector<DomTreeNodeBase *> Children;
   mutable unsigned DFSNumIn = ~0;
   mutable unsigned DFSNumOut = ~0;
 
void PrintDomTree(const DomTreeNodeBase<NodeT> *N, raw_ostream &O,
   O.indent(2 * Lev) << "[" << Lev << "] " << N;
   for (typename DomTreeNodeBase<NodeT>::const_iterator I = N->begin(),
                                                        E = N->end();
        I != E; ++I)
     PrintDomTree<NodeT>(*I, O, Lev + 1);
 }
 
 namespace DomTreeBuilder {
-template <typename DomTreeT>
-struct SemiNCAInfo;
-
-// The calculate routine is provided in a separate header but referenced here.
+// The routines below are provided in a separate header but referenced here.
 template <typename DomTreeT, typename FuncT>
 void Calculate(DomTreeT &DT, FuncT &F);
 
-// The verify function is provided in a separate header but referenced here.
+template <class DomTreeT>
+void InsertEdge(DomTreeT &DT, typename DomTreeT::NodePtr From,
+                typename DomTreeT::NodePtr To);
+
 template <typename DomTreeT>
 bool Verify(const DomTreeT &DT);
 }  // namespace DomTreeBuilder
 
 /// \brief Core dominator tree base class.
 ///
 /// This class is a generic template over graph nodes. It is instantiated for
 /// various graphs in the LLVM IR or in the code generator.
#endif
   bool isVirtualRoot(const DomTreeNodeBase<NodeT> *A) const {
     return isPostDominator() && !A->getBlock();
   }
 
   //===--------------------------------------------------------------------===//
   // API to update (Post)DominatorTree information based on modifications to
   // the CFG...
 
+  /// Inform the dominator tree about a CFG edge insertion and update the tree.
+  ///
+  /// This function has to be called just before or just after making the update
+  /// on the actual CFG. There cannot be any other updates that the dominator
+  /// tree doesn't know about.
+  /// Note that for postdominators it automatically takes care of inserting
+  /// a reverse edge internally (so there's no need to swap the parameters).
+  ///
+  void insertEdge(NodeT *From, NodeT *To) {
+    assert(From);
+    assert(To);
+    DomTreeBuilder::InsertEdge(*this, From, To);
+  }
+
   /// Add a new node to the dominator tree information.
   ///
   /// This creates a new node as a child of DomBB dominator node, linking it
   /// into the children list of the immediate dominator.
   ///
   /// \param BB New node in CFG.
   /// \param DomBB CFG node that is dominator for BB.
   /// \returns New dominator tree node that represents new CFG node.

include/llvm/Support/GenericDomTreeConstruction.h

 ///   Linear-Time Algorithms for Dominators and Related Problems
 ///   Loukas Georgiadis, Princeton University, November 2005, pp. 21-23:
 ///   ftp://ftp.cs.princeton.edu/reports/2005/737.pdf
 ///
 /// This implements the O(n*log(n)) versions of EVAL and LINK, because it turns
 /// out that the theoretically slower O(n*log(n)) implementation is actually
 /// faster than the almost-linear O(n*alpha(n)) version, even for large CFGs.
 ///
+/// The file uses the Depth Based Search algorithm to perform incremental
+/// upates (insertion and deletions). The implemented algorithm is based on this
+/// publication:
+///
+///   An Experimental Study of Dynamic Dominators
+///   Loukas Georgiadis, et al., April 12 2016, pp. 5-7, 9-10:
+///   https://arxiv.org/pdf/1604.02711.pdf
+///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
 #define LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
 
+#include <queue>
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/GenericDomTree.h"
 
 #define DEBUG_TYPE "dom-tree-builder"
 
 namespace llvm {
struct ChildrenGetter<NodePtr, true> {
   }
 };
 
 template <typename DomTreeT>
 struct SemiNCAInfo {
   using NodePtr = typename DomTreeT::NodePtr;
   using NodeT = typename DomTreeT::NodeType;
   using TreeNodePtr = DomTreeNodeBase<NodeT> *;
+  static constexpr bool IsPostDom = DomTreeT::IsPostDominator;
 
