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/iterator_range.h"
22#include <iterator>
23#include <set>
24#include <utility>
25#include <vector>
26
27namespace llvm {
28
29// The po_iterator_storage template provides access to the set of already
30// visited nodes during the po_iterator's depth-first traversal.
31//
32// The default implementation simply contains a set of visited nodes, while
33// the External=true version uses a reference to an external set.
34//
35// It is possible to prune the depth-first traversal in several ways:
36//
37// - When providing an external set that already contains some graph nodes,
38// those nodes won't be visited again. This is useful for restarting a
39// post-order traversal on a graph with nodes that aren't dominated by a
40// single node.
41//
42// - By providing a custom SetType class, unwanted graph nodes can be excluded
43// by having the insert() function return false. This could for example
44// confine a CFG traversal to blocks in a specific loop.
45//
46// - Finally, by specializing the po_iterator_storage template itself, graph
47// edges can be pruned by returning false in the insertEdge() function. This
48// could be used to remove loop back-edges from the CFG seen by po_iterator.
49//
50// A specialized po_iterator_storage class can observe both the pre-order and
51// the post-order. The insertEdge() function is called in a pre-order, while
52// the finishPostorder() function is called just before the po_iterator moves
53// on to the next node.
54
55/// Default po_iterator_storage implementation with an internal set object.
56template<class SetType, bool External>
57class po_iterator_storage {
58 SetType Visited;
59
60public:
61 // Return true if edge destination should be visited.
62 template <typename NodeRef>
63 bool insertEdge(Optional<NodeRef> From, NodeRef To) {
64 return Visited.insert(To).second;
65 }
66
67 // Called after all children of BB have been visited.
68 template <typename NodeRef> void finishPostorder(NodeRef BB) {}
69};
70
71/// Specialization of po_iterator_storage that references an external set.
72template<class SetType>
73class po_iterator_storage<SetType, true> {
74 SetType &Visited;
75
76public:
77 po_iterator_storage(SetType &VSet) : Visited(VSet) {}
78 po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {}
79
80 // Return true if edge destination should be visited, called with From = 0 for
81 // the root node.
82 // Graph edges can be pruned by specializing this function.
83 template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) {
84 return Visited.insert(To).second;
85 }
86
87 // Called after all children of BB have been visited.
88 template <class NodeRef> void finishPostorder(NodeRef BB) {}
89};
90
91template <class GraphT,
92 class SetType =
93 SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
94 bool ExtStorage = false, class GT = GraphTraits<GraphT>>
95class po_iterator
96 : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
97 public po_iterator_storage<SetType, ExtStorage> {
98 using super = std::iterator<std::forward_iterator_tag, typename GT::NodeRef>;
99 using NodeRef = typename GT::NodeRef;
100 using ChildItTy = typename GT::ChildIteratorType;
101
102 // VisitStack - Used to maintain the ordering. Top = current block
103 // First element is basic block pointer, second is the 'next child' to visit
104 std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
105
106 po_iterator(NodeRef BB) {
107 this->insertEdge(Optional<NodeRef>(), BB);
108 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
109 traverseChild();
110 }
111
112 po_iterator() = default; // End is when stack is empty.
113
114 po_iterator(NodeRef BB, SetType &S)
115 : po_iterator_storage<SetType, ExtStorage>(S) {
116 if (this->insertEdge(Optional<NodeRef>(), BB)) {
117 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
118 traverseChild();
119 }
120 }
121
122 po_iterator(SetType &S)
123 : po_iterator_storage<SetType, ExtStorage>(S) {
124 } // End is when stack is empty.
125
126 void traverseChild() {
127 while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
128 NodeRef BB = *VisitStack.back().second++;
129 if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
130 // If the block is not visited...
131 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
132 }
133 }
134 }
135
136public:
137 using pointer = typename super::pointer;
138
139 // Provide static "constructors"...
140 static po_iterator begin(GraphT G) {
141 return po_iterator(GT::getEntryNode(G));
142 }
143 static po_iterator end(GraphT G) { return po_iterator(); }
144
145 static po_iterator begin(GraphT G, SetType &S) {
146 return po_iterator(GT::getEntryNode(G), S);
147 }
148 static po_iterator end(GraphT G, SetType &S) { return po_iterator(S); }
149
150 bool operator==(const po_iterator &x) const {
151 return VisitStack == x.VisitStack;
152 }
153 bool operator!=(const po_iterator &x) const { return !(*this == x); }
154
155 const NodeRef &operator*() const { return VisitStack.back().first; }
156
157 // This is a nonstandard operator-> that dereferences the pointer an extra
158 // time... so that you can actually call methods ON the BasicBlock, because
159 // the contained type is a pointer. This allows BBIt->getTerminator() f.e.
