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
28namespace 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.
57template<class SetType, bool External>
58class po_iterator_storage {
59 SetType Visited;
60
61public:
62 // Return true if edge destination should be visited.
63 template <typename NodeRef>
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.
69 template <typename NodeRef> void finishPostorder(NodeRef BB) {}
70};
71
72/// Specialization of po_iterator_storage that references an external set.
73template<class SetType>
74class po_iterator_storage<SetType, true> {
75 SetType &Visited;
76
77public:
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> 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.
89 template <class NodeRef> void finishPostorder(NodeRef BB) {}
90};
91
92template <class GraphT,
93 class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
94 bool ExtStorage = false, class GT = GraphTraits<GraphT>>
95class po_iterator : public po_iterator_storage<SetType, ExtStorage> {
96public:
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
103private:
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
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
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
127 po_iterator(SetType &S)
128 : po_iterator_storage<SetType, ExtStorage>(S) {
129 } // End is when stack is empty.
130
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
141public:
142 // Provide static "constructors"...
143 static po_iterator begin(const GraphT &G) {
144 return po_iterator(GT::getEntryNode(G));
145 }
146 static po_iterator end(const GraphT &G) { return po_iterator(); }
147
148 static po_iterator begin(const GraphT &G, SetType &S) {
149 return po_iterator(GT::getEntryNode(G), S);
150 }
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//
183template <class T>
184po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
185template <class T>
186po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
187
188template <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...
193template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
194struct po_ext_iterator : public po_iterator<T, SetType, true> {
195 po_ext_iterator(const po_iterator<T, SetType, true> &V) :
196 po_iterator<T, SetType, true>(V) {}
197};
198
199template<class T, class SetType>
200po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
201 return po_ext_iterator<T, SetType>::begin(G, S);
202}
203
204template<class T, class SetType>
205po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
206 return po_ext_iterator<T, SetType>::end(G, S);
207}
208
209template <class T, class SetType>
210iterator_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...
215template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
216 bool External = false>
217struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
218 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
219 po_iterator<Inverse<T>, SetType, External> (V) {}
220};
221
222template <class T>
223ipo_iterator<T> ipo_begin(const T &G) {
224 return ipo_iterator<T>::begin(G);
225}
226
227template <class T>
228ipo_iterator<T> ipo_end(const T &G){
229 return ipo_iterator<T>::end(G);
230}
231
232template <class T>
233iterator_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...
238template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
239struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
240 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
241 ipo_iterator<T, SetType, true>(V) {}
242 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
243 ipo_iterator<T, SetType, true>(V) {}
244};
245
246template <class T, class SetType>
247ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
248 return ipo_ext_iterator<T, SetType>::begin(G, S);
249}
250
251template <class T, class SetType>
252ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
253 return ipo_ext_iterator<T, SetType>::end(G, S);
254}
255
256template <class T, class SetType>
257iterator_range<ipo_ext_iterator<T, SetType>>
258inverse_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
289template<class GraphT, class GT = GraphTraits<GraphT>>
290class ReversePostOrderTraversal {
291 using NodeRef = typename GT::NodeRef;
292
293 std::vector<NodeRef> Blocks; // Block list in normal PO order
294
295 void Initialize(const GraphT &G) {
296 std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks));
297 }
298
299public:
300 using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
301 using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
302
303 ReversePostOrderTraversal(const GraphT &G) { Initialize(G); }
304
305 // Because we want a reverse post order, use reverse iterators from the vector
306 rpo_iterator begin() { return Blocks.rbegin(); }
307 const_rpo_iterator begin() const { return Blocks.crbegin(); }
308 rpo_iterator end() { return Blocks.rend(); }
309 const_rpo_iterator end() const { return Blocks.crend(); }
310};
311
312} // end namespace llvm
313
314#endif // LLVM_ADT_POSTORDERITERATOR_H
315