[clang-diff] Add initial implementation

This is the first commit for the "Clang-based C/C++ diff tool" GSoC project.

ASTDiff is a new library that computes a structural AST diff between two ASTs
using the gumtree algorithm. Clang-diff is a new Clang tool that will show
the structural code changes between different ASTs.

Patch by Johannes Altmanninger!

Differential Revision: https://reviews.llvm.org/D34329


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308731 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 923 insertions, 0 deletions

diff --git a/lib/Tooling/ASTDiff/ASTDiff.cpp b/lib/Tooling/ASTDiff/ASTDiff.cpp
new file mode 100644
index 0000000000..d4c6702030
--- /dev/null
+++ b/lib/Tooling/ASTDiff/ASTDiff.cpp
@@ -0,0 +1,912 @@
+//===- ASTDiff.cpp - AST differencing implementation-----------*- C++ -*- -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains definitons for the AST differencing interface.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/ASTDiff/ASTDiff.h"
+
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/Lex/Lexer.h"
+#include "llvm/ADT/PriorityQueue.h"
+
+#include <limits>
+#include <memory>
+#include <unordered_set>
+
+using namespace llvm;
+using namespace clang;
+
+namespace clang {
+namespace diff {
+
+class ASTDiff::Impl {
+public:
+  SyntaxTreeImpl &T1, &T2;
+  bool IsMappingDone = false;
+  Mapping TheMapping;
+
+  Impl(SyntaxTreeImpl &T1, SyntaxTreeImpl &T2, const ComparisonOptions &Options)
+      : T1(T1), T2(T2), Options(Options) {}
+
+  /// Matches nodes one-by-one based on their similarity.
+  void computeMapping();
+
+  std::vector<Match> getMatches(Mapping &M);
+
+  /// Finds an edit script that converts T1 to T2.
+  std::vector<Change> computeChanges(Mapping &M);
+
+  void printChangeImpl(raw_ostream &OS, const Change &Chg) const;
+  void printMatchImpl(raw_ostream &OS, const Match &M) const;
+
+  // Returns a mapping of isomorphic subtrees.
+  Mapping matchTopDown() const;
+
+private:
+  // Returns true if the two subtrees are identical.
+  bool isomorphic(NodeId Id1, NodeId Id2) const;
+
+  bool canBeAddedToMapping(const Mapping &M, NodeId Id1, NodeId Id2) const;
+
+  // Returns false if the nodes must not be mached.
+  bool isMatchingPossible(NodeId Id1, NodeId Id2) const;
+
+  // Adds all corresponding subtrees of the two nodes to the mapping.
+  // The two nodes must be isomorphic.
+  void addIsomorphicSubTrees(Mapping &M, NodeId Id1, NodeId Id2) const;
+
+  // Uses an optimal albeit slow algorithm to compute a mapping between two
+  // subtrees, but only if both have fewer nodes than MaxSize.
+  void addOptimalMapping(Mapping &M, NodeId Id1, NodeId Id2) const;
+
+  // Computes the ratio of common descendants between the two nodes.
+  // Descendants are only considered to be equal when they are mapped in M.
+  double getSimilarity(const Mapping &M, NodeId Id1, NodeId Id2) const;
+
+  // Returns the node that has the highest degree of similarity.
+  NodeId findCandidate(const Mapping &M, NodeId Id1) const;
+
+  // Tries to match any yet unmapped nodes, in a bottom-up fashion.
+  void matchBottomUp(Mapping &M) const;
+
+  const ComparisonOptions &Options;
+
+  friend class ZhangShashaMatcher;
+};
+
+template <class T>
+static bool isNodeExcluded(const SourceManager &SrcMgr, T *N) {
+  if (!N)
+    return true;
+  SourceLocation SLoc = N->getLocStart();
+  return SLoc.isValid() && SrcMgr.isInSystemHeader(SLoc);
+}
+
+namespace {
+/// Counts the number of nodes that will be compared.
+struct NodeCountVisitor : public RecursiveASTVisitor<NodeCountVisitor> {
+  int Count = 0;
+  const SyntaxTreeImpl &Root;
+  NodeCountVisitor(const SyntaxTreeImpl &Root) : Root(Root) {}
+  bool TraverseDecl(Decl *D) {
+    if (isNodeExcluded(Root.AST.getSourceManager(), D))
+      return true;
+    ++Count;
+    RecursiveASTVisitor<NodeCountVisitor>::TraverseDecl(D);
+    return true;
+  }
+  bool TraverseStmt(Stmt *S) {
+    if (isNodeExcluded(Root.AST.getSourceManager(), S))
+      return true;
+    ++Count;
+    RecursiveASTVisitor<NodeCountVisitor>::TraverseStmt(S);
+    return true;
+  }
+  bool TraverseType(QualType T) { return true; }
+};
+} // end anonymous namespace
+
+namespace {
+// Sets Height, Parent and Children for each node.
