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>
insertEdge(std::optional<NodeRef> From,NodeRef To)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.
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.
84 template <class NodeRef>
insertEdge(std::optional<NodeRef> From,NodeRef To)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.
finishPostorder(NodeRef BB)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
po_iterator(NodeRef BB)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
po_iterator(NodeRef BB,SetType & S)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
po_iterator(SetType & S)129 po_iterator(SetType &S)
130 : po_iterator_storage<SetType, ExtStorage>(S) {
131 } // End is when stack is empty.
132
traverseChild()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"...
begin(const GraphT & G)148 static po_iterator begin(const GraphT &G) {
149 return po_iterator(GT::getEntryNode(G));
150 }
end(const GraphT & G)151 static po_iterator end(const GraphT &G) { return po_iterator(); }
152
begin(const GraphT & G,SetType & S)153 static po_iterator begin(const GraphT &G, SetType &S) {
154 return po_iterator(GT::getEntryNode(G), S);
155 }
end(const GraphT & G,SetType & S)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>
po_begin(const T & G)189 po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
190 template <class T>
po_end(const T & G)191 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
192
post_order(const T & G)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> {
po_ext_iteratorpo_ext_iterator200 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>
po_ext_begin(T G,SetType & S)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>
po_ext_end(T G,SetType & S)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>
post_order_ext(const T & G,SetType & S)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> {
ipo_iteratoripo_iterator223 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
224 po_iterator<Inverse<T>, SetType, External> (V) {}
225 };
226
227 template <class T>
ipo_begin(const T & G)228 ipo_iterator<T> ipo_begin(const T &G) {
229 return ipo_iterator<T>::begin(G);
230 }
231
232 template <class T>
ipo_end(const T & G)233 ipo_iterator<T> ipo_end(const T &G){
234 return ipo_iterator<T>::end(G);
235 }
236
237 template <class T>
inverse_post_order(const T & G)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> {
ipo_ext_iteratoripo_ext_iterator245 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
246 ipo_iterator<T, SetType, true>(V) {}
ipo_ext_iteratoripo_ext_iterator247 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>
ipo_ext_begin(const T & G,SetType & S)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>
ipo_ext_end(const T & G,SetType & S)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>>
inverse_post_order_ext(const T & G,SetType & S)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
Initialize(const GraphT & G)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
ReversePostOrderTraversal(const GraphT & G)309 ReversePostOrderTraversal(const GraphT &G) { Initialize(G); }
310
311 // Because we want a reverse post order, use reverse iterators from the vector
begin()312 rpo_iterator begin() { return Blocks.rbegin(); }
begin()313 const_rpo_iterator begin() const { return Blocks.rbegin(); }
end()314 rpo_iterator end() { return Blocks.rend(); }
end()315 const_rpo_iterator end() const { return Blocks.rend(); }
316 };
317
318 } // end namespace llvm
319
320 #endif // LLVM_ADT_POSTORDERITERATOR_H
321