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 | /// \file |
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" |
22 | #include "llvm/ADT/SmallVector.h" |
23 | #include "llvm/ADT/iterator_range.h" |
24 | #include <iterator> |
25 | #include <set> |
26 | #include <utility> |
27 | #include <vector> |
28 | |
29 | namespace llvm { |
30 | |
31 | // The po_iterator_storage template provides access to the set of already |
32 | // visited nodes during the po_iterator's depth-first traversal. |
33 | // |
34 | // The default implementation simply contains a set of visited nodes, while |
35 | // the External=true version uses a reference to an external set. |
36 | // |
37 | // It is possible to prune the depth-first traversal in several ways: |
38 | // |
39 | // - When providing an external set that already contains some graph nodes, |
40 | // those nodes won't be visited again. This is useful for restarting a |
41 | // post-order traversal on a graph with nodes that aren't dominated by a |
42 | // single node. |
43 | // |
44 | // - By providing a custom SetType class, unwanted graph nodes can be excluded |
45 | // by having the insert() function return false. This could for example |
46 | // confine a CFG traversal to blocks in a specific loop. |
47 | // |
48 | // - Finally, by specializing the po_iterator_storage template itself, graph |
49 | // edges can be pruned by returning false in the insertEdge() function. This |
50 | // could be used to remove loop back-edges from the CFG seen by po_iterator. |
51 | // |
52 | // A specialized po_iterator_storage class can observe both the pre-order and |
53 | // the post-order. The insertEdge() function is called in a pre-order, while |
54 | // the finishPostorder() function is called just before the po_iterator moves |
55 | // on to the next node. |
56 | |
57 | /// Default po_iterator_storage implementation with an internal set object. |
58 | template<class SetType, bool External> |
59 | class po_iterator_storage { |
60 | SetType Visited; |
61 | |
62 | public: |
63 | // Return true if edge destination should be visited. |
64 | template <typename NodeRef> |
65 | bool insertEdge(Optional<NodeRef> From, NodeRef To) { |
66 | return Visited.insert(To).second; |
67 | } |
68 | |
69 | // Called after all children of BB have been visited. |
70 | template <typename NodeRef> void finishPostorder(NodeRef BB) {} |
71 | }; |
72 | |
73 | /// Specialization of po_iterator_storage that references an external set. |
74 | template<class SetType> |
75 | class po_iterator_storage<SetType, true> { |
76 | SetType &Visited; |
77 | |
78 | public: |
79 | po_iterator_storage(SetType &VSet) : Visited(VSet) {} |
80 | po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {} |
81 | |
82 | // Return true if edge destination should be visited, called with From = 0 for |
83 | // the root node. |
84 | // Graph edges can be pruned by specializing this function. |
85 | template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) { |
86 | return Visited.insert(To).second; |
87 | } |
88 | |
89 | // Called after all children of BB have been visited. |
90 | template <class NodeRef> void finishPostorder(NodeRef BB) {} |
91 | }; |
92 | |
93 | template <class GraphT, |
94 | class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>, |
95 | bool ExtStorage = false, class GT = GraphTraits<GraphT>> |
96 | class po_iterator : public po_iterator_storage<SetType, ExtStorage> { |
97 | public: |
98 | using iterator_category = std::forward_iterator_tag; |
99 | using value_type = typename GT::NodeRef; |
100 | using difference_type = std::ptrdiff_t; |
101 | using pointer = value_type *; |
102 | using reference = value_type &; |
103 | |
104 | private: |
105 | using NodeRef = typename GT::NodeRef; |
106 | using ChildItTy = typename GT::ChildIteratorType; |
107 | |
108 | // VisitStack - Used to maintain the ordering. Top = current block |
109 | // First element is basic block pointer, second is the 'next child' to visit |
110 | SmallVector<std::pair<NodeRef, ChildItTy>, 8> VisitStack; |
111 | |
112 | po_iterator(NodeRef BB) { |
113 | this->insertEdge(Optional<NodeRef>(), BB); |
114 | VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); |
115 | traverseChild(); |
116 | } |
117 | |
118 | po_iterator() = default; // End is when stack is empty. |
119 | |
120 | po_iterator(NodeRef BB, SetType &S) |
121 | : po_iterator_storage<SetType, ExtStorage>(S) { |
122 | if (this->insertEdge(Optional<NodeRef>(), BB)) { |
123 | VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); |
124 | traverseChild(); |
125 | } |
126 | } |
127 | |
128 | po_iterator(SetType &S) |
129 | : po_iterator_storage<SetType, ExtStorage>(S) { |
130 | } // End is when stack is empty. |
131 | |
132 | void traverseChild() { |
133 | while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) { |
134 | NodeRef BB = *VisitStack.back().second++; |
135 | if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) { |
136 | // If the block is not visited... |
137 | VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); |
138 | } |
139 | } |
140 | } |
141 | |
142 | public: |
143 | // Provide static "constructors"... |
144 | static po_iterator begin(const GraphT &G) { |
145 | return po_iterator(GT::getEntryNode(G)); |
146 | } |
147 | static po_iterator end(const GraphT &G) { return po_iterator(); } |
148 | |
149 | static po_iterator begin(const GraphT &G, SetType &S) { |
150 | return po_iterator(GT::getEntryNode(G), S); |
151 | } |
152 | static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); } |
153 | |
154 | bool operator==(const po_iterator &x) const { |
155 | return VisitStack == x.VisitStack; |
156 | } |
157 | bool operator!=(const po_iterator &x) const { return !(*this == x); } |
158 | |
159 | const NodeRef &operator*() const { return VisitStack.back().first; } |
160 | |
161 | // This is a nonstandard operator-> that dereferences the pointer an extra |
162 | // time... so that you can actually call methods ON the BasicBlock, because |
163 | // the contained type is a pointer. This allows BBIt->getTerminator() f.e. |
