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
29namespace 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.
58template<class SetType, bool External>
59class po_iterator_storage {
60 SetType Visited;
61
62public:
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.
74template<class SetType>
75class po_iterator_storage<SetType, true> {
76 SetType &Visited;
77
78public:
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
93template <class GraphT,
94 class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
95 bool ExtStorage = false, class GT = GraphTraits<GraphT>>
96class po_iterator : public po_iterator_storage<SetType, ExtStorage> {
97public:
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
104private:
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
142public:
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//
184template <class T>
185po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
186template <class T>
187po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
188
189template <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...
194template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
195struct 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
200template<class T, class SetType>
201po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
202 return po_ext_iterator<T, SetType>::begin(G, S);
203}
204
205template<class T, class SetType>
206po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
207 return po_ext_iterator<T, SetType>::end(G, S);
208}
209
210template <class T, class SetType>
211iterator_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...
216template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
217 bool External = false>
218struct 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
223template <class T>
224ipo_iterator<T> ipo_begin(const T &G) {
225 return ipo_iterator<T>::begin(G);
226}
227
228template <class T>
229ipo_iterator<T> ipo_end(const T &G){
230 return ipo_iterator<T>::end(G);
231}
232
233template <class T>
234iterator_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...
239template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
240struct 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
247template <class T, class SetType>
248ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
249 return ipo_ext_iterator<T, SetType>::begin(G, S);
250}
251
252template <class T, class SetType>
253ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
254 return ipo_ext_iterator<T, SetType>::end(G, S);
255}
256
257template <class T, class SetType>
258iterator_range<ipo_ext_iterator<T, SetType>>
259inverse_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
290template<class GraphT, class GT = GraphTraits<GraphT>>
291class 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
300public:
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

source code of llvm/include/llvm/ADT/PostOrderIterator.h