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PostOrderIterator.h
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1 //===- llvm/ADT/PostOrderIterator.h - PostOrder iterator --------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file builds on the ADT/GraphTraits.h file to build a generic graph
10 // post order iterator. This should work over any graph type that has a
11 // GraphTraits specialization.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_ADT_POSTORDERITERATOR_H
16 #define LLVM_ADT_POSTORDERITERATOR_H
17 
18 #include "llvm/ADT/GraphTraits.h"
19 #include "llvm/ADT/Optional.h"
20 #include "llvm/ADT/SmallPtrSet.h"
22 #include <iterator>
23 #include <set>
24 #include <utility>
25 #include <vector>
26 
27 namespace llvm {
28 
29 // The po_iterator_storage template provides access to the set of already
30 // visited nodes during the po_iterator's depth-first traversal.
31 //
32 // The default implementation simply contains a set of visited nodes, while
33 // the External=true version uses a reference to an external set.
34 //
35 // It is possible to prune the depth-first traversal in several ways:
36 //
37 // - When providing an external set that already contains some graph nodes,
38 // those nodes won't be visited again. This is useful for restarting a
39 // post-order traversal on a graph with nodes that aren't dominated by a
40 // single node.
41 //
42 // - By providing a custom SetType class, unwanted graph nodes can be excluded
43 // by having the insert() function return false. This could for example
44 // confine a CFG traversal to blocks in a specific loop.
45 //
46 // - Finally, by specializing the po_iterator_storage template itself, graph
47 // edges can be pruned by returning false in the insertEdge() function. This
48 // could be used to remove loop back-edges from the CFG seen by po_iterator.
49 //
50 // A specialized po_iterator_storage class can observe both the pre-order and
51 // the post-order. The insertEdge() function is called in a pre-order, while
52 // the finishPostorder() function is called just before the po_iterator moves
53 // on to the next node.
54 
55 /// Default po_iterator_storage implementation with an internal set object.
56 template<class SetType, bool External>
58  SetType Visited;
59 
60 public:
61  // Return true if edge destination should be visited.
62  template <typename NodeRef>
63  bool insertEdge(Optional<NodeRef> From, NodeRef To) {
64  return Visited.insert(To).second;
65  }
66 
67  // Called after all children of BB have been visited.
68  template <typename NodeRef> void finishPostorder(NodeRef BB) {}
69 };
70 
71 /// Specialization of po_iterator_storage that references an external set.
72 template<class SetType>
73 class po_iterator_storage<SetType, true> {
74  SetType &Visited;
75 
76 public:
77  po_iterator_storage(SetType &VSet) : Visited(VSet) {}
78  po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {}
79 
80  // Return true if edge destination should be visited, called with From = 0 for
81  // the root node.
82  // Graph edges can be pruned by specializing this function.
83  template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) {
84  return Visited.insert(To).second;
85  }
86 
87  // Called after all children of BB have been visited.
88  template <class NodeRef> void finishPostorder(NodeRef BB) {}
89 };
90 
91 template <class GraphT,
92  class SetType =
94  bool ExtStorage = false, class GT = GraphTraits<GraphT>>
96  : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
97  public po_iterator_storage<SetType, ExtStorage> {
98  using super = std::iterator<std::forward_iterator_tag, typename GT::NodeRef>;
99  using NodeRef = typename GT::NodeRef;
100  using ChildItTy = typename GT::ChildIteratorType;
101 
102  // VisitStack - Used to maintain the ordering. Top = current block
103  // First element is basic block pointer, second is the 'next child' to visit
104  std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
105 
106  po_iterator(NodeRef BB) {
107  this->insertEdge(Optional<NodeRef>(), BB);
108  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
109  traverseChild();
110  }
111 
112  po_iterator() = default; // End is when stack is empty.
113 
114  po_iterator(NodeRef BB, SetType &S)
116  if (this->insertEdge(Optional<NodeRef>(), BB)) {
117  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
118  traverseChild();
119  }
120  }
121 
122  po_iterator(SetType &S)
124  } // End is when stack is empty.
125 
126  void traverseChild() {
127  while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
128  NodeRef BB = *VisitStack.back().second++;
129  if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
130  // If the block is not visited...
131  VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
132  }
133  }
134  }
135 
136 public:
137  using pointer = typename super::pointer;
138 
139  // Provide static "constructors"...