+struct PreorderVisitor : public RecursiveASTVisitor<PreorderVisitor> {
+  int Id = 0, Depth = 0;
+  NodeId Parent;
+  SyntaxTreeImpl &Root;
+
+  PreorderVisitor(SyntaxTreeImpl &Root) : Root(Root) {}
+
+  template <class T> std::tuple<NodeId, NodeId> PreTraverse(T *ASTNode) {
+    NodeId MyId = Id;
+    Node &N = Root.getMutableNode(MyId);
+    N.Parent = Parent;
+    N.Depth = Depth;
+    N.ASTNode = DynTypedNode::create(*ASTNode);
+    assert(!N.ASTNode.getNodeKind().isNone() &&
+           "Expected nodes to have a valid kind.");
+    if (Parent.isValid()) {
+      Node &P = Root.getMutableNode(Parent);
+      P.Children.push_back(MyId);
+    }
+    Parent = MyId;
+    ++Id;
+    ++Depth;
+    return {MyId, Root.getNode(MyId).Parent};
+  }
+  void PostTraverse(std::tuple<NodeId, NodeId> State) {
+    NodeId MyId, PreviousParent;
+    std::tie(MyId, PreviousParent) = State;
+    assert(MyId.isValid() && "Expecting to only traverse valid nodes.");
+    Parent = PreviousParent;
+    --Depth;
+    Node &N = Root.getMutableNode(MyId);
+    N.RightMostDescendant = Id;
+    if (N.isLeaf())
+      Root.Leaves.push_back(MyId);
+    N.Height = 1;
+    for (NodeId Child : N.Children)
+      N.Height = std::max(N.Height, 1 + Root.getNode(Child).Height);
+  }
+  bool TraverseDecl(Decl *D) {
+    if (isNodeExcluded(Root.AST.getSourceManager(), D))
+      return true;
+    auto SavedState = PreTraverse(D);
+    RecursiveASTVisitor<PreorderVisitor>::TraverseDecl(D);
+    PostTraverse(SavedState);
+    return true;
+  }
+  bool TraverseStmt(Stmt *S) {
+    if (isNodeExcluded(Root.AST.getSourceManager(), S))
+      return true;
+    auto SavedState = PreTraverse(S);
+    RecursiveASTVisitor<PreorderVisitor>::TraverseStmt(S);
+    PostTraverse(SavedState);
+    return true;
+  }
+  bool TraverseType(QualType T) { return true; }
+};
+} // end anonymous namespace
+
+SyntaxTreeImpl::SyntaxTreeImpl(SyntaxTree *Parent, const ASTContext &AST)
+    : SyntaxTreeImpl(Parent, AST.getTranslationUnitDecl(), AST) {}
+
+SyntaxTreeImpl::SyntaxTreeImpl(SyntaxTree *Parent, Decl *N,
+                               const ASTContext &AST)
+    : Parent(Parent), AST(AST) {
+  NodeCountVisitor NodeCounter(*this);
+  NodeCounter.TraverseDecl(N);
+  Nodes.resize(NodeCounter.Count);
+  PreorderVisitor PreorderWalker(*this);
+  PreorderWalker.TraverseDecl(N);
+  initTree();
+}
+
+SyntaxTreeImpl::SyntaxTreeImpl(SyntaxTree *Parent, Stmt *N,
+                               const ASTContext &AST)
+    : Parent(Parent), AST(AST) {
+  NodeCountVisitor NodeCounter(*this);
+  NodeCounter.TraverseStmt(N);
+  Nodes.resize(NodeCounter.Count);
+  PreorderVisitor PreorderWalker(*this);
+  PreorderWalker.TraverseStmt(N);
+  initTree();
+}
+
+void SyntaxTreeImpl::initTree() {
+  setLeftMostDescendants();
+  int PostorderId = 0;
+  PostorderIds.resize(getSize());
+  std::function<void(NodeId)> PostorderTraverse = [&](NodeId Id) {
+    for (NodeId Child : getNode(Id).Children)
+      PostorderTraverse(Child);
+    PostorderIds[Id] = PostorderId;
+    ++PostorderId;
+  };
+  PostorderTraverse(root());
+}
+
+void SyntaxTreeImpl::setLeftMostDescendants() {
+  for (NodeId Leaf : Leaves) {
+    getMutableNode(Leaf).LeftMostDescendant = Leaf;
+    NodeId Parent, Cur = Leaf;
+    while ((Parent = getNode(Cur).Parent).isValid() &&
+           getNode(Parent).Children[0] == Cur) {
+      Cur = Parent;
+      getMutableNode(Cur).LeftMostDescendant = Leaf;
+    }
+  }
+}
+
+static std::vector<NodeId> getSubtreePostorder(const SyntaxTreeImpl &Tree,
+                                               NodeId Root) {
+  std::vector<NodeId> Postorder;
+  std::function<void(NodeId)> Traverse = [&](NodeId Id) {
+    const Node &N = Tree.getNode(Id);
+    for (NodeId Child : N.Children)
+      Traverse(Child);
+    Postorder.push_back(Id);
+  };
+  Traverse(Root);
+  return Postorder;
+}
+
+static std::vector<NodeId> getSubtreeBfs(const SyntaxTreeImpl &Tree,