164 | // |
165 | NodeRef operator->() const { return **this; } |
166 | |
167 | po_iterator &operator++() { // Preincrement |
168 | this->finishPostorder(VisitStack.back().first); |
169 | VisitStack.pop_back(); |
170 | if (!VisitStack.empty()) |
171 | traverseChild(); |
172 | return *this; |
173 | } |
174 | |
175 | po_iterator operator++(int) { // Postincrement |
176 | po_iterator tmp = *this; |
177 | ++*this; |
178 | return tmp; |
179 | } |
180 | }; |
181 | |
182 | // Provide global constructors that automatically figure out correct types... |
183 | // |
184 | template <class T> |
185 | po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); } |
186 | template <class T> |
187 | po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); } |
188 | |
189 | template <class T> iterator_range<po_iterator<T>> post_order(const T &G) { |
190 | return make_range(po_begin(G), po_end(G)); |
191 | } |
192 | |
193 | // Provide global definitions of external postorder iterators... |
194 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> |
195 | struct po_ext_iterator : public po_iterator<T, SetType, true> { |
196 | po_ext_iterator(const po_iterator<T, SetType, true> &V) : |
197 | po_iterator<T, SetType, true>(V) {} |
198 | }; |
199 | |
200 | template<class T, class SetType> |
201 | po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) { |
202 | return po_ext_iterator<T, SetType>::begin(G, S); |
203 | } |
204 | |
205 | template<class T, class SetType> |
206 | po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) { |
207 | return po_ext_iterator<T, SetType>::end(G, S); |
208 | } |
209 | |
210 | template <class T, class SetType> |
211 | iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) { |
212 | return make_range(po_ext_begin(G, S), po_ext_end(G, S)); |
213 | } |
214 | |
215 | // Provide global definitions of inverse post order iterators... |
216 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>, |
217 | bool External = false> |
218 | struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> { |
219 | ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) : |
220 | po_iterator<Inverse<T>, SetType, External> (V) {} |
221 | }; |
222 | |
223 | template <class T> |
224 | ipo_iterator<T> ipo_begin(const T &G) { |
225 | return ipo_iterator<T>::begin(G); |
226 | } |
227 | |
228 | template <class T> |
229 | ipo_iterator<T> ipo_end(const T &G){ |
230 | return ipo_iterator<T>::end(G); |
231 | } |
232 | |
233 | template <class T> |
234 | iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) { |
235 | return make_range(ipo_begin(G), ipo_end(G)); |
236 | } |
237 | |
238 | // Provide global definitions of external inverse postorder iterators... |
239 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> |
240 | struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> { |
241 | ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) : |
242 | ipo_iterator<T, SetType, true>(V) {} |
243 | ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) : |
244 | ipo_iterator<T, SetType, true>(V) {} |
245 | }; |
246 | |
247 | template <class T, class SetType> |
248 | ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) { |
249 | return ipo_ext_iterator<T, SetType>::begin(G, S); |
250 | } |
251 | |
252 | template <class T, class SetType> |
253 | ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) { |
254 | return ipo_ext_iterator<T, SetType>::end(G, S); |
255 | } |
256 | |
257 | template <class T, class SetType> |
258 | iterator_range<ipo_ext_iterator<T, SetType>> |
259 | inverse_post_order_ext(const T &G, SetType &S) { |
260 | return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S)); |
261 | } |
262 | |
263 | //===--------------------------------------------------------------------===// |
264 | // Reverse Post Order CFG iterator code |
265 | //===--------------------------------------------------------------------===// |
266 | // |
267 | // This is used to visit basic blocks in a method in reverse post order. This |
268 | // class is awkward to use because I don't know a good incremental algorithm to |
269 | // computer RPO from a graph. Because of this, the construction of the |
270 | // ReversePostOrderTraversal object is expensive (it must walk the entire graph |
271 | // with a postorder iterator to build the data structures). The moral of this |
272 | // story is: Don't create more ReversePostOrderTraversal classes than necessary. |
273 | // |
274 | // Because it does the traversal in its constructor, it won't invalidate when |
275 | // BasicBlocks are removed, *but* it may contain erased blocks. Some places |
276 | // rely on this behavior (i.e. GVN). |
277 | // |
278 | // This class should be used like this: |
279 | // { |
280 | // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create |
281 | // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { |
282 | // ... |
283 | // } |
284 | // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { |
285 | // ... |
286 | // } |
287 | // } |
288 | // |
289 | |
290 | template<class GraphT, class GT = GraphTraits<GraphT>> |
291 | class ReversePostOrderTraversal { |
292 | using NodeRef = typename GT::NodeRef; |
293 | |
294 | std::vector<NodeRef> Blocks; // Block list in normal PO order |
295 | |
296 | void Initialize(const GraphT &G) { |
297 | std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks)); |
298 | } |
299 | |
300 | public: |
301 | using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator; |
302 | using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator; |
303 | |
304 | ReversePostOrderTraversal(const GraphT &G) { Initialize(G); } |
305 | |
306 | // Because we want a reverse post order, use reverse iterators from the vector |
307 | rpo_iterator begin() { return Blocks.rbegin(); } |
308 | const_rpo_iterator begin() const { return Blocks.crbegin(); } |
309 | rpo_iterator end() { return Blocks.rend(); } |
310 | const_rpo_iterator end() const { return Blocks.crend(); } |
311 | }; |
312 | |
313 | } // end namespace llvm |
314 | |
315 | #endif // LLVM_ADT_POSTORDERITERATOR_H |
316 | |