[Dominators] Introduce batch updates

Summary:
This patch introduces a way of informing the (Post)DominatorTree about multiple CFG updates that happened since the last tree update. This makes performing tree updates much easier, as it internally takes care of applying the updates in lockstep with the (virtual) updates to the CFG, which is done by reverse-applying future CFG updates.

The batch updater is able to remove redundant updates that cancel each other out. In the future, it should be also possible to reorder updates to reduce the amount of work needed to perform the updates.

Reviewers: dberlin, sanjoy, grosser, davide, brzycki

Reviewed By: brzycki

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D36167

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311015 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Dominators.h

@@ -41,6 +41,10 @@ namespace DomTreeBuilder {
 using BBDomTree = DomTreeBase<BasicBlock>;
 using BBPostDomTree = PostDomTreeBase<BasicBlock>;
 
+extern template struct Update<BasicBlock *>;
+
+using BBUpdates = ArrayRef<Update<BasicBlock *>>;
+
 extern template void Calculate<BBDomTree>(BBDomTree &DT);
 extern template void Calculate<BBPostDomTree>(BBPostDomTree &DT);
 
@@ -56,6 +60,9 @@ extern template void DeleteEdge<BBPostDomTree>(BBPostDomTree &DT,
                                                BasicBlock *From,
                                                BasicBlock *To);
 
+extern template void ApplyUpdates<BBDomTree>(BBDomTree &DT, BBUpdates);
+extern template void ApplyUpdates<BBPostDomTree>(BBPostDomTree &DT, BBUpdates);
+
 extern template bool Verify<BBDomTree>(const BBDomTree &DT);
 extern template bool Verify<BBPostDomTree>(const BBPostDomTree &DT);
 }  // namespace DomTreeBuilder

include/llvm/Support/GenericDomTree.h

@@ -24,12 +24,6 @@
 #ifndef LLVM_SUPPORT_GENERICDOMTREE_H
 #define LLVM_SUPPORT_GENERICDOMTREE_H
 
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/GraphTraits.h"
-#include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
@@ -38,14 +32,21 @@
 #include <type_traits>
 #include <utility>
 #include <vector>
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/GraphTraits.h"
+#include "llvm/ADT/PointerIntPair.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
 
 template <typename NodeT, bool IsPostDom>
 class DominatorTreeBase;
 
 namespace DomTreeBuilder {
-template <class DomTreeT>
+template <typename DomTreeT>
 struct SemiNCAInfo;
 }  // namespace DomTreeBuilder
 
@@ -190,14 +191,48 @@ namespace DomTreeBuilder {
 template <typename DomTreeT>
 void Calculate(DomTreeT &DT);
 
-template <class DomTreeT>
+template <typename DomTreeT>
 void InsertEdge(DomTreeT &DT, typename DomTreeT::NodePtr From,
                 typename DomTreeT::NodePtr To);
 
-template <class DomTreeT>
+template <typename DomTreeT>
 void DeleteEdge(DomTreeT &DT, typename DomTreeT::NodePtr From,
                 typename DomTreeT::NodePtr To);
 
+// UpdateKind and Update are used by the batch update API and it's easiest to
+// define them here.
+enum class UpdateKind : unsigned char { Insert, Delete };
+
+template <typename NodePtr>
+struct Update {
+  using NodeKindPair = PointerIntPair<NodePtr, 1, UpdateKind>;
+
+  NodePtr From;
+  NodeKindPair ToAndKind;
+
+  Update(UpdateKind Kind, NodePtr From, NodePtr To)
+      : From(From), ToAndKind(To, Kind) {}
+
+  UpdateKind getKind() const { return ToAndKind.getInt(); }
+  NodePtr getFrom() const { return From; }
+  NodePtr getTo() const { return ToAndKind.getPointer(); }
+  bool operator==(const Update &RHS) const {
+    return From == RHS.From && ToAndKind == RHS.ToAndKind;
+  }
+
+  friend raw_ostream &operator<<(raw_ostream &OS, const Update &U) {
+    OS << (U.getKind() == UpdateKind::Insert ? "Insert " : "Delete ");
+    U.getFrom()->printAsOperand(OS, false);
+    OS << " -> ";
+    U.getTo()->printAsOperand(OS, false);
+    return OS;
+  }
+};
+
+template <typename DomTreeT>
+void ApplyUpdates(DomTreeT &DT,
+                  ArrayRef<typename DomTreeT::UpdateType> Updates);
+
 template <typename DomTreeT>
 bool Verify(const DomTreeT &DT);
 }  // namespace DomTreeBuilder
@@ -219,6 +254,11 @@ class DominatorTreeBase {
   using ParentType = typename std::remove_pointer<ParentPtr>::type;
   static constexpr bool IsPostDominator = IsPostDom;
 