+                                         NodeId Root) {
+  std::vector<NodeId> Ids;
+  size_t Expanded = 0;
+  Ids.push_back(Root);
+  while (Expanded < Ids.size())
+    for (NodeId Child : Tree.getNode(Ids[Expanded++]).Children)
+      Ids.push_back(Child);
+  return Ids;
+}
+
+int SyntaxTreeImpl::getNumberOfDescendants(NodeId Id) const {
+  return getNode(Id).RightMostDescendant - Id + 1;
+}
+
+bool SyntaxTreeImpl::isInSubtree(NodeId Id, NodeId SubtreeRoot) const {
+  NodeId Lower = SubtreeRoot;
+  NodeId Upper = getNode(SubtreeRoot).RightMostDescendant;
+  return Id >= Lower && Id <= Upper;
+}
+
+std::string SyntaxTreeImpl::getNodeValueImpl(NodeId Id) const {
+  return getNodeValueImpl(getNode(Id).ASTNode);
+}
+
+std::string SyntaxTreeImpl::getNodeValueImpl(const DynTypedNode &DTN) const {
+  if (auto *X = DTN.get<BinaryOperator>())
+    return X->getOpcodeStr();
+  if (auto *X = DTN.get<AccessSpecDecl>()) {
+    CharSourceRange Range(X->getSourceRange(), false);
+    return Lexer::getSourceText(Range, AST.getSourceManager(),
+                                AST.getLangOpts());
+  }
+  if (auto *X = DTN.get<IntegerLiteral>()) {
+    SmallString<256> Str;
+    X->getValue().toString(Str, /*Radix=*/10, /*Signed=*/false);
+    return Str.str();
+  }
+  if (auto *X = DTN.get<StringLiteral>())
+    return X->getString();
+  if (auto *X = DTN.get<ValueDecl>())
+    return X->getNameAsString() + "(" + X->getType().getAsString() + ")";
+  if (auto *X = DTN.get<DeclStmt>())
+    return "";
+  if (auto *X = DTN.get<TranslationUnitDecl>())
+    return "";
+  std::string Value;
+  if (auto *X = DTN.get<DeclRefExpr>()) {
+    if (X->hasQualifier()) {
+      llvm::raw_string_ostream OS(Value);
+      PrintingPolicy PP(AST.getLangOpts());
+      X->getQualifier()->print(OS, PP);
+    }
+    Value += X->getDecl()->getNameAsString();
+    return Value;
+  }
+  if (auto *X = DTN.get<NamedDecl>())
+    Value += X->getNameAsString() + ";";
+  if (auto *X = DTN.get<TypedefNameDecl>())
+    return Value + X->getUnderlyingType().getAsString() + ";";
+  if (auto *X = DTN.get<NamespaceDecl>())
+    return Value;
+  if (auto *X = DTN.get<TypeDecl>())
+    if (X->getTypeForDecl())
+      Value +=
+          X->getTypeForDecl()->getCanonicalTypeInternal().getAsString() + ";";
+  if (auto *X = DTN.get<Decl>())
+    return Value;
+  if (auto *X = DTN.get<Stmt>())
+    return "";
+  llvm_unreachable("Fatal: unhandled AST node.\n");
+}
+
+void SyntaxTreeImpl::printTree() const { printTree(root()); }
+void SyntaxTreeImpl::printTree(NodeId Root) const {
+  printTree(llvm::outs(), Root);
+}
+
+void SyntaxTreeImpl::printTree(raw_ostream &OS, NodeId Root) const {
+  const Node &N = getNode(Root);
+  for (int I = 0; I < N.Depth; ++I)
+    OS << " ";
+  printNode(OS, Root);
+  OS << "\n";
+  for (NodeId Child : N.Children)
+    printTree(OS, Child);
+}
+
+void SyntaxTreeImpl::printNode(raw_ostream &OS, NodeId Id) const {
+  if (Id.isInvalid()) {
+    OS << "None";
+    return;
+  }
+  OS << getNode(Id).getTypeLabel();
+  if (getNodeValueImpl(Id) != "")
+    OS << ": " << getNodeValueImpl(Id);
+  OS << "(" << PostorderIds[Id] << ")";
+}
+
+void SyntaxTreeImpl::printNodeAsJson(raw_ostream &OS, NodeId Id) const {
+  auto N = getNode(Id);
+  OS << R"({"type":")" << N.getTypeLabel() << R"(")";
+  if (getNodeValueImpl(Id) != "")
+    OS << R"(,"value":")" << getNodeValueImpl(Id) << R"(")";
+  OS << R"(,"children":[)";
+  if (N.Children.size() > 0) {
+    printNodeAsJson(OS, N.Children[0]);
+    for (size_t I = 1, E = N.Children.size(); I < E; ++I) {
+      OS << ",";
+      printNodeAsJson(OS, N.Children[I]);
+    }
+  }
+  OS << "]}";
+}
+
+void SyntaxTreeImpl::printAsJsonImpl(raw_ostream &OS) const {
+  OS << R"({"root":)";
+  printNodeAsJson(OS, root());
+  OS << "}\n";
+}
+
+/// Identifies a node in a subtree by its postorder offset, starting at 1.