160 //
161 NodeRef operator->() const { return **this; }
162
163 po_iterator &operator++() { // Preincrement
164 this->finishPostorder(VisitStack.back().first);
165 VisitStack.pop_back();
166 if (!VisitStack.empty())
167 traverseChild();
168 return *this;
169 }
170
171 po_iterator operator++(int) { // Postincrement
172 po_iterator tmp = *this;
173 ++*this;
174 return tmp;
175 }
176};
177
178// Provide global constructors that automatically figure out correct types...
179//
180template <class T>
181po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
182template <class T>
183po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
184
185template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
186 return make_range(po_begin(G), po_end(G));
187}
188
189// Provide global definitions of external postorder iterators...
190template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
191struct po_ext_iterator : public po_iterator<T, SetType, true> {
192 po_ext_iterator(const po_iterator<T, SetType, true> &V) :
193 po_iterator<T, SetType, true>(V) {}
194};
195
196template<class T, class SetType>
197po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
198 return po_ext_iterator<T, SetType>::begin(G, S);
199}
200
201template<class T, class SetType>
202po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
203 return po_ext_iterator<T, SetType>::end(G, S);
204}
205
206template <class T, class SetType>
207iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) {
208 return make_range(po_ext_begin(G, S), po_ext_end(G, S));
209}
210
211// Provide global definitions of inverse post order iterators...
212template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
213 bool External = false>
214struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
215 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
216 po_iterator<Inverse<T>, SetType, External> (V) {}
217};
218
219template <class T>
220ipo_iterator<T> ipo_begin(const T &G) {
221 return ipo_iterator<T>::begin(G);
222}
223
224template <class T>
225ipo_iterator<T> ipo_end(const T &G){
226 return ipo_iterator<T>::end(G);
227}
228
229template <class T>
230iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) {
231 return make_range(ipo_begin(G), ipo_end(G));
232}
233
234// Provide global definitions of external inverse postorder iterators...
235template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
236struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
237 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
238 ipo_iterator<T, SetType, true>(V) {}
239 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
240 ipo_iterator<T, SetType, true>(V) {}
241};
242
243template <class T, class SetType>
244ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
245 return ipo_ext_iterator<T, SetType>::begin(G, S);
246}
247
248template <class T, class SetType>
249ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
250 return ipo_ext_iterator<T, SetType>::end(G, S);
251}
252
253template <class T, class SetType>
254iterator_range<ipo_ext_iterator<T, SetType>>
255inverse_post_order_ext(const T &G, SetType &S) {
256 return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
257}
258
259//===--------------------------------------------------------------------===//
260// Reverse Post Order CFG iterator code
261//===--------------------------------------------------------------------===//
262//
263// This is used to visit basic blocks in a method in reverse post order. This
264// class is awkward to use because I don't know a good incremental algorithm to
265// computer RPO from a graph. Because of this, the construction of the
266// ReversePostOrderTraversal object is expensive (it must walk the entire graph
267// with a postorder iterator to build the data structures). The moral of this
268// story is: Don't create more ReversePostOrderTraversal classes than necessary.
269//
270// Because it does the traversal in its constructor, it won't invalidate when
271// BasicBlocks are removed, *but* it may contain erased blocks. Some places
272// rely on this behavior (i.e. GVN).
273//
274// This class should be used like this:
275// {
276// ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
277// for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
278// ...
279// }
280// for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
281// ...
282// }
283// }
284//
285
286template<class GraphT, class GT = GraphTraits<GraphT>>
287class ReversePostOrderTraversal {
288 using NodeRef = typename GT::NodeRef;
289
290 std::vector<NodeRef> Blocks; // Block list in normal PO order
291
292 void Initialize(NodeRef BB) {
293 std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
294 }
295
296public:
297 using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
298 using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
299
300 ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
301
302 // Because we want a reverse post order, use reverse iterators from the vector
303 rpo_iterator begin() { return Blocks.rbegin(); }
304 const_rpo_iterator begin() const { return Blocks.crbegin(); }
305 rpo_iterator end() { return Blocks.rend(); }
306 const_rpo_iterator end() const { return Blocks.crend(); }
307};
308
309} // end namespace llvm
310
311#endif // LLVM_ADT_POSTORDERITERATOR_H
312