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/SmallPtrSet.h" |
21 | #include "llvm/ADT/SmallVector.h" |
22 | #include "llvm/ADT/iterator_range.h" |
23 | #include <iterator> |
24 | #include <optional> |
25 | #include <set> |
26 | #include <utility> |
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> |
64 | bool insertEdge(std::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> |
85 | bool insertEdge(std::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 = const value_type &; |
103 | |
104 | private: |
105 | using NodeRef = typename GT::NodeRef; |
106 | using ChildItTy = typename GT::ChildIteratorType; |
107 | |
108 | /// Used to maintain the ordering. |
109 | /// First element is basic block pointer, second is iterator for the next |
110 | /// child to visit, third is the end iterator. |
111 | SmallVector<std::tuple<NodeRef, ChildItTy, ChildItTy>, 8> VisitStack; |
112 | |
113 | po_iterator(NodeRef BB) { |
114 | this->insertEdge(std::optional<NodeRef>(), BB); |
115 | VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); |
116 | traverseChild(); |
117 | } |
118 | |
119 | po_iterator() = default; // End is when stack is empty. |
120 | |
121 | po_iterator(NodeRef BB, SetType &S) |
122 | : po_iterator_storage<SetType, ExtStorage>(S) { |
123 | if (this->insertEdge(std::optional<NodeRef>(), BB)) { |
124 | VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); |
125 | traverseChild(); |
126 | } |
127 | } |
128 | |
129 | po_iterator(SetType &S) |
130 | : po_iterator_storage<SetType, ExtStorage>(S) { |
131 | } // End is when stack is empty. |
132 | |
133 | void traverseChild() { |
134 | while (true) { |
135 | auto &Entry = VisitStack.back(); |
136 | if (std::get<1>(Entry) == std::get<2>(Entry)) |
137 | break; |
138 | NodeRef BB = *std::get<1>(Entry)++; |
139 | if (this->insertEdge(std::optional<NodeRef>(std::get<0>(Entry)), BB)) { |
140 | // If the block is not visited... |
141 | VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); |
142 | } |
143 | } |
144 | } |
145 | |
146 | public: |
147 | // Provide static "constructors"... |
148 | static po_iterator begin(const GraphT &G) { |
149 | return po_iterator(GT::getEntryNode(G)); |
150 | } |
151 | static po_iterator end(const GraphT &G) { return po_iterator(); } |
152 | |
153 | static po_iterator begin(const GraphT &G, SetType &S) { |
154 | return po_iterator(GT::getEntryNode(G), S); |
155 | } |
156 | static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); } |
157 | |
158 | bool operator==(const po_iterator &x) const { |
159 | return VisitStack == x.VisitStack; |
160 | } |
161 | bool operator!=(const po_iterator &x) const { return !(*this == x); } |
162 | |
163 | reference operator*() const { return std::get<0>(VisitStack.back()); } |
164 | |
165 | // This is a nonstandard operator-> that dereferences the pointer an extra |
166 | // time... so that you can actually call methods ON the BasicBlock, because |
167 | // the contained type is a pointer. This allows BBIt->getTerminator() f.e. |
168 | // |
169 | NodeRef operator->() const { return **this; } |
170 | |
171 | po_iterator &operator++() { // Preincrement |
172 | this->finishPostorder(std::get<0>(VisitStack.back())); |
173 | VisitStack.pop_back(); |
174 | if (!VisitStack.empty()) |
175 | traverseChild(); |
176 | return *this; |
177 | } |
178 | |
179 | po_iterator operator++(int) { // Postincrement |
180 | po_iterator tmp = *this; |
181 | ++*this; |
182 | return tmp; |
183 | } |
184 | }; |
185 | |
186 | // Provide global constructors that automatically figure out correct types... |
187 | // |
188 | template <class T> |
189 | po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); } |
190 | template <class T> |
191 | po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); } |
192 | |
193 | template <class T> iterator_range<po_iterator<T>> post_order(const T &G) { |
194 | return make_range(po_begin(G), po_end(G)); |
195 | } |
196 | |
197 | // Provide global definitions of external postorder iterators... |
198 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> |
199 | struct po_ext_iterator : public po_iterator<T, SetType, true> { |
200 | po_ext_iterator(const po_iterator<T, SetType, true> &V) : |
201 | po_iterator<T, SetType, true>(V) {} |
202 | }; |
203 | |
204 | template<class T, class SetType> |
205 | po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) { |
206 | return po_ext_iterator<T, SetType>::begin(G, S); |
207 | } |
208 | |
209 | template<class T, class SetType> |
210 | po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) { |
