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 28 namespace 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. 57 template<class SetType, bool External> 58 class po_iterator_storage { 59 SetType Visited; 60 61 public: 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. 73 template<class SetType> 74 class po_iterator_storage<SetType, true> { 75 SetType &Visited; 76 77 public: 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 92 template <class GraphT, 93 class SetType = 94 SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>, 95 bool ExtStorage = false, class GT = GraphTraits<GraphT>> 96 class 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 } 136 137 public: 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 // 181 template <class T> 182 po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); } 183 template <class T> 184 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); } 185 186 template <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... 191 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 192 struct 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 }; 196 197 template<class T, class SetType> 198 po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) { 199 return po_ext_iterator<T, SetType>::begin(G, S); 200 } 201 202 template<class T, class SetType> 203 po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) { 204 return po_ext_iterator<T, SetType>::end(G, S); 205 } 206 207 template <class T, class SetType> 208 iterator_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... 213 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>, 214 bool External = false> 215 struct 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 }; 219 220 template <class T> 221 ipo_iterator<T> ipo_begin(const T &G) { 222 return ipo_iterator<T>::begin(G); 223 } 224 225 template <class T> 226 ipo_iterator<T> ipo_end(const T &G