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+//===- RegionIterator.h - Iterators to iteratate over Regions ---*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// This file defines the iterators to iterate over the elements of a Region.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_REGIONITERATOR_H
+#define LLVM_ANALYSIS_REGIONITERATOR_H
+
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/GraphTraits.h"
+#include "llvm/ADT/PointerIntPair.h"
+#include "llvm/Analysis/RegionInfo.h"
+#include "llvm/IR/CFG.h"
+#include <cassert>
+#include <iterator>
+#include <type_traits>
+
+namespace llvm {
+
+class BasicBlock;
+
+//===----------------------------------------------------------------------===//
+/// @brief Hierarchical RegionNode successor iterator.
+///
+/// This iterator iterates over all successors of a RegionNode.
+///
+/// For a BasicBlock RegionNode it skips all BasicBlocks that are not part of
+/// the parent Region.  Furthermore for BasicBlocks that start a subregion, a
+/// RegionNode representing the subregion is returned.
+///
+/// For a subregion RegionNode there is just one successor. The RegionNode
+/// representing the exit of the subregion.
+template <class NodeRef, class BlockT, class RegionT>
+class RNSuccIterator
+    : public std::iterator<std::forward_iterator_tag, NodeRef> {
+  using super = std::iterator<std::forward_iterator_tag, NodeRef>;
+  using BlockTraits = GraphTraits<BlockT *>;
+  using SuccIterTy = typename BlockTraits::ChildIteratorType;
+
+  // The iterator works in two modes, bb mode or region mode.
+  enum ItMode {
+    // In BB mode it returns all successors of this BasicBlock as its
+    // successors.
+    ItBB,
+    // In region mode there is only one successor, thats the regionnode mapping
+    // to the exit block of the regionnode
+    ItRgBegin, // At the beginning of the regionnode successor.
+    ItRgEnd    // At the end of the regionnode successor.
+  };
+
+  static_assert(std::is_pointer<NodeRef>::value,
+                "FIXME: Currently RNSuccIterator only supports NodeRef as "
+                "pointers due to the use of pointer-specific data structures "
+                "(e.g. PointerIntPair and SmallPtrSet) internally. Generalize "
+                "it to support non-pointer types");
+
+  // Use two bit to represent the mode iterator.
+  PointerIntPair<NodeRef, 2, ItMode> Node;
+
+  // The block successor iterator.
+  SuccIterTy BItor;
+
+  // advanceRegionSucc - A region node has only one successor. It reaches end
+  // once we advance it.
+  void advanceRegionSucc() {
+    assert(Node.getInt() == ItRgBegin && "Cannot advance region successor!");
+    Node.setInt(ItRgEnd);
+  }
+
+  NodeRef getNode() const { return Node.getPointer(); }
+
+  // isRegionMode - Is the current iterator in region mode?
+  bool isRegionMode() const { return Node.getInt() != ItBB; }
+
+  // Get the immediate successor. This function may return a Basic Block
+  // RegionNode or a subregion RegionNode.
+  NodeRef getISucc(BlockT *BB) const {
+    NodeRef succ;
+    succ = getNode()->getParent()->getNode(BB);
+    assert(succ && "BB not in Region or entered subregion!");
+    return succ;
+  }
+
+  // getRegionSucc - Return the successor basic block of a SubRegion RegionNode.
+  inline BlockT* getRegionSucc() const {
+    assert(Node.getInt() == ItRgBegin && "Cannot get the region successor!");
+    return getNode()->template getNodeAs<RegionT>()->getExit();
+  }
+
+  // isExit - Is this the exit BB of the Region?
+  inline bool isExit(BlockT* BB) const {
+    return getNode()->getParent()->getExit() == BB;
+  }
+
+public:
+  using Self = RNSuccIterator<NodeRef, BlockT, RegionT>;
+  using value_type = typename super::value_type;
+
+  /// @brief Create begin iterator of a RegionNode.
+  inline RNSuccIterator(NodeRef node)
+      : Node(node, node->isSubRegion() ? ItRgBegin : ItBB),
+        BItor(BlockTraits::child_begin(node->getEntry())) {
+    // Skip the exit block
+    if (!isRegionMode())
+      while (BlockTraits::child_end(node->getEntry()) != BItor && isExit(*BItor))
+        ++BItor;
+
+    if (isRegionMode() && isExit(getRegionSucc()))
+      advanceRegionSucc();
+  }
+
+  /// @brief Create an end iterator.
+  inline RNSuccIterator(NodeRef node, bool)
+      : Node(node, node->isSubRegion() ? ItRgEnd : ItBB),
+        BItor(BlockTraits::child_end(node->getEntry())) {}
+
+  inline bool operator==(const Self& x) const {
+    assert(isRegionMode() == x.isRegionMode() && "Broken iterator!");
+    if (isRegionMode())
+      return Node.getInt() == x.Node.getInt();
+    else
+      return BItor == x.BItor;
+  }
+
+  inline bool operator!=(const Self& x) const { return !operator==(x); }
+
+  inline value_type operator*() const {
+    BlockT *BB = isRegionMode() ? getRegionSucc() : *BItor;
+    assert(!isExit(BB) && "Iterator out of range!");
+    return getISucc(BB);
+  }
+
+  inline Self& operator++() {
+    if(isRegionMode()) {
+      // The Region only has 1 successor.
