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"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/ADT/iterator_range.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>
58 class po_iterator_storage {
59 SetType Visited;
60
61 public:
62 // Return true if edge destination should be visited.
63 template <typename NodeRef>
insertEdge(Optional<NodeRef> From,NodeRef To)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.
finishPostorder(NodeRef BB)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:
po_iterator_storage(SetType & VSet)78 po_iterator_storage(SetType &VSet) : Visited(VSet) {}
po_iterator_storage(const po_iterator_storage & S)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.
insertEdge(Optional<NodeRef> From,NodeRef To)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.
finishPostorder(NodeRef BB)89 template <class NodeRef> void finishPostorder(NodeRef BB) {}
90 };
91
92 template <class GraphT,
93 class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
94 bool ExtStorage = false, class GT = GraphTraits<GraphT>>
95 class po_iterator : public po_iterator_storage<SetType, ExtStorage> {
96 public:
97 using iterator_category = std::forward_iterator_tag;
98 using value_type = typename GT::NodeRef;
99 using difference_type = std::ptrdiff_t;
100 using pointer = value_type *;
101 using reference = value_type &;
102
103 private:
104 using NodeRef = typename GT::NodeRef;
105 using ChildItTy = typename GT::ChildIteratorType;
106
107 // VisitStack - Used to maintain the ordering. Top = current block
108 // First element is basic block pointer, second is the 'next child' to visit
109 SmallVector<std::pair<NodeRef, ChildItTy>, 8> VisitStack;
110
po_iterator(NodeRef BB)111 po_iterator(NodeRef BB) {
112 this->insertEdge(Optional<NodeRef>(), BB);
113 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
114 traverseChild();
115 }
116
117 po_iterator() = default; // End is when stack is empty.
118
po_iterator(NodeRef BB,SetType & S)119 po_iterator(NodeRef BB, SetType &S)
120 : po_iterator_storage<SetType, ExtStorage>(S) {
121 if (this->insertEdge(Optional<NodeRef>(), BB)) {
122 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
123 traverseChild();
124 }
125 }
126
po_iterator(SetType & S)127 po_iterator(SetType &S)
128 : po_iterator_storage<SetType, ExtStorage>(S) {
129 } // End is when stack is empty.
130
traverseChild()131 void traverseChild() {
132 while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
133 NodeRef BB = *VisitStack.back().second++;
134 if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
135 // If the block is not visited...
136 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
137 }
138 }
139 }
140
141 public:
142 // Provide static "constructors"...
begin(const GraphT & G)143 static po_iterator begin(const GraphT &G) {
144 return po_iterator(GT::getEntryNode(G));
145 }
end(const GraphT & G)146 static po_iterator end(const GraphT &G) { return po_iterator(); }
147
begin(const GraphT & G,SetType & S)148 static po_iterator begin(const GraphT &G, SetType &S) {
149 return po_iterator(GT::getEntryNode(G), S);
150 }
end(const GraphT & G,SetType & S)151 static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); }
152
153 bool operator==(const po_iterator &x) const {
154 return VisitStack == x.VisitStack;
155 }
156 bool operator!=(const po_iterator &x) const { return !(*this == x); }
157
158 const NodeRef &operator*() const { return VisitStack.back().first; }
159
160 // This is a nonstandard operator-> that dereferences the pointer an extra
161 // time... so that you can actually call methods ON the BasicBlock, because
162 // the contained type is a pointer. This allows BBIt->getTerminator() f.e.
163 //
164 NodeRef operator->() const { return **this; }
165
166 po_iterator &operator++() { // Preincrement
167 this->finishPostorder(VisitStack.back().first);
168 VisitStack.pop_back();
169 if (!VisitStack.empty())
170 traverseChild();
171 return *this;
172 }
173
174 po_iterator operator++(int) { // Postincrement
175 po_iterator tmp = *this;
176 ++*this;
177 return tmp;
178 }
179 };
180
181 // Provide global constructors that automatically figure out correct types...
182 //
183 template <class T>
po_begin(const T & G)184 po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
185 template <class T>
po_end(const T & G)186 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
187
post_order(const T & G)188 template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
189 return make_range(po_begin(G), po_end(G));
190 }
191
192 // Provide global definitions of external postorder iterators...
