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PostOrderIterator.h
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1 //===- llvm/ADT/PostOrderIterator.h - PostOrder iterator --------*- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file builds on the ADT/GraphTraits.h file to build a generic graph
11 // post order iterator. This should work over any graph type that has a
12 // GraphTraits specialization.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #ifndef LLVM_ADT_POSTORDERITERATOR_H
17 #define LLVM_ADT_POSTORDERITERATOR_H
18 
19 #include "llvm/ADT/GraphTraits.h"
20 #include "llvm/ADT/Optional.h"
21 #include "llvm/ADT/SmallPtrSet.h"
23 #include <iterator>
24 #include <set>
25 #include <utility>
26 #include <vector>
27 
28 namespace llvm {
29 
30 // The po_iterator_storage template provides access to the set of already
31 // visited nodes during the po_iterator's depth-first traversal.
32 //
33 // The default implementation simply contains a set of visited nodes, while
34 // the External=true version uses a reference to an external set.
35 //
36 // It is possible to prune the depth-first traversal in several ways:
37 //
38 // - When providing an external set that already contains some graph nodes,
39 // those nodes won't be visited again. This is useful for restarting a
40 // post-order traversal on a graph with nodes that aren't dominated by a
41 // single node.
42 //
43 // - By providing a custom SetType class, unwanted graph nodes can be excluded
44 // by having the insert() function return false. This could for example
45 // confine a CFG traversal to blocks in a specific loop.
46 //
47 // - Finally, by specializing the po_iterator_storage template itself, graph
48 // edges can be pruned by returning false in the insertEdge() function. This
49 // could be used to remove loop back-edges from the CFG seen by po_iterator.
50 //
51 // A specialized po_iterator_storage class can observe both the pre-order and
52 // the post-order. The insertEdge() function is called in a pre-order, while
53 // the finishPostorder() function is called just before the po_iterator moves
54 // on to the next node.
55 
56 /// Default po_iterator_storage implementation with an internal set object.
57 template<class SetType, bool External>
59  SetType Visited;
60 
61 public:
62  // Return true if edge destination should be visited.
63  template <typename NodeRef>
64  bool insertEdge(Optional<NodeRef> From, NodeRef To) {
65  return Visited.insert(To).second;
66  }
67 
68  // Called after all children of BB have been visited.
69  template <typename NodeRef> void finishPostorder(NodeRef BB) {}
70 };
71 
72 /// Specialization of po_iterator_storage that references an external set.
73 template<class SetType>
74 class po_iterator_storage<SetType, true> {
75  SetType &Visited;
76 
77 public:
78  po_iterator_storage(SetType &VSet) : Visited(VSet) {}
79  po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {}
80 
81  // Return true if edge destination should be visited, called with From = 0 for
82  // the root node.
83  // Graph edges can be pruned by specializing this function.
84  template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) {
85  return Visited.insert(To).second;
86  }
87 
88  // Called after all children of BB have been visited.
89  template <class NodeRef> void finishPostorder(NodeRef BB) {}
90 };
91 
92 template <class GraphT,
93  class SetType =
95  bool ExtStorage = false, class GT = GraphTraits<GraphT>>
97  : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
98  public po_iterator_storage<SetType, ExtStorage> {
99  using super = std::iterator<std::forward_iterator_tag, typename GT::NodeRef>;
100  using NodeRef = typename GT::NodeRef;
101  using ChildItTy = typename GT::ChildIteratorType;
102 
103  // VisitStack - Used to maintain the ordering. Top = current block
104  // First element is basic block pointer, second is the 'next child' to visit
105  std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
106 
107  po_iterator(NodeRef BB) {
108  this->insertEdge(Optional<NodeRef>(), BB);
109  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
110  traverseChild();
111  }
112 
113  po_iterator() = default; // End is when stack is empty.
114 
115  po_iterator(NodeRef BB, SetType &S)
117  if (this->insertEdge(Optional<NodeRef>(), BB)) {
118  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
119  traverseChild();
120  }
121  }
122 
123  po_iterator(SetType &S)
125  } // End is when stack is empty.
126 
127  void traverseChild() {
128  while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
129  NodeRef BB = *VisitStack.back().second++;
130  if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
131  // If the block is not visited...
132  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
133  }
134  }
135  }
136 
137 public:
138  using pointer = typename super::pointer;
139 
140  // Provide static "constructors"...
141  static po_iterator begin(GraphT G) {
142  return po_iterator(GT::getEntryNode(G));
143  }
144  static po_iterator end(GraphT G) { return po_iterator(); }
145 
146  static po_iterator begin(GraphT G, SetType &S) {
147  return po_iterator(GT::getEntryNode(G), S);
148  }
149  static po_iterator end(GraphT G, SetType &S) { return po_iterator(S); }
150 
151  bool operator==(const po_iterator &x) const {
152  return VisitStack == x.VisitStack;
153  }
154  bool operator!=(const po_iterator &x) const { return !(*this == x); }
155 
156  const NodeRef &operator*() const { return VisitStack.back().first; }
157 
158  // This is a nonstandard operator-> that dereferences the pointer an extra
159  // time... so that you can actually call methods ON the BasicBlock, because
160  // the contained type is a pointer. This allows BBIt->getTerminator() f.e.
161  //
162  NodeRef operator->() const { return **this; }
163 
164  po_iterator &operator++() { // Preincrement
165  this->finishPostorder(VisitStack.back().first);
166  VisitStack.pop_back();
167  if (!VisitStack.empty())
168  traverseChild();
169  return *this;
170  }
171 
172  po_iterator operator++(int) { // Postincrement
173  po_iterator tmp = *this;
174  ++*this;
175  return tmp;
176  }
177 };
178 
179 // Provide global constructors that automatically figure out correct types...