   // Information record used by Semi-NCA during tree construction.
   struct InfoRec {
     unsigned DFSNum = 0;
     unsigned Parent = 0;
     unsigned Semi = 0;
     NodePtr Label = nullptr;
     NodePtr IDom = nullptr;
while (!WorkList.empty()) {
         auto &SuccInfo = NodeToInfo[Succ];
         WorkList.push_back(Succ);
         SuccInfo.Parent = LastNum;
         SuccInfo.ReverseChildren.push_back(BB);
       }
     }
 
     return LastNum;
   }
 
   NodePtr eval(NodePtr VIn, unsigned LastLinked) {
     auto &VInInfo = NodeToInfo[VIn];
     if (VInInfo.DFSNum < LastLinked)
       return VIn;
 
     SmallVector<NodePtr, 32> Work;
     SmallPtrSet<NodePtr, 32> Visited;
while (!Work.empty()) {
       if (NodeToInfo[VAncestorLabel].Semi < NodeToInfo[VLabel].Semi)
         VInfo.Label = VAncestorLabel;
       VInfo.Parent = VAInfo.Parent;
     }
 
     return VInInfo.Label;
   }
 
+  // This function requires DFS to be run before calling it.
   void runSemiNCA(DomTreeT &DT, const unsigned MinLevel = 0) {
     const unsigned NextDFSNum(NumToNode.size());
     // Initialize IDoms to spanning tree parents.
     for (unsigned i = 1; i < NextDFSNum; ++i) {
       const NodePtr V = NumToNode[i];
       auto &VInfo = NodeToInfo[V];
       VInfo.IDom = NumToNode[VInfo.Parent];
     }
for (size_t i = 1, e = NumToNode.size(); i != e; ++i) {
 
       // Add a new tree node for this BasicBlock, and link it as a child of
       // IDomNode
       DT.DomTreeNodes[W] = IDomNode->addChild(
           llvm::make_unique<DomTreeNodeBase<NodeT>>(W, IDomNode));
     }
   }
 