211 | return po_ext_iterator<T, SetType>::end(G, S); |
212 | } |
213 | |
214 | template <class T, class SetType> |
215 | iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) { |
216 | return make_range(po_ext_begin(G, S), po_ext_end(G, S)); |
217 | } |
218 | |
219 | // Provide global definitions of inverse post order iterators... |
220 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>, |
221 | bool External = false> |
222 | struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> { |
223 | ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) : |
224 | po_iterator<Inverse<T>, SetType, External> (V) {} |
225 | }; |
226 | |
227 | template <class T> |
228 | ipo_iterator<T> ipo_begin(const T &G) { |
229 | return ipo_iterator<T>::begin(G); |
230 | } |
231 | |
232 | template <class T> |
233 | ipo_iterator<T> ipo_end(const T &G){ |
234 | return ipo_iterator<T>::end(G); |
235 | } |
236 | |
237 | template <class T> |
238 | iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) { |
239 | return make_range(ipo_begin(G), ipo_end(G)); |
240 | } |
241 | |
242 | // Provide global definitions of external inverse postorder iterators... |
243 | template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> |
244 | struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> { |
245 | ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) : |
246 | ipo_iterator<T, SetType, true>(V) {} |
247 | ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) : |
248 | ipo_iterator<T, SetType, true>(V) {} |
249 | }; |
250 | |
251 | template <class T, class SetType> |
252 | ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) { |
253 | return ipo_ext_iterator<T, SetType>::begin(G, S); |
254 | } |
255 | |
256 | template <class T, class SetType> |
257 | ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) { |
258 | return ipo_ext_iterator<T, SetType>::end(G, S); |
259 | } |
260 | |
261 | template <class T, class SetType> |
262 | iterator_range<ipo_ext_iterator<T, SetType>> |
263 | inverse_post_order_ext(const T &G, SetType &S) { |
264 | return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S)); |
265 | } |
266 | |
267 | //===--------------------------------------------------------------------===// |
268 | // Reverse Post Order CFG iterator code |
269 | //===--------------------------------------------------------------------===// |
270 | // |
271 | // This is used to visit basic blocks in a method in reverse post order. This |
272 | // class is awkward to use because I don't know a good incremental algorithm to |
273 | // computer RPO from a graph. Because of this, the construction of the |
274 | // ReversePostOrderTraversal object is expensive (it must walk the entire graph |
275 | // with a postorder iterator to build the data structures). The moral of this |
276 | // story is: Don't create more ReversePostOrderTraversal classes than necessary. |
277 | // |
278 | // Because it does the traversal in its constructor, it won't invalidate when |
279 | // BasicBlocks are removed, *but* it may contain erased blocks. Some places |
280 | // rely on this behavior (i.e. GVN). |
281 | // |
282 | // This class should be used like this: |
283 | // { |
284 | // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create |
285 | // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { |
286 | // ... |
287 | // } |
288 | // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { |
289 | // ... |
290 | // } |
291 | // } |
292 | // |
293 | |
294 | template<class GraphT, class GT = GraphTraits<GraphT>> |
295 | class ReversePostOrderTraversal { |
296 | using NodeRef = typename GT::NodeRef; |
297 | |
298 | using VecTy = SmallVector<NodeRef, 8>; |
299 | VecTy Blocks; // Block list in normal PO order |
300 | |
301 | void Initialize(const GraphT &G) { |
302 | std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks)); |
303 | } |
304 | |
305 | public: |
306 | using rpo_iterator = typename VecTy::reverse_iterator; |
307 | using const_rpo_iterator = typename VecTy::const_reverse_iterator; |
308 | |
309 | ReversePostOrderTraversal(const GraphT &G) { Initialize(G); } |
310 | |
311 | // Because we want a reverse post order, use reverse iterators from the vector |
312 | rpo_iterator begin() { return Blocks.rbegin(); } |
313 | const_rpo_iterator begin() const { return Blocks.rbegin(); } |
314 | rpo_iterator end() { return Blocks.rend(); } |
315 | const_rpo_iterator end() const { return Blocks.rend(); } |
316 | }; |
317 | |
318 | } // end namespace llvm |
319 | |
320 | #endif // LLVM_ADT_POSTORDERITERATOR_H |
321 | |