180 //
181 template <class T>
183 template <class T>
184 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
185 
186 template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
187  return make_range(po_begin(G), po_end(G));
188 }
189 
190 // Provide global definitions of external postorder iterators...
191 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
192 struct po_ext_iterator : public po_iterator<T, SetType, true> {
194  po_iterator<T, SetType, true>(V) {}
195 };
196 
197 template<class T, class SetType>
200 }
201 
202 template<class T, class SetType>
205 }
206 
207 template <class T, class SetType>
209  return make_range(po_ext_begin(G, S), po_ext_end(G, S));
210 }
211 
212 // Provide global definitions of inverse post order iterators...
213 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
214  bool External = false>
215 struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
216  ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
217  po_iterator<Inverse<T>, SetType, External> (V) {}
218 };
219 
220 template <class T>
222  return ipo_iterator<T>::begin(G);
223 }
224 
225 template <class T>
227  return ipo_iterator<T>::end(G);
228 }
229 
230 template <class T>
232  return make_range(ipo_begin(G), ipo_end(G));
233 }
234 
235 // Provide global definitions of external inverse postorder iterators...
236 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
237 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
239  ipo_iterator<T, SetType, true>(V) {}
240  ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
241  ipo_iterator<T, SetType, true>(V) {}
242 };
243 
244 template <class T, class SetType>
247 }
248 
249 template <class T, class SetType>
252 }
253 
254 template <class T, class SetType>
256 inverse_post_order_ext(const T &G, SetType &S) {
257  return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
258 }
259 
260 //===--------------------------------------------------------------------===//
261 // Reverse Post Order CFG iterator code
262 //===--------------------------------------------------------------------===//
263 //
264 // This is used to visit basic blocks in a method in reverse post order. This
265 // class is awkward to use because I don't know a good incremental algorithm to
266 // computer RPO from a graph. Because of this, the construction of the
267 // ReversePostOrderTraversal object is expensive (it must walk the entire graph
268 // with a postorder iterator to build the data structures). The moral of this
269 // story is: Don't create more ReversePostOrderTraversal classes than necessary.
270 //
271 // Because it does the traversal in its constructor, it won't invalidate when
272 // BasicBlocks are removed, *but* it may contain erased blocks. Some places
273 // rely on this behavior (i.e. GVN).
274 //
275 // This class should be used like this:
276 // {
277 // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
278 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
279 // ...
280 // }
281 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
282 // ...
283 // }
284 // }
285 //
286 
287 template<class GraphT, class GT = GraphTraits<GraphT>>
289  using NodeRef = typename GT::NodeRef;
290 
291  std::vector<NodeRef> Blocks; // Block list in normal PO order
292 
293  void Initialize(NodeRef BB) {
294  std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
295  }
296 
297 public:
298  using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
299 
300  ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
301 
302  // Because we want a reverse post order, use reverse iterators from the vector
303  rpo_iterator begin() { return Blocks.rbegin(); }
304  rpo_iterator end() { return Blocks.rend(); }
305 };
306 
307 } // end namespace llvm
308 
309 #endif // LLVM_ADT_POSTORDERITERATOR_H
po_iterator & operator++()
po_ext_iterator< T, SetType > po_ext_end(T G, SetType &S)
static po_iterator end(GraphT G, SetType &S)
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
ipo_ext_iterator(const ipo_iterator< T, SetType, true > &V)
This provides a very simple, boring adaptor for a begin and end iterator into a range type...
static po_iterator begin(GraphT G)
ipo_ext_iterator(const po_iterator< Inverse< T >, SetType, true > &V)
po_iterator operator++(int)
bool operator==(const po_iterator &x) const
po_ext_iterator(const po_iterator< T, SetType, true > &V)
static po_iterator begin(GraphT G, SetType &S)
bool insertEdge(Optional< NodeRef > From, NodeRef To)
ipo_iterator< T > ipo_end(const T &G)
void finishPostorder(NodeRef BB)
ipo_ext_iterator< T, SetType > ipo_ext_begin(const T &G, SetType &S)
ipo_ext_iterator< T, SetType > ipo_ext_end(const T &G, SetType &S)
static po_iterator end(GraphT G)
po_iterator_storage(const po_iterator_storage &S)
Default po_iterator_storage implementation with an internal set object.
po_iterator< T > po_end(const T &G)
iterator_range< po_iterator< T > > post_order(const T &G)
bool insertEdge(Optional< NodeRef > From, NodeRef To)
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:418
bool operator!=(const po_iterator &x) const
const DataFlowGraph & G
Definition: RDFGraph.cpp:211
iterator_range< po_ext_iterator< T, SetType > > post_order_ext(const T &G, SetType &S)
NodeRef operator->() const
A range adaptor for a pair of iterators.
ipo_iterator< T > ipo_begin(const T &G)
const NodeRef & operator*() const
iterator_range< ipo_ext_iterator< T, SetType > > inverse_post_order_ext(const T &G, SetType &S)
Basic Alias true
po_ext_iterator< T, SetType > po_ext_begin(T G, SetType &S)
typename std::vector< NodeRef >::reverse_iterator rpo_iterator
ipo_iterator(const po_iterator< Inverse< T >, SetType, External > &V)
iterator_range< ipo_iterator< T > > inverse_post_order(const T &G)
po_iterator< T > po_begin(const T &G)