+  // Helper struct used during edge insertions.
+  struct InsertionInfo {
+    using BucketElementTy = std::pair<unsigned, TreeNodePtr>;
+    struct DecreasingLevel {
+      bool operator()(const BucketElementTy &First,
+                      const BucketElementTy &Second) const {
+        return First.first > Second.first;
+      }
+    };
+
+    std::priority_queue<BucketElementTy, SmallVector<BucketElementTy, 8>,
+                        DecreasingLevel>
+        Bucket;  // Queue of tree nodes sorted by level in descending order.
+    SmallDenseSet<TreeNodePtr, 8> Affected;
+    SmallDenseSet<TreeNodePtr, 8> Visited;
+    SmallVector<TreeNodePtr, 8> AffectedQueue;
+    SmallVector<TreeNodePtr, 8> VisitedNotAffectedQueue;
+  };
+
+  static void InsertEdge(DomTreeT &DT, const NodePtr From, const NodePtr To) {
+    assert(From && To && "Cannot connect nullptrs");
+    DEBUG(dbgs() << "Inserting edge " << BlockNamePrinter(From) << " -> "
+                 << BlockNamePrinter(To) << "\n");
+    const TreeNodePtr FromTN = DT.getNode(From);
+
+    // Ignore edges from unreachable nodes.
+    if (!FromTN) return;
+
+    DT.DFSInfoValid = false;
+
+    const TreeNodePtr ToTN = DT.getNode(To);
+    if (!ToTN)
+      InsertUnreachable(DT, FromTN, To);
+    else
+      InsertReachable(DT, FromTN, ToTN);
+  }
+
+  // Handles insertion to a node already in the dominator tree.
+  static void InsertReachable(DomTreeT &DT, const TreeNodePtr From,
+                              const TreeNodePtr To) {
+    DEBUG(dbgs() << "\tReachable " << BlockNamePrinter(From->getBlock())
+                 << " -> " << BlockNamePrinter(To->getBlock()) << "\n");
+    const NodePtr NCDBlock =
+        DT.findNearestCommonDominator(From->getBlock(), To->getBlock());
+    assert(NCDBlock || DT.isPostDominator());
+    const TreeNodePtr NCD = DT.getNode(NCDBlock);
+    assert(NCD);
+
+    DEBUG(dbgs() << "\t\tNCA == " << BlockNamePrinter(NCD) << "\n");
+    const TreeNodePtr ToIDom = To->getIDom();
+
+    // Nothing affected -- NCA property holds.
+    // (Based on the lemma 2.5 from the second paper.)
+    if (NCD == To || NCD == ToIDom) return;
+
+    // Identify and collect affected nodes.
+    InsertionInfo II;
+    DEBUG(dbgs() << "Marking " << BlockNamePrinter(To) << " as affected\n");
+    II.Affected.insert(To);
+    const unsigned ToLevel = To->getLevel();
+    DEBUG(dbgs() << "Putting " << BlockNamePrinter(To) << " into a Bucket\n");
+    II.Bucket.push({ToLevel, To});
+
+    while (!II.Bucket.empty()) {
+      const TreeNodePtr CurrentNode = II.Bucket.top().second;
+      II.Bucket.pop();
+      DEBUG(dbgs() << "\tAdding to Visited and AffectedQueue: "
+                   << BlockNamePrinter(CurrentNode) << "\n");
+      II.Visited.insert(CurrentNode);
+      II.AffectedQueue.push_back(CurrentNode);
+
+      // Discover and collect affected successors of the current node.
+      VisitInsertion(DT, CurrentNode, ToLevel, NCD, II);
+    }
+
+    // Finish by updating immediate dominators and levels.
+    UpdateInsertion(DT, NCD, II);
+  }
+
+  // Visits an affected node and collect its affected successors.
+  static void VisitInsertion(DomTreeT &DT, const TreeNodePtr TN,
+                             const unsigned RootLevel, const TreeNodePtr NCD,
+                             InsertionInfo &II) {
+    const unsigned NCDLevel = NCD->getLevel();
+    DEBUG(dbgs() << "Visiting " << BlockNamePrinter(TN) << "\n");
+
+    assert(TN->getBlock());
+    for (const NodePtr Succ :
+         ChildrenGetter<NodePtr, IsPostDom>::Get(TN->getBlock())) {
+      const TreeNodePtr SuccTN = DT.getNode(Succ);
+      assert(SuccTN && "Unreachable successor found at reachable insertion");
+      const unsigned SuccLevel = SuccTN->getLevel();
+
+      DEBUG(dbgs() << "\tSuccessor " << BlockNamePrinter(Succ)
+                   << ", level = " << SuccLevel << "\n");
+
+      // Succ dominated by subtree From -- not affected.
+      // (Based on the lemma 2.5 from the second paper.)
+      if (SuccLevel > RootLevel) {