140  static po_iterator begin(GraphT G) {
141  return po_iterator(GT::getEntryNode(G));
142  }
143  static po_iterator end(GraphT G) { return po_iterator(); }
144 
145  static po_iterator begin(GraphT G, SetType &S) {
146  return po_iterator(GT::getEntryNode(G), S);
147  }
148  static po_iterator end(GraphT G, SetType &S) { return po_iterator(S); }
149 
150  bool operator==(const po_iterator &x) const {
151  return VisitStack == x.VisitStack;
152  }
153  bool operator!=(const po_iterator &x) const { return !(*this == x); }
154 
155  const NodeRef &operator*() const { return VisitStack.back().first; }
156 
157  // This is a nonstandard operator-> that dereferences the pointer an extra
158  // time... so that you can actually call methods ON the BasicBlock, because
159  // the contained type is a pointer. This allows BBIt->getTerminator() f.e.
160  //
161  NodeRef operator->() const { return **this; }
162 
163  po_iterator &operator++() { // Preincrement
164  this->finishPostorder(VisitStack.back().first);
165  VisitStack.pop_back();
166  if (!VisitStack.empty())
167  traverseChild();
168  return *this;
169  }
170 
171  po_iterator operator++(int) { // Postincrement
172  po_iterator tmp = *this;
173  ++*this;
174  return tmp;
175  }
176 };
177 
178 // Provide global constructors that automatically figure out correct types...
179 //
180 template <class T>
182 template <class T>
183 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
184 
185 template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
186  return make_range(po_begin(G), po_end(G));
187 }
188 
189 // Provide global definitions of external postorder iterators...
190 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
191 struct po_ext_iterator : public po_iterator<T, SetType, true> {
193  po_iterator<T, SetType, true>(V) {}
194 };
195 
196 template<class T, class SetType>
199 }
200 
201 template<class T, class SetType>
204 }
205 
206 template <class T, class SetType>
208  return make_range(po_ext_begin(G, S), po_ext_end(G, S));
209 }
210 
211 // Provide global definitions of inverse post order iterators...
212 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
213  bool External = false>
214 struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
215  ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
216  po_iterator<Inverse<T>, SetType, External> (V) {}
217 };
218 
219 template <class T>
221  return ipo_iterator<T>::begin(G);
222 }
223 
224 template <class T>
226  return ipo_iterator<T>::end(G);
227 }
228 
229 template <class T>
231  return make_range(ipo_begin(G), ipo_end(G));
232 }
233 
234 // Provide global definitions of external inverse postorder iterators...
235 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
236 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
238  ipo_iterator<T, SetType, true>(V) {}
239  ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
240  ipo_iterator<T, SetType, true>(V) {}
241 };
242 
243 template <class T, class SetType>
246 }
247 
248 template <class T, class SetType>
251 }
252 
253 template <class T, class SetType>
255 inverse_post_order_ext(const T &G, SetType &S) {
256  return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
257 }
258 
259 //===--------------------------------------------------------------------===//
260 // Reverse Post Order CFG iterator code
261 //===--------------------------------------------------------------------===//
262 //
263 // This is used to visit basic blocks in a method in reverse post order. This
264 // class is awkward to use because I don't know a good incremental algorithm to
265 // computer RPO from a graph. Because of this, the construction of the
266 // ReversePostOrderTraversal object is expensive (it must walk the entire graph
267 // with a postorder iterator to build the data structures). The moral of this
268 // story is: Don't create more ReversePostOrderTraversal classes than necessary.
269 //
270 // Because it does the traversal in its constructor, it won't invalidate when
271 // BasicBlocks are removed, *but* it may contain erased blocks. Some places
272 // rely on this behavior (i.e. GVN).
273 //
274 // This class should be used like this:
275 // {
276 // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
277 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
278 // ...
279 // }
280 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
281 // ...
282 // }
283 // }
284 //
285 
286 template<class GraphT, class GT = GraphTraits<GraphT>>
288  using NodeRef = typename GT::NodeRef;
289 
290  std::vector<NodeRef> Blocks; // Block list in normal PO order
291 
292  void Initialize(NodeRef BB) {
293  std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
294  }
295 
296 public:
297  using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
298  using const_rpo_iterator = typename std::vector<NodeRef>::const_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  const_rpo_iterator begin() const { return Blocks.crbegin(); }
305  rpo_iterator end() { return Blocks.rend(); }
306  const_rpo_iterator end() const { return Blocks.crend(); }
307 };
308 
309 } // end namespace llvm
310 
311 #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)
This class represents lattice values for constants.
Definition: AllocatorList.h:23
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
block Block Frequency true
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)
const_rpo_iterator end() const
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:417
BlockVerifier::State From
bool operator!=(const po_iterator &x) const
const DataFlowGraph & G
Definition: RDFGraph.cpp:202
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)
typename std::vector< NodeRef >::const_reverse_iterator const_rpo_iterator
const_rpo_iterator begin() const
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)
OutputIt copy(R &&Range, OutputIt Out)
Definition: STLExtras.h:1229
po_iterator< T > po_begin(const T &G)