+  using UpdateType = DomTreeBuilder::Update<NodePtr>;
+  using UpdateKind = DomTreeBuilder::UpdateKind;
+  static constexpr UpdateKind Insert = UpdateKind::Insert;
+  static constexpr UpdateKind Delete = UpdateKind::Delete;
+
  protected:
   // Dominators always have a single root, postdominators can have more.
   SmallVector<NodeT *, IsPostDom ? 4 : 1> Roots;
@@ -463,6 +503,39 @@ class DominatorTreeBase {
   // API to update (Post)DominatorTree information based on modifications to
   // the CFG...
 
+  /// Inform the dominator tree about a sequence of CFG edge insertions and
+  /// deletions and perform a batch update on the tree.
+  ///
+  /// This function should be used when there were multiple CFG updates after
+  /// the last dominator tree update. It takes care of performing the updates
+  /// in sync with the CFG and optimizes away the redundant operations that
+  /// cancel each other.
+  /// The functions expects the sequence of updates to be balanced. Eg.:
+  ///  - {{Insert, A, B}, {Delete, A, B}, {Insert, A, B}} is fine, because
+  ///    logically it results in a single insertions.
+  ///  - {{Insert, A, B}, {Insert, A, B}} is invalid, because it doesn't make
+  ///    sense to insert the same edge twice.
+  ///
+  /// What's more, the functions assumes that it's safe to ask every node in the
+  /// CFG about its children and inverse children. This implies that deletions
+  /// of CFG edges must not delete the CFG nodes before calling this function.
+  ///
+  /// Batch updates should be generally faster when performing longer sequences
+  /// of updates than calling insertEdge/deleteEdge manually multiple times, as
+  /// they can reorder the updates and remove redundant ones internally.
+  ///
+  /// Note that for postdominators it automatically takes care of applying
+  /// updates on reverse edges internally (so there's no need to swap the
+  /// From and To pointers when constructing DominatorTree::UpdateType).
+  /// The type of updates is the same for DomTreeBase<T> and PostDomTreeBase<T>
+  /// with the same template parameter T.
+  ///
+  /// \param Updates An unordered sequence of updates to perform.
+  ///
+  void applyUpdates(ArrayRef<UpdateType> Updates) {
+    DomTreeBuilder::ApplyUpdates(*this, Updates);
+  }
+
   /// 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
@@ -487,11 +560,6 @@ class DominatorTreeBase {
   /// DEBUG mode. There cannot be any other updates that the
   /// dominator tree doesn't know about.
   ///
-  /// However, it is fine to perform multiple CFG deletions that make different
-  /// subtrees forward-unreachable and to inform the DomTree about them all at
-  /// the same time, as the incremental algorithm doesn't walk the tree above
-  /// the NearestCommonDominator of a deleted edge
-  ///
   /// Note that for postdominators it automatically takes care of deleting
   /// a reverse edge internally (so there's no need to swap the parameters).
   ///
@@ -678,7 +746,6 @@ class DominatorTreeBase {
 
   /// recalculate - compute a dominator tree for the given function
   void recalculate(ParentType &Func) {
-    reset();
     Parent = &Func;
     DomTreeBuilder::Calculate(*this);
   }