+        DEBUG(dbgs() << "\t\tDominated by subtree From\n");
+        if (II.Visited.count(SuccTN) != 0) continue;
+
+        DEBUG(dbgs() << "\t\tMarking visited not affected "
+                     << BlockNamePrinter(Succ) << "\n");
+        II.Visited.insert(SuccTN);
+        II.VisitedNotAffectedQueue.push_back(SuccTN);
+        VisitInsertion(DT, SuccTN, RootLevel, NCD, II);
+      } else if ((SuccLevel > NCDLevel + 1) && II.Affected.count(SuccTN) == 0) {
+        DEBUG(dbgs() << "\t\tMarking affected and adding "
+                     << BlockNamePrinter(Succ) << " to a Bucket\n");
+        II.Affected.insert(SuccTN);
+        II.Bucket.push({SuccLevel, SuccTN});
+      }
+    }
+  }
+
+  // Updates immediate dominators and levels after insertion.
+  static void UpdateInsertion(DomTreeT &DT, const TreeNodePtr NCD,
+                              InsertionInfo &II) {
+    DEBUG(dbgs() << "Updating NCD = " << BlockNamePrinter(NCD) << "\n");
+
+    for (const TreeNodePtr TN : II.AffectedQueue) {
+      DEBUG(dbgs() << "\tIDom(" << BlockNamePrinter(TN)
+                   << ") = " << BlockNamePrinter(NCD) << "\n");
+      TN->setIDom(NCD);
+    }
+
+    UpdateLevelsAfterInsertion(II);
+  }
+
+  static void UpdateLevelsAfterInsertion(InsertionInfo &II) {
+    DEBUG(dbgs() << "Updating levels for visited but not affected nodes\n");
+
+    for (const TreeNodePtr TN : II.VisitedNotAffectedQueue) {
+      DEBUG(dbgs() << "\tlevel(" << BlockNamePrinter(TN) << ") = ("
+                   << BlockNamePrinter(TN->getIDom()) << ") "
+                   << TN->getIDom()->getLevel() << " + 1\n");
+      TN->UpdateLevel();
+    }
+  }
+
+  // Handles insertion to previousely unreachable nodes.
+  static void InsertUnreachable(DomTreeT &DT, const TreeNodePtr From,
+                                const NodePtr To) {
+    DEBUG(dbgs() << "Inserting " << BlockNamePrinter(From)
+                 << " -> (unreachable) " << BlockNamePrinter(To) << "\n");
+
+    // Collect discovered edges to already reachable nodes.
+    SmallVector<std::pair<NodePtr, TreeNodePtr>, 8> DiscoveredEdgesToReachable;
+    // Discover and connect nodes that became reachable with the insertion.
+    ComputeUnreachableDominators(DT, To, From, DiscoveredEdgesToReachable);
+
+    DEBUG(dbgs() << "Inserted " << BlockNamePrinter(From)
+                 << " -> (prev unreachable) " << BlockNamePrinter(To) << "\n");
+
+    DEBUG(DT.print(dbgs()));
+
+    // Used the discovered edges and inset discovered connecting (incoming)
+    // edges.
+    for (const auto &Edge : DiscoveredEdgesToReachable) {
+      DEBUG(dbgs() << "\tInserting discovered connecting edge "
+                   << BlockNamePrinter(Edge.first) << " -> "
+                   << BlockNamePrinter(Edge.second) << "\n");
+      InsertReachable(DT, DT.getNode(Edge.first), Edge.second);
+    }
+  }
+
+  // Connects nodes that become reachable with an insertion.
+  static void ComputeUnreachableDominators(
+      DomTreeT &DT, const NodePtr Root, const TreeNodePtr Incoming,
+      SmallVectorImpl<std::pair<NodePtr, TreeNodePtr>>
+          &DiscoveredConnectingEdges) {
+    assert(!DT.getNode(Root) && "Root must not be reachable");
+
+    // Visit only previously unreachable nodes.
+    auto UnreachableDescender = [&DT, &DiscoveredConnectingEdges](NodePtr From,
+                                                                  NodePtr To) {
+      const TreeNodePtr ToTN = DT.getNode(To);
+      if (!ToTN) return true;
+
+      DiscoveredConnectingEdges.push_back({From, ToTN});
+      return false;
+    };
+
+    SemiNCAInfo SNCA;
+    SNCA.runDFS<IsPostDom>(Root, 0, UnreachableDescender, 0);
+    SNCA.runSemiNCA(DT);
+    SNCA.attachNewSubtree(DT, Incoming);
+
+    DEBUG(dbgs() << "After adding unreachable nodes\n");
+    DEBUG(DT.print(dbgs()));
+  }
+
   // Checks if the tree contains all reachable nodes in the input graph.
   bool verifyReachability(const DomTreeT &DT) {
     clear();
     doFullDFSWalk(DT, AlwaysDescend);
 