+      advanceRegionSucc();
+    } else {
+      // Skip the exit.
+      do
+        ++BItor;
+      while (BItor != BlockTraits::child_end(getNode()->getEntry())
+          && isExit(*BItor));
+    }
+    return *this;
+  }
+
+  inline Self operator++(int) {
+    Self tmp = *this;
+    ++*this;
+    return tmp;
+  }
+};
+
+//===----------------------------------------------------------------------===//
+/// @brief Flat RegionNode iterator.
+///
+/// The Flat Region iterator will iterate over all BasicBlock RegionNodes that
+/// are contained in the Region and its subregions. This is close to a virtual
+/// control flow graph of the Region.
+template <class NodeRef, class BlockT, class RegionT>
+class RNSuccIterator<FlatIt<NodeRef>, BlockT, RegionT>
+    : public std::iterator<std::forward_iterator_tag, NodeRef> {
+  using super = std::iterator<std::forward_iterator_tag, NodeRef>;
+  using BlockTraits = GraphTraits<BlockT *>;
+  using SuccIterTy = typename BlockTraits::ChildIteratorType;
+
+  NodeRef Node;
+  SuccIterTy Itor;
+
+public:
+  using Self = RNSuccIterator<FlatIt<NodeRef>, BlockT, RegionT>;
+  using value_type = typename super::value_type;
+
+  /// @brief Create the iterator from a RegionNode.
+  ///
+  /// Note that the incoming node must be a bb node, otherwise it will trigger
+  /// an assertion when we try to get a BasicBlock.
+  inline RNSuccIterator(NodeRef node)
+      : Node(node), Itor(BlockTraits::child_begin(node->getEntry())) {
+    assert(!Node->isSubRegion() &&
+           "Subregion node not allowed in flat iterating mode!");
+    assert(Node->getParent() && "A BB node must have a parent!");
+
+    // Skip the exit block of the iterating region.
+    while (BlockTraits::child_end(Node->getEntry()) != Itor &&
+           Node->getParent()->getExit() == *Itor)
+      ++Itor;
+  }
+
+  /// @brief Create an end iterator
+  inline RNSuccIterator(NodeRef node, bool)
+      : Node(node), Itor(BlockTraits::child_end(node->getEntry())) {
+    assert(!Node->isSubRegion() &&
+           "Subregion node not allowed in flat iterating mode!");
+  }
+
+  inline bool operator==(const Self& x) const {
+    assert(Node->getParent() == x.Node->getParent()
+           && "Cannot compare iterators of different regions!");
+
+    return Itor == x.Itor && Node == x.Node;
+  }
+
+  inline bool operator!=(const Self& x) const { return !operator==(x); }
+
+  inline value_type operator*() const {
+    BlockT *BB = *Itor;
+
+    // Get the iterating region.
+    RegionT *Parent = Node->getParent();
+
+    // The only case that the successor reaches out of the region is it reaches
+    // the exit of the region.
+    assert(Parent->getExit() != BB && "iterator out of range!");
+
+    return Parent->getBBNode(BB);
+  }
+
+  inline Self& operator++() {
+    // Skip the exit block of the iterating region.
+    do
+      ++Itor;
+    while (Itor != succ_end(Node->getEntry())
+        && Node->getParent()->getExit() == *Itor);
+
+    return *this;
+  }
+
+  inline Self operator++(int) {
+    Self tmp = *this;
+    ++*this;
+    return tmp;
+  }
+};
+
+template <class NodeRef, class BlockT, class RegionT>
+inline RNSuccIterator<NodeRef, BlockT, RegionT> succ_begin(NodeRef Node) {
+  return RNSuccIterator<NodeRef, BlockT, RegionT>(Node);
+}
+
+template <class NodeRef, class BlockT, class RegionT>
+inline RNSuccIterator<NodeRef, BlockT, RegionT> succ_end(NodeRef Node) {
+  return RNSuccIterator<NodeRef, BlockT, RegionT>(Node, true);
+}
+
+//===--------------------------------------------------------------------===//
+// RegionNode GraphTraits specialization so the bbs in the region can be
+// iterate by generic graph iterators.
+//
+// NodeT can either be region node or const region node, otherwise child_begin
+// and child_end fail.