+struct SNodeId {
+  int Id = 0;
+
+  explicit SNodeId(int Id) : Id(Id) {}
+  explicit SNodeId() = default;
+
+  operator int() const { return Id; }
+  SNodeId &operator++() { return ++Id, *this; }
+  SNodeId &operator--() { return --Id, *this; }
+  SNodeId operator+(int Other) const { return SNodeId(Id + Other); }
+};
+
+class Subtree {
+private:
+  /// The parent tree.
+  const SyntaxTreeImpl &Tree;
+  /// Maps SNodeIds to original ids.
+  std::vector<NodeId> RootIds;
+  /// Maps subtree nodes to their leftmost descendants wtihin the subtree.
+  std::vector<SNodeId> LeftMostDescendants;
+
+public:
+  std::vector<SNodeId> KeyRoots;
+
+  Subtree(const SyntaxTreeImpl &Tree, NodeId SubtreeRoot) : Tree(Tree) {
+    RootIds = getSubtreePostorder(Tree, SubtreeRoot);
+    int NumLeaves = setLeftMostDescendants();
+    computeKeyRoots(NumLeaves);
+  }
+  int getSize() const { return RootIds.size(); }
+  NodeId getIdInRoot(SNodeId Id) const {
+    assert(Id > 0 && Id <= getSize() && "Invalid subtree node index.");
+    return RootIds[Id - 1];
+  }
+  const Node &getNode(SNodeId Id) const {
+    return Tree.getNode(getIdInRoot(Id));
+  }
+  SNodeId getLeftMostDescendant(SNodeId Id) const {
+    assert(Id > 0 && Id <= getSize() && "Invalid subtree node index.");
+    return LeftMostDescendants[Id - 1];
+  }
+  /// Returns the postorder index of the leftmost descendant in the subtree.
+  NodeId getPostorderOffset() const {
+    return Tree.PostorderIds[getIdInRoot(SNodeId(1))];
+  }
+
+private:
+  /// Returns the number of leafs in the subtree.
+  int setLeftMostDescendants() {
+    int NumLeaves = 0;
+    LeftMostDescendants.resize(getSize());
+    for (int I = 0; I < getSize(); ++I) {
+      SNodeId SI(I + 1);
+      const Node &N = getNode(SI);
+      NumLeaves += N.isLeaf();
+      assert(I == Tree.PostorderIds[getIdInRoot(SI)] - getPostorderOffset() &&
+             "Postorder traversal in subtree should correspond to traversal in "
+             "the root tree by a constant offset.");
+      LeftMostDescendants[I] = SNodeId(Tree.PostorderIds[N.LeftMostDescendant] -
+                                       getPostorderOffset());
+    }
+    return NumLeaves;
+  }
+  void computeKeyRoots(int Leaves) {
+    KeyRoots.resize(Leaves);
+    std::unordered_set<int> Visited;
+    int K = Leaves - 1;
+    for (SNodeId I(getSize()); I > 0; --I) {
+      SNodeId LeftDesc = getLeftMostDescendant(I);
+      if (Visited.count(LeftDesc))
+        continue;
+      assert(K >= 0 && "K should be non-negative");
+      KeyRoots[K] = I;
+      Visited.insert(LeftDesc);
+      --K;
+    }
+  }
+};
+
+/// Implementation of Zhang and Shasha's Algorithm for tree edit distance.
+/// Computes an optimal mapping between two trees using only insertion,
+/// deletion and update as edit actions (similar to the Levenshtein distance).