     for (auto &NodeToTN : DT.DomTreeNodes) {
       const TreeNodePtr TN = NodeToTN.second.get();
       const NodePtr BB = TN->getBlock();

 
 template <class DomTreeT, class FuncT>
 void Calculate(DomTreeT &DT, FuncT &F) {
   SemiNCAInfo<DomTreeT> SNCA;
   SNCA.calculateFromScratch(DT, GraphTraits<FuncT *>::size(&F));
 }
 
 template <class DomTreeT>
+void InsertEdge(DomTreeT &DT, typename DomTreeT::NodePtr From,
+                typename DomTreeT::NodePtr To) {
+  if (DT.isPostDominator()) std::swap(From, To);
+  SemiNCAInfo<DomTreeT>::InsertEdge(DT, From, To);
+}
+
+template <class DomTreeT>
 bool Verify(const DomTreeT &DT) {
   SemiNCAInfo<DomTreeT> SNCA;
   return SNCA.verifyReachability(DT) && SNCA.VerifyLevels(DT) &&
          SNCA.verifyNCD(DT) && SNCA.verifyParentProperty(DT) &&
          SNCA.verifySiblingProperty(DT);
 }
 
 }  // namespace DomTreeBuilder
 }  // namespace llvm
 
 #undef DEBUG_TYPE
 
 #endif

lib/IR/Dominators.cpp

 
 template void
 llvm::DomTreeBuilder::Calculate<DomTreeBuilder::BBDomTree, Function>(
     DomTreeBuilder::BBDomTree &DT, Function &F);
 template void
 llvm::DomTreeBuilder::Calculate<DomTreeBuilder::BBPostDomTree, Function>(
     DomTreeBuilder::BBPostDomTree &DT, Function &F);
 
+template void llvm::DomTreeBuilder::InsertEdge<DomTreeBuilder::BBDomTree>(
+    DomTreeBuilder::BBDomTree &DT, BasicBlock *From, BasicBlock *To);
+template void llvm::DomTreeBuilder::InsertEdge<DomTreeBuilder::BBPostDomTree>(
+    DomTreeBuilder::BBPostDomTree &DT, BasicBlock *From, BasicBlock *To);
+
 template bool llvm::DomTreeBuilder::Verify<DomTreeBuilder::BBDomTree>(
     const DomTreeBuilder::BBDomTree &DT);
 template bool llvm::DomTreeBuilder::Verify<DomTreeBuilder::BBPostDomTree>(
     const DomTreeBuilder::BBPostDomTree &DT);
 
 bool DominatorTree::invalidate(Function &F, const PreservedAnalyses &PA,
                                FunctionAnalysisManager::Invalidator &) {
   // Check whether the analysis, all analyses on functions, or the function's

unittests/IR/DominatorTreeTest.cpp

 #include "llvm/Analysis/PostDominators.h"
 #include "llvm/AsmParser/Parser.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Support/SourceMgr.h"
+#include "CFGBuilder.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
 
 struct PostDomTree : DominatorTreeBase<BasicBlock, true> {
   PostDomTree(Function &F) { recalculate(F); }
 };
 
runWithDomTree(
 
         EXPECT_FALSE(DT->dominates(Edge_BB0_BB1_a, BB1));
         EXPECT_FALSE(DT->dominates(Edge_BB0_BB1_b, BB1));
 