193 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
194 struct po_ext_iterator : public po_iterator<T, SetType, true> {
po_ext_iteratorpo_ext_iterator195 po_ext_iterator(const po_iterator<T, SetType, true> &V) :
196 po_iterator<T, SetType, true>(V) {}
197 };
198
199 template<class T, class SetType>
po_ext_begin(T G,SetType & S)200 po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
201 return po_ext_iterator<T, SetType>::begin(G, S);
202 }
203
204 template<class T, class SetType>
po_ext_end(T G,SetType & S)205 po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
206 return po_ext_iterator<T, SetType>::end(G, S);
207 }
208
209 template <class T, class SetType>
post_order_ext(const T & G,SetType & S)210 iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) {
211 return make_range(po_ext_begin(G, S), po_ext_end(G, S));
212 }
213
214 // Provide global definitions of inverse post order iterators...
215 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
216 bool External = false>
217 struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
ipo_iteratoripo_iterator218 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
219 po_iterator<Inverse<T>, SetType, External> (V) {}
220 };
221
222 template <class T>
ipo_begin(const T & G)223 ipo_iterator<T> ipo_begin(const T &G) {
224 return ipo_iterator<T>::begin(G);
225 }
226
227 template <class T>
ipo_end(const T & G)228 ipo_iterator<T> ipo_end(const T &G){
229 return ipo_iterator<T>::end(G);
230 }
231
232 template <class T>
inverse_post_order(const T & G)233 iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) {
234 return make_range(ipo_begin(G), ipo_end(G));
235 }
236
237 // Provide global definitions of external inverse postorder iterators...
238 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
239 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
ipo_ext_iteratoripo_ext_iterator240 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
241 ipo_iterator<T, SetType, true>(V) {}
ipo_ext_iteratoripo_ext_iterator242 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
243 ipo_iterator<T, SetType, true>(V) {}
244 };
245
246 template <class T, class SetType>
ipo_ext_begin(const T & G,SetType & S)247 ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
248 return ipo_ext_iterator<T, SetType>::begin(G, S);
249 }
250
251 template <class T, class SetType>
ipo_ext_end(const T & G,SetType & S)252 ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
253 return ipo_ext_iterator<T, SetType>::end(G, S);
254 }
255
256 template <class T, class SetType>
257 iterator_range<ipo_ext_iterator<T, SetType>>
inverse_post_order_ext(const T & G,SetType & S)258 inverse_post_order_ext(const T &G, SetType &S) {
259 return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
260 }
261
262 //===--------------------------------------------------------------------===//
263 // Reverse Post Order CFG iterator code
264 //===--------------------------------------------------------------------===//
265 //
266 // This is used to visit basic blocks in a method in reverse post order. This
267 // class is awkward to use because I don't know a good incremental algorithm to
268 // computer RPO from a graph. Because of this, the construction of the
269 // ReversePostOrderTraversal object is expensive (it must walk the entire graph
270 // with a postorder iterator to build the data structures). The moral of this
271 // story is: Don't create more ReversePostOrderTraversal classes than necessary.
272 //
273 // Because it does the traversal in its constructor, it won't invalidate when
274 // BasicBlocks are removed, *but* it may contain erased blocks. Some places
275 // rely on this behavior (i.e. GVN).
276 //
277 // This class should be used like this:
278 // {
279 // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
280 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
281 // ...
282 // }
283 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
284 // ...
285 // }
286 // }
287 //
288
289 template<class GraphT, class GT = GraphTraits<GraphT>>
290 class ReversePostOrderTraversal {
291 using NodeRef = typename GT::NodeRef;
292
293 std::vector<NodeRef> Blocks; // Block list in normal PO order
294
Initialize(const GraphT & G)295 void Initialize(const GraphT &G) {
296 std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks));
297 }
298
299 public:
300 using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
301 using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
302
ReversePostOrderTraversal(const GraphT & G)303 ReversePostOrderTraversal(const GraphT &G) { Initialize(G); }
304
305 // Because we want a reverse post order, use reverse iterators from the vector
begin()306 rpo_iterator begin() { return Blocks.rbegin(); }
begin()307 const_rpo_iterator begin() const { return Blocks.crbegin(); }
end()308 rpo_iterator end() { return Blocks.rend(); }
end()309 const_rpo_iterator end() const { return Blocks.crend(); }
310 };
311
312 } // end namespace llvm
313
314 #endif // LLVM_ADT_POSTORDERITERATOR_H
315