+class ZhangShashaMatcher {
+  const ASTDiff::Impl &DiffImpl;
+  Subtree S1;
+  Subtree S2;
+  std::unique_ptr<std::unique_ptr<double[]>[]> TreeDist, ForestDist;
+
+public:
+  ZhangShashaMatcher(const ASTDiff::Impl &DiffImpl, const SyntaxTreeImpl &T1,
+                     const SyntaxTreeImpl &T2, NodeId Id1, NodeId Id2)
+      : DiffImpl(DiffImpl), S1(T1, Id1), S2(T2, Id2) {
+    TreeDist = llvm::make_unique<std::unique_ptr<double[]>[]>(
+        size_t(S1.getSize()) + 1);
+    ForestDist = llvm::make_unique<std::unique_ptr<double[]>[]>(
+        size_t(S1.getSize()) + 1);
+    for (int I = 0, E = S1.getSize() + 1; I < E; ++I) {
+      TreeDist[I] = llvm::make_unique<double[]>(size_t(S2.getSize()) + 1);
+      ForestDist[I] = llvm::make_unique<double[]>(size_t(S2.getSize()) + 1);
+    }
+  }
+
+  std::vector<std::pair<NodeId, NodeId>> getMatchingNodes() {
+    std::vector<std::pair<NodeId, NodeId>> Matches;
+    std::vector<std::pair<SNodeId, SNodeId>> TreePairs;
+
+    computeTreeDist();
+
+    bool RootNodePair = true;
+
+    TreePairs.emplace_back(S1.getSize(), S2.getSize());
+
+    while (!TreePairs.empty()) {
+      SNodeId LastRow, LastCol, FirstRow, FirstCol, Row, Col;
+      std::tie(LastRow, LastCol) = TreePairs.back();
+      TreePairs.pop_back();
+
+      if (!RootNodePair) {
+        computeForestDist(LastRow, LastCol);
+      }
+
+      RootNodePair = false;
+
+      FirstRow = S1.getLeftMostDescendant(LastRow);
+      FirstCol = S2.getLeftMostDescendant(LastCol);
+
+      Row = LastRow;
+      Col = LastCol;
+
+      while (Row > FirstRow || Col > FirstCol) {
+        if (Row > FirstRow &&
+            ForestDist[Row - 1][Col] + 1 == ForestDist[Row][Col]) {
+          --Row;
+        } else if (Col > FirstCol &&
+                   ForestDist[Row][Col - 1] + 1 == ForestDist[Row][Col]) {
+          --Col;
+        } else {
+          SNodeId LMD1 = S1.getLeftMostDescendant(Row);
+          SNodeId LMD2 = S2.getLeftMostDescendant(Col);
+          if (LMD1 == S1.getLeftMostDescendant(LastRow) &&
+              LMD2 == S2.getLeftMostDescendant(LastCol)) {
+            NodeId Id1 = S1.getIdInRoot(Row);
+            NodeId Id2 = S2.getIdInRoot(Col);
+            assert(DiffImpl.isMatchingPossible(Id1, Id2) &&
+                   "These nodes must not be matched.");
+            Matches.emplace_back(Id1, Id2);
+            --Row;
+            --Col;
+          } else {
+            TreePairs.emplace_back(Row, Col);
+            Row = LMD1;
+            Col = LMD2;
+          }
+        }
+      }
+    }
+    return Matches;
+  }
+
+private:
+  /// Simple cost model for edit actions.
+  /// The values range between 0 and 1, or infinity if this edit action should
+  /// always be avoided.
+
+  /// These costs could be modified to better model the estimated cost of /
+  /// inserting / deleting the current node.
+  static constexpr double DeletionCost = 1;
+  static constexpr double InsertionCost = 1;
+
+  double getUpdateCost(SNodeId Id1, SNodeId Id2) {
+    const DynTypedNode &DTN1 = S1.getNode(Id1).ASTNode,
+                       &DTN2 = S2.getNode(Id2).ASTNode;
+    if (!DiffImpl.Options.isMatchingAllowed(DTN1, DTN2))
+      return std::numeric_limits<double>::max();
+    return DiffImpl.Options.getNodeDistance(*DiffImpl.T1.Parent, DTN1,
+                                            *DiffImpl.T2.Parent, DTN2);
+  }
+
+  void computeTreeDist() {
+    for (SNodeId Id1 : S1.KeyRoots)
+      for (SNodeId Id2 : S2.KeyRoots)
+        computeForestDist(Id1, Id2);
+  }
+
+  void computeForestDist(SNodeId Id1, SNodeId Id2) {
+    assert(Id1 > 0 && Id2 > 0 && "Expecting offsets greater than 0.");
+    SNodeId LMD1 = S1.getLeftMostDescendant(Id1);
+    SNodeId LMD2 = S2.getLeftMostDescendant(Id2);
+
+    ForestDist[LMD1][LMD2] = 0;
+    for (SNodeId D1 = LMD1 + 1; D1 <= Id1; ++D1) {
+      ForestDist[D1][LMD2] = ForestDist[D1 - 1][LMD2] + DeletionCost;
+      for (SNodeId D2 = LMD2 + 1; D2 <= Id2; ++D2) {
+        ForestDist[LMD1][D2] = ForestDist[LMD1][D2 - 1] + InsertionCost;
+        SNodeId DLMD1 = S1.getLeftMostDescendant(D1);
+        SNodeId DLMD2 = S2.getLeftMostDescendant(D2);
+        if (DLMD1 == LMD1 && DLMD2 == LMD2) {
+          double UpdateCost = getUpdateCost(D1, D2);
+          ForestDist[D1][D2] =
+              std::min({ForestDist[D1 - 1][D2] + DeletionCost,
+                        ForestDist[D1][D2 - 1] + InsertionCost,
+                        ForestDist[D1 - 1][D2 - 1] + UpdateCost});
+          TreeDist[D1][D2] = ForestDist[D1][D2];
+        } else {
+          ForestDist[D1][D2] =
+              std::min({ForestDist[D1 - 1][D2] + DeletionCost,
+                        ForestDist[D1][D2 - 1] + InsertionCost,
+                        ForestDist[DLMD1][DLMD2] + TreeDist[D1][D2]});
+        }
+      }
+    }
+  }
+};
+
+namespace {
+// Compares nodes by their depth.
