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
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(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.
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>
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
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 = const value_type &;
103
104private:
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
146public:
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//
188template <class T>
189po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
190template <class T>
191po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
192
193template <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...
198template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
199struct 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
204template<class T, class SetType>
205po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
206 return po_ext_iterator<T, SetType>::begin(G, S);
207}
208
209template<class T, class SetType>
210po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
211 return po_ext_iterator<T, SetType>::end(G, S);
212}
213
214template <class T, class SetType>
215iterator_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...
220template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
221 bool External = false>
222struct 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
227template <class T>
228ipo_iterator<T> ipo_begin(const T &G) {
229 return ipo_iterator<T>::begin(G);
230}
231
232template <class T>
233ipo_iterator<T> ipo_end(const T &G){
234 return ipo_iterator<T>::end(G);
235}
236
237template <class T>
238iterator_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...
243template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
244struct 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
251template <class T, class SetType>
252ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
253 return ipo_ext_iterator<T, SetType>::begin(G, S);
254}
255
256template <class T, class SetType>
257ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
258 return ipo_ext_iterator<T, SetType>::end(G, S);
259}
260
261template <class T, class SetType>
262iterator_range<ipo_ext_iterator<T, SetType>>
263inverse_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
294template<class GraphT, class GT = GraphTraits<GraphT>>
295class 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
305public:
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

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