1//===- llvm/ADT/PostOrderIterator.h - PostOrder iterator --------*- C++ -*-===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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/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 =
94 SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
95 bool ExtStorage = false, class GT = GraphTraits<GraphT>>
96class po_iterator
97 : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
98 public po_iterator_storage<SetType, ExtStorage> {
99 using super = std::iterator<std::forward_iterator_tag, typename GT::NodeRef>;
100 using NodeRef = typename GT::NodeRef;
101 using ChildItTy = typename GT::ChildIteratorType;
102
103 // VisitStack - Used to maintain the ordering. Top = current block
104 // First element is basic block pointer, second is the 'next child' to visit
105 std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
106
107 po_iterator(NodeRef BB) {
108 this->insertEdge(Optional<NodeRef>(), BB);
109 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
110 traverseChild();
111 }
112
113 po_iterator() = default; // End is when stack is empty.
114
115 po_iterator(NodeRef BB, SetType &S)
116 : po_iterator_storage<SetType, ExtStorage>(S) {
117 if (this->insertEdge(Optional<NodeRef>(), BB)) {
118 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
119 traverseChild();
120 }
121 }
122
123 po_iterator(SetType &S)
124 : po_iterator_storage<SetType, ExtStorage>(S) {
125 } // End is when stack is empty.
126
127 void traverseChild() {
128 while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
129 NodeRef BB = *VisitStack.back().second++;
130 if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
131 // If the block is not visited...
132 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
133 }
134 }
135 }
136
137public:
138 using pointer = typename super::pointer;
139
140 // Provide static "constructors"...
141 static po_iterator begin(GraphT G) {
142 return po_iterator(GT::getEntryNode(G));
143 }
144 static po_iterator end(GraphT G) { return po_iterator(); }
145
146 static po_iterator begin(GraphT G, SetType &S) {
147 return po_iterator(GT::getEntryNode(G), S);
148 }
149 static po_iterator end(GraphT G, SetType &S) { return po_iterator(S); }
150
151 bool operator==(const po_iterator &x) const {
152 return VisitStack == x.VisitStack;
153 }
154 bool operator!=(const po_iterator &x) const { return !(*this == x); }
155
156 const NodeRef &operator*() const { return VisitStack.back().first; }
157
158 // This is a nonstandard operator-> that dereferences the pointer an extra
159 // time... so that you can actually call methods ON the BasicBlock, because
160 // the contained type is a pointer. This allows BBIt->getTerminator() f.e.
161 //
162 NodeRef operator->() const { return **this; }
163
164 po_iterator &operator++() { // Preincrement
165 this->finishPostorder(VisitStack.back().first);
166 VisitStack.pop_back();
167 if (!VisitStack.empty())
168 traverseChild();
169 return *this;
170 }
171
172 po_iterator operator++(int) { // Postincrement
173 po_iterator tmp = *this;
174 ++*this;
175 return tmp;
176 }
177};
178
179// Provide global constructors that automatically figure out correct types...
180//
181template <class T>
182po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
183template <class T>
184po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); }
185
186template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
187 return make_range(po_begin(G), po_end(G));
188}
189
190// Provide global definitions of external postorder iterators...
191template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
192struct po_ext_iterator : public po_iterator<T, SetType, true> {
193 po_ext_iterator(const po_iterator<T, SetType, true> &V) :
194 po_iterator<T, SetType, true>(V) {}
195};
196
197template<class T, class SetType>
198po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
199 return po_ext_iterator<T, SetType>::begin(G, S);
200}
201
202template<class T, class SetType>
203po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
204 return po_ext_iterator<T, SetType>::end(G, S);
205}
206
207template <class T, class SetType>
208iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) {
209 return make_range(po_ext_begin(G, S), po_ext_end(G, S));
210}
211
212// Provide global definitions of inverse post order iterators...
213template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
214 bool External = false>
215struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
216 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
217 po_iterator<Inverse<T>, SetType, External> (V) {}
218};
219
220template <class T>
221ipo_iterator<T> ipo_begin(const T &G) {
222 return ipo_iterator<T>::begin(G);
223}
224
225template <class T>
226ipo_iterator<T> ipo_end(const T &G){
227 return ipo_iterator<T>::end(G);
228}
229
230template <class T>
231iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) {
232 return make_range(ipo_begin(G), ipo_end(G));
233}
234
235// Provide global definitions of external inverse postorder iterators...
236template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
237struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
238 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
239 ipo_iterator<T, SetType, true>(V) {}
240 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
241 ipo_iterator<T, SetType, true>(V) {}
242};
243
244template <class T, class SetType>
245ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
246 return ipo_ext_iterator<T, SetType>::begin(G, S);
247}
248
249template <class T, class SetType>
250ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
251 return ipo_ext_iterator<T, SetType>::end(G, S);
252}
253
254template <class T, class SetType>
255iterator_range<ipo_ext_iterator<T, SetType>>
256inverse_post_order_ext(const T &G, SetType &S) {
257 return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
258}
259
260//===--------------------------------------------------------------------===//
261// Reverse Post Order CFG iterator code
262//===--------------------------------------------------------------------===//
263//
264// This is used to visit basic blocks in a method in reverse post order. This
265// class is awkward to use because I don't know a good incremental algorithm to
266// computer RPO from a graph. Because of this, the construction of the
267// ReversePostOrderTraversal object is expensive (it must walk the entire graph
268// with a postorder iterator to build the data structures). The moral of this
269// story is: Don't create more ReversePostOrderTraversal classes than necessary.
270//
271// Because it does the traversal in its constructor, it won't invalidate when
272// BasicBlocks are removed, *but* it may contain erased blocks. Some places
273// rely on this behavior (i.e. GVN).
274//
275// This class should be used like this:
276// {
277// ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
278// for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
279// ...
280// }
281// for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
282// ...
283// }
284// }
285//
286
287template<class GraphT, class GT = GraphTraits<GraphT>>
288class ReversePostOrderTraversal {
289 using NodeRef = typename GT::NodeRef;
290
291 std::vector<NodeRef> Blocks; // Block list in normal PO order
292
293 void Initialize(NodeRef BB) {
294 std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
295 }
296
297public:
298 using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
299 using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
300
301 ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
302
303 // Because we want a reverse post order, use reverse iterators from the vector
304 rpo_iterator begin() { return Blocks.rbegin(); }
305 const_rpo_iterator begin() const { return Blocks.crbegin(); }
306 rpo_iterator end() { return Blocks.rend(); }
307 const_rpo_iterator end() const { return Blocks.crend(); }
308};
309
310} // end namespace llvm
311
312#endif // LLVM_ADT_POSTORDERITERATOR_H
313