+struct HeightLess {
+  const SyntaxTreeImpl &Tree;
+  HeightLess(const SyntaxTreeImpl &Tree) : Tree(Tree) {}
+  bool operator()(NodeId Id1, NodeId Id2) const {
+    return Tree.getNode(Id1).Height < Tree.getNode(Id2).Height;
+  }
+};
+} // end anonymous namespace
+
+// Priority queue for nodes, sorted descendingly by their height.
+class PriorityList {
+  const SyntaxTreeImpl &Tree;
+  HeightLess Cmp;
+  std::vector<NodeId> Container;
+  PriorityQueue<NodeId, std::vector<NodeId>, HeightLess> List;
+
+public:
+  PriorityList(const SyntaxTreeImpl &Tree)
+      : Tree(Tree), Cmp(Tree), List(Cmp, Container) {}
+
+  void push(NodeId id) { List.push(id); }
+
+  std::vector<NodeId> pop() {
+    int Max = peekMax();
+    std::vector<NodeId> Result;
+    if (Max == 0)
+      return Result;
+    while (peekMax() == Max) {
+      Result.push_back(List.top());
+      List.pop();
+    }
+    // TODO this is here to get a stable output, not a good heuristic
+    std::sort(Result.begin(), Result.end());
+    return Result;
+  }
+  int peekMax() const {
+    if (List.empty())
+      return 0;
+    return Tree.getNode(List.top()).Height;
+  }
+  void open(NodeId Id) {
+    for (NodeId Child : Tree.getNode(Id).Children)
+      push(Child);
+  }
+};
+
+bool ASTDiff::Impl::isomorphic(NodeId Id1, NodeId Id2) const {
+  const Node &N1 = T1.getNode(Id1);
+  const Node &N2 = T2.getNode(Id2);
+  if (N1.Children.size() != N2.Children.size() ||
+      !isMatchingPossible(Id1, Id2) ||
+      Options.getNodeDistance(*T1.Parent, N1.ASTNode, *T2.Parent, N2.ASTNode) !=
+          0)
+    return false;
+  for (size_t Id = 0, E = N1.Children.size(); Id < E; ++Id)
+    if (!isomorphic(N1.Children[Id], N2.Children[Id]))
+      return false;
+  return true;
+}
+
+bool ASTDiff::Impl::canBeAddedToMapping(const Mapping &M, NodeId Id1,
+                                        NodeId Id2) const {
+  assert(isMatchingPossible(Id1, Id2) &&
+         "Matching must be possible in the first place.");
+  if (M.hasSrcDst(Id1, Id2))
+    return false;
+  if (Options.EnableMatchingWithUnmatchableParents)
+    return true;
+  const Node &N1 = T1.getNode(Id1);
+  const Node &N2 = T2.getNode(Id2);
+  NodeId P1 = N1.Parent;
+  NodeId P2 = N2.Parent;
+  // Only allow matching if parents can be matched.
+  return (P1.isInvalid() && P2.isInvalid()) ||
+         (P1.isValid() && P2.isValid() && isMatchingPossible(P1, P2));
+}
+
+bool ASTDiff::Impl::isMatchingPossible(NodeId Id1, NodeId Id2) const {
+  return Options.isMatchingAllowed(T1.getNode(Id1).ASTNode,
+                                   T2.getNode(Id2).ASTNode);
+}
+
+void ASTDiff::Impl::addIsomorphicSubTrees(Mapping &M, NodeId Id1,
+                                          NodeId Id2) const {
+  assert(isomorphic(Id1, Id2) && "Can only be called on isomorphic subtrees.");
+  M.link(Id1, Id2);
+  const Node &N1 = T1.getNode(Id1);
+  const Node &N2 = T2.getNode(Id2);
+  for (size_t Id = 0, E = N1.Children.size(); Id < E; ++Id)
+    addIsomorphicSubTrees(M, N1.Children[Id], N2.Children[Id]);
+}
+
+void ASTDiff::Impl::addOptimalMapping(Mapping &M, NodeId Id1,
+                                      NodeId Id2) const {
+  if (std::max(T1.getNumberOfDescendants(Id1),
+               T2.getNumberOfDescendants(Id2)) >= Options.MaxSize)
+    return;
+  ZhangShashaMatcher Matcher(*this, T1, T2, Id1, Id2);
+  std::vector<std::pair<NodeId, NodeId>> R = Matcher.getMatchingNodes();
+  for (const auto Tuple : R) {
+    NodeId Src = Tuple.first;
+    NodeId Dst = Tuple.second;
+    if (canBeAddedToMapping(M, Src, Dst))
+      M.link(Src, Dst);
+  }
+}
+
+double ASTDiff::Impl::getSimilarity(const Mapping &M, NodeId Id1,
+                                    NodeId Id2) const {
+  if (Id1.isInvalid() || Id2.isInvalid())
+    return 0.0;
+  int CommonDescendants = 0;
+  const Node &N1 = T1.getNode(Id1);
+  for (NodeId Id = Id1 + 1; Id <= N1.RightMostDescendant; ++Id)
+    CommonDescendants += int(T2.isInSubtree(M.getDst(Id), Id2));
+  return 2.0 * CommonDescendants /
+         (T1.getNumberOfDescendants(Id1) + T2.getNumberOfDescendants(Id2));
+}
+