         EXPECT_FALSE(DT->dominates(Edge_BB0_BB1_a, BB2));
         EXPECT_FALSE(DT->dominates(Edge_BB0_BB1_b, BB2));
       });
 }
+
+namespace {
+const auto Insert = CFGBuilder::ActionKind::Insert;
+
+bool CompUpdates(const CFGBuilder::Update &A, const CFGBuilder::Update &B) {
+  return std::tie(A.Action, A.Edge.From, A.Edge.To) <
+         std::tie(B.Action, B.Edge.From, B.Edge.To);
+};
+}  // namespace
+
+TEST(DominatorTree, InsertReachable) {
+  CFGHolder Holder;
+  std::vector<CFGBuilder::Arc> Arcs = {
+      {"1", "2"}, {"2", "3"}, {"3", "4"},  {"4", "5"},  {"5", "6"},  {"5", "7"},
+      {"3", "8"}, {"8", "9"}, {"9", "10"}, {"8", "11"}, {"11", "12"}};
+
+  std::vector<CFGBuilder::Update> Updates = {{Insert, {"12", "10"}},
+                                             {Insert, {"10", "9"}},
+                                             {Insert, {"7", "6"}},
+                                             {Insert, {"7", "5"}}};
+  CFGBuilder B(Holder.F, Arcs, Updates);
+  DominatorTree DT(*Holder.F);
+  EXPECT_TRUE(DT.verify());
+  PostDomTree PDT(*Holder.F);
+  EXPECT_TRUE(PDT.verify());
+
+  Optional<CFGBuilder::Update> LastUpdate;
+  while ((LastUpdate = B.applyUpdate())) {
+    EXPECT_EQ(LastUpdate->Action, Insert);
+    BasicBlock *From = B.getOrAddBlock(LastUpdate->Edge.From);
+    BasicBlock *To = B.getOrAddBlock(LastUpdate->Edge.To);
+    DT.insertEdge(From, To);
+    EXPECT_TRUE(DT.verify());
+    PDT.insertEdge(From, To);
+    EXPECT_TRUE(PDT.verify());
+  }
+}
+
+TEST(DominatorTree, InsertUnreachable) {
+  CFGHolder Holder;
+  std::vector<CFGBuilder::Arc> Arcs = {{"1", "2"},  {"2", "3"},  {"3", "4"},
+                                       {"5", "6"},  {"5", "7"},  {"3", "8"},
+                                       {"9", "10"}, {"11", "12"}};
+
+  std::vector<CFGBuilder::Update> Updates = {{Insert, {"4", "5"}},
+                                             {Insert, {"8", "9"}},
+                                             {Insert, {"10", "12"}},
+                                             {Insert, {"10", "11"}}};
+  CFGBuilder B(Holder.F, Arcs, Updates);
+  DominatorTree DT(*Holder.F);
+  EXPECT_TRUE(DT.verify());
+  PostDomTree PDT(*Holder.F);
+  EXPECT_TRUE(PDT.verify());
+
+  Optional<CFGBuilder::Update> LastUpdate;
+  while ((LastUpdate = B.applyUpdate())) {
+    EXPECT_EQ(LastUpdate->Action, Insert);
+    BasicBlock *From = B.getOrAddBlock(LastUpdate->Edge.From);
+    BasicBlock *To = B.getOrAddBlock(LastUpdate->Edge.To);
+    DT.insertEdge(From, To);
+    EXPECT_TRUE(DT.verify());
+    PDT.insertEdge(From, To);
+    EXPECT_TRUE(PDT.verify());
+  }
+}
+
+TEST(DominatorTree, InsertMixed) {
+  CFGHolder Holder;
+  std::vector<CFGBuilder::Arc> Arcs = {
+      {"1", "2"}, {"2", "3"},  {"3", "4"},  {"5", "6"},   {"5", "7"},
+      {"8", "9"}, {"9", "10"}, {"8", "11"}, {"11", "12"}, {"7", "3"}};
+
+  std::vector<CFGBuilder::Update> Updates = {
+      {Insert, {"4", "5"}},   {Insert, {"2", "5"}},   {Insert, {"10", "9"}},
+      {Insert, {"12", "10"}}, {Insert, {"12", "10"}}, {Insert, {"7", "8"}},
+      {Insert, {"7", "5"}}};
+  CFGBuilder B(Holder.F, Arcs, Updates);
+  DominatorTree DT(*Holder.F);
+  EXPECT_TRUE(DT.verify());
+  PostDomTree PDT(*Holder.F);
+  EXPECT_TRUE(PDT.verify());
+
+  Optional<CFGBuilder::Update> LastUpdate;
+  while ((LastUpdate = B.applyUpdate())) {
+    EXPECT_EQ(LastUpdate->Action, Insert);
+    BasicBlock *From = B.getOrAddBlock(LastUpdate->Edge.From);
+    BasicBlock *To = B.getOrAddBlock(LastUpdate->Edge.To);
+    DT.insertEdge(From, To);
+    EXPECT_TRUE(DT.verify());
+    PDT.insertEdge(From, To);
+    EXPECT_TRUE(PDT.verify());
+  }
+}
+
+TEST(DominatorTree, InsertPermut) {
+  std::vector<CFGBuilder::Arc> Arcs = {
+      {"1", "2"}, {"2", "3"},  {"3", "4"},  {"5", "6"},   {"5", "7"},
+      {"8", "9"}, {"9", "10"}, {"8", "11"}, {"11", "12"}, {"7", "3"}};
+
+  std::vector<CFGBuilder::Update> Updates = {{Insert, {"4", "5"}},
+                                             {Insert, {"2", "5"}},
+                                             {Insert, {"10", "9"}},
+                                             {Insert, {"12", "10"}}};
+
+  while (std::next_permutation(Updates.begin(), Updates.end(), CompUpdates)) {
+    CFGHolder Holder;
+    CFGBuilder B(Holder.F, Arcs, Updates);
+    DominatorTree DT(*Holder.F);
+    EXPECT_TRUE(DT.verify());
+    PostDomTree PDT(*Holder.F);
+    EXPECT_TRUE(PDT.verify());
+
+    Optional<CFGBuilder::Update> LastUpdate;
+    while ((LastUpdate = B.applyUpdate())) {
+      EXPECT_EQ(LastUpdate->Action, Insert);
+      BasicBlock *From = B.getOrAddBlock(LastUpdate->Edge.From);
+      BasicBlock *To = B.getOrAddBlock(LastUpdate->Edge.To);
+      DT.insertEdge(From, To);
+      EXPECT_TRUE(DT.verify());
+      PDT.insertEdge(From, To);
+      EXPECT_TRUE(PDT.verify());
+    }
+  }
+}