+
+#define RegionNodeGraphTraits(NodeT, BlockT, RegionT)                          \
+  template <> struct GraphTraits<NodeT *> {                                    \
+    using NodeRef = NodeT *;                                                   \
+    using ChildIteratorType = RNSuccIterator<NodeRef, BlockT, RegionT>;        \
+    static NodeRef getEntryNode(NodeRef N) { return N; }                       \
+    static inline ChildIteratorType child_begin(NodeRef N) {                   \
+      return RNSuccIterator<NodeRef, BlockT, RegionT>(N);                      \
+    }                                                                          \
+    static inline ChildIteratorType child_end(NodeRef N) {                     \
+      return RNSuccIterator<NodeRef, BlockT, RegionT>(N, true);                \
+    }                                                                          \
+  };                                                                           \
+  template <> struct GraphTraits<FlatIt<NodeT *>> {                            \
+    using NodeRef = NodeT *;                                                   \
+    using ChildIteratorType =                                                  \
+        RNSuccIterator<FlatIt<NodeRef>, BlockT, RegionT>;                      \
+    static NodeRef getEntryNode(NodeRef N) { return N; }                       \
+    static inline ChildIteratorType child_begin(NodeRef N) {                   \
+      return RNSuccIterator<FlatIt<NodeRef>, BlockT, RegionT>(N);              \
+    }                                                                          \
+    static inline ChildIteratorType child_end(NodeRef N) {                     \
+      return RNSuccIterator<FlatIt<NodeRef>, BlockT, RegionT>(N, true);        \
+    }                                                                          \
+  }
+
+#define RegionGraphTraits(RegionT, NodeT)                                      \
+  template <> struct GraphTraits<RegionT *> : public GraphTraits<NodeT *> {    \
+    using nodes_iterator = df_iterator<NodeRef>;                               \
+    static NodeRef getEntryNode(RegionT *R) {                                  \
+      return R->getNode(R->getEntry());                                        \
+    }                                                                          \
+    static nodes_iterator nodes_begin(RegionT *R) {                            \
+      return nodes_iterator::begin(getEntryNode(R));                           \
+    }                                                                          \
+    static nodes_iterator nodes_end(RegionT *R) {                              \
+      return nodes_iterator::end(getEntryNode(R));                             \
+    }                                                                          \
+  };                                                                           \
+  template <>                                                                  \
+  struct GraphTraits<FlatIt<RegionT *>>                                        \
+      : public GraphTraits<FlatIt<NodeT *>> {                                  \
+    using nodes_iterator =                                                     \
+        df_iterator<NodeRef, df_iterator_default_set<NodeRef>, false,          \
+                    GraphTraits<FlatIt<NodeRef>>>;                             \
+    static NodeRef getEntryNode(RegionT *R) {                                  \
+      return R->getBBNode(R->getEntry());                                      \
+    }                                                                          \
+    static nodes_iterator nodes_begin(RegionT *R) {                            \
+      return nodes_iterator::begin(getEntryNode(R));                           \
+    }                                                                          \
+    static nodes_iterator nodes_end(RegionT *R) {                              \
+      return nodes_iterator::end(getEntryNode(R));                             \
+    }                                                                          \
+  }
+
+RegionNodeGraphTraits(RegionNode, BasicBlock, Region);
+RegionNodeGraphTraits(const RegionNode, BasicBlock, Region);
+
+RegionGraphTraits(Region, RegionNode);
+RegionGraphTraits(const Region, const RegionNode);
+
+template <> struct GraphTraits<RegionInfo*>
+  : public GraphTraits<FlatIt<RegionNode*>> {
+  using nodes_iterator =
+      df_iterator<NodeRef, df_iterator_default_set<NodeRef>, false,
+                  GraphTraits<FlatIt<NodeRef>>>;
+
+  static NodeRef getEntryNode(RegionInfo *RI) {
+    return GraphTraits<FlatIt<Region*>>::getEntryNode(RI->getTopLevelRegion());
+  }
+
+  static nodes_iterator nodes_begin(RegionInfo* RI) {
+    return nodes_iterator::begin(getEntryNode(RI));
+  }
+
+  static nodes_iterator nodes_end(RegionInfo *RI) {
+    return nodes_iterator::end(getEntryNode(RI));
+  }
+};
+
+template <> struct GraphTraits<RegionInfoPass*>
+  : public GraphTraits<RegionInfo *> {
+  using nodes_iterator =
+      df_iterator<NodeRef, df_iterator_default_set<NodeRef>, false,
+                  GraphTraits<FlatIt<NodeRef>>>;
+
+  static NodeRef getEntryNode(RegionInfoPass *RI) {
+    return GraphTraits<RegionInfo*>::getEntryNode(&RI->getRegionInfo());
+  }
+
+  static nodes_iterator nodes_begin(RegionInfoPass* RI) {
+    return GraphTraits<RegionInfo*>::nodes_begin(&RI->getRegionInfo());
+  }
+
+  static nodes_iterator nodes_end(RegionInfoPass *RI) {
+    return GraphTraits<RegionInfo*>::nodes_end(&RI->getRegionInfo());
+  }
+};
+
+} // end namespace llvm
+
+#endif // LLVM_ANALYSIS_REGIONITERATOR_H