+NodeId ASTDiff::Impl::findCandidate(const Mapping &M, NodeId Id1) const {
+  NodeId Candidate;
+  double MaxSimilarity = 0.0;
+  for (NodeId Id2 = 0, E = T2.getSize(); Id2 < E; ++Id2) {
+    if (!isMatchingPossible(Id1, Id2))
+      continue;
+    if (M.hasDst(Id2))
+      continue;
+    double Similarity = getSimilarity(M, Id1, Id2);
+    if (Similarity > MaxSimilarity) {
+      MaxSimilarity = Similarity;
+      Candidate = Id2;
+    }
+  }
+  return Candidate;
+}
+
+void ASTDiff::Impl::matchBottomUp(Mapping &M) const {
+  std::vector<NodeId> Postorder = getSubtreePostorder(T1, T1.root());
+  for (NodeId Id1 : Postorder) {
+    if (Id1 == T1.root()) {
+      if (isMatchingPossible(T1.root(), T2.root())) {
+        M.link(T1.root(), T2.root());
+        addOptimalMapping(M, T1.root(), T2.root());
+      }
+      break;
+    }
+    const Node &N1 = T1.getNode(Id1);
+    bool Matched = M.hasSrc(Id1);
+    bool MatchedChildren =
+        std::any_of(N1.Children.begin(), N1.Children.end(),
+                    [&](NodeId Child) { return M.hasSrc(Child); });
+    if (Matched || !MatchedChildren)
+      continue;
+    NodeId Id2 = findCandidate(M, Id1);
+    if (Id2.isInvalid() || !canBeAddedToMapping(M, Id1, Id2) ||
+        getSimilarity(M, Id1, Id2) < Options.MinSimilarity)
+      continue;
+    M.link(Id1, Id2);
+    addOptimalMapping(M, Id1, Id2);
+  }
+}
+
+Mapping ASTDiff::Impl::matchTopDown() const {
+  PriorityList L1(T1);
+  PriorityList L2(T2);
+
+  Mapping M(T1.getSize(), T2.getSize());
+
+  L1.push(T1.root());
+  L2.push(T2.root());
+
+  int Max1, Max2;
+  while (std::min(Max1 = L1.peekMax(), Max2 = L2.peekMax()) >
+         Options.MinHeight) {
+    if (Max1 > Max2) {
+      for (NodeId Id : L1.pop())
+        L1.open(Id);
+      continue;
+    }
+    if (Max2 > Max1) {
+      for (NodeId Id : L2.pop())
+        L2.open(Id);
+      continue;
+    }
+    std::vector<NodeId> H1, H2;
+    H1 = L1.pop();
+    H2 = L2.pop();
+    for (NodeId Id1 : H1) {
+      for (NodeId Id2 : H2)
+        if (isomorphic(Id1, Id2) && canBeAddedToMapping(M, Id1, Id2))
+          addIsomorphicSubTrees(M, Id1, Id2);
+    }
+    for (NodeId Id1 : H1) {
+      if (!M.hasSrc(Id1))
+        L1.open(Id1);
+    }
+    for (NodeId Id2 : H2) {
+      if (!M.hasDst(Id2))
+        L2.open(Id2);
+    }
+  }
+  return M;
+}
+
+void ASTDiff::Impl::computeMapping() {
+  if (IsMappingDone)
+    return;
+  TheMapping = matchTopDown();
+  matchBottomUp(TheMapping);
+  IsMappingDone = true;
+}
+
+std::vector<Match> ASTDiff::Impl::getMatches(Mapping &M) {
+  std::vector<Match> Matches;
+  for (NodeId Id1 = 0, Id2, E = T1.getSize(); Id1 < E; ++Id1)
+    if ((Id2 = M.getDst(Id1)).isValid())
+      Matches.push_back({Id1, Id2});
+  return Matches;
+}
+
+std::vector<Change> ASTDiff::Impl::computeChanges(Mapping &M) {
+  std::vector<Change> Changes;
+  for (NodeId Id2 : getSubtreeBfs(T2, T2.root())) {
+    const Node &N2 = T2.getNode(Id2);
+    NodeId Id1 = M.getSrc(Id2);
+    if (Id1.isValid()) {
+      assert(isMatchingPossible(Id1, Id2) && "Invalid matching.");
+      if (T1.getNodeValueImpl(Id1) != T2.getNodeValueImpl(Id2)) {
+        Changes.emplace_back(Update, Id1, Id2);
+      }
+      continue;
+    }
+    NodeId P2 = N2.Parent;
+    NodeId P1 = M.getSrc(P2);
+    assert(P1.isValid() &&
+           "Parents must be matched for determining the change type.");
+    Node &Parent1 = T1.getMutableNode(P1);
+    const Node &Parent2 = T2.getNode(P2);
+    auto &Siblings1 = Parent1.Children;
+    const auto &Siblings2 = Parent2.Children;
+    size_t Position;
+    for (Position = 0; Position < Siblings2.size(); ++Position)
+      if (Siblings2[Position] == Id2 || Position >= Siblings1.size())
+        break;
+    Changes.emplace_back(Insert, Id2, P2, Position);
+    Node PatchNode;
+    PatchNode.Parent = P1;
+    PatchNode.LeftMostDescendant = N2.LeftMostDescendant;
+    PatchNode.RightMostDescendant = N2.RightMostDescendant;
+    PatchNode.Depth = N2.Depth;
+    PatchNode.ASTNode = N2.ASTNode;
+    // TODO update Depth if needed
+    NodeId PatchNodeId = T1.getSize();
+    // TODO maybe choose a different data structure for Children.
+    Siblings1.insert(Siblings1.begin() + Position, PatchNodeId);
+    T1.addNode(PatchNode);
+    M.link(PatchNodeId, Id2);
+  }
+  for (NodeId Id1 = 0; Id1 < T1.getSize(); ++Id1) {
+    NodeId Id2 = M.getDst(Id1);
+    if (Id2.isInvalid())
+      Changes.emplace_back(Delete, Id1, Id2);
+  }
+  return Changes;
+}
+
+void ASTDiff::Impl::printChangeImpl(raw_ostream &OS, const Change &Chg) const {
+  switch (Chg.Kind) {
+  case Delete:
+    OS << "Delete ";
+    T1.printNode(OS, Chg.Src);
+    OS << "\n";
+    break;
+  case Update:
+    OS << "Update ";
+    T1.printNode(OS, Chg.Src);
+    OS << " to " << T2.getNodeValueImpl(Chg.Dst) << "\n";
+    break;
+  case Insert:
+    OS << "Insert ";
+    T2.printNode(OS, Chg.Src);
+    OS << " into ";
+    T2.printNode(OS, Chg.Dst);
+    OS << " at " << Chg.Position << "\n";
+    break;
+  case Move:
+    llvm_unreachable("TODO");
+    break;
+  };
+}
+
+void ASTDiff::Impl::printMatchImpl(raw_ostream &OS, const Match &M) const {
+  OS << "Match ";
+  T1.printNode(OS, M.Src);
+  OS << " to ";
+  T2.printNode(OS, M.Dst);
+  OS << "\n";
+}
+
+ASTDiff::ASTDiff(SyntaxTree &T1, SyntaxTree &T2,
+                 const ComparisonOptions &Options)
+    : DiffImpl(llvm::make_unique<Impl>(*T1.TreeImpl, *T2.TreeImpl, Options)) {}
+
+ASTDiff::~ASTDiff() {}
+
+SyntaxTree::SyntaxTree(const ASTContext &AST)
+    : TreeImpl(llvm::make_unique<SyntaxTreeImpl>(
+          this, AST.getTranslationUnitDecl(), AST)) {}
+
+std::vector<Match> ASTDiff::getMatches() {
+  DiffImpl->computeMapping();
+  return DiffImpl->getMatches(DiffImpl->TheMapping);
+}
+
+std::vector<Change> ASTDiff::getChanges() {
+  DiffImpl->computeMapping();
+  return DiffImpl->computeChanges(DiffImpl->TheMapping);
+}
+
+void ASTDiff::printChange(raw_ostream &OS, const Change &Chg) const {
+  DiffImpl->printChangeImpl(OS, Chg);
+}
+
+void ASTDiff::printMatch(raw_ostream &OS, const Match &M) const {
+  DiffImpl->printMatchImpl(OS, M);
+}
+
+void SyntaxTree::printAsJson(raw_ostream &OS) { TreeImpl->printAsJsonImpl(OS); }
+
+std::string SyntaxTree::getNodeValue(const DynTypedNode &DTN) const {
+  return TreeImpl->getNodeValueImpl(DTN);
+}
+
+} // end namespace diff
+} // end namespace clang
diff --git a/lib/Tooling/ASTDiff/CMakeLists.txt b/lib/Tooling/ASTDiff/CMakeLists.txt
new file mode 100644
index 0000000000..578d8ca0cb
--- /dev/null
+++ b/lib/Tooling/ASTDiff/CMakeLists.txt
@@ -0,0 +1,11 @@
+set(LLVM_LINK_COMPONENTS
+  Support
+  )
+
+add_clang_library(clangToolingASTDiff
+  ASTDiff.cpp
+  LINK_LIBS
+  clangBasic
+  clangAST
+  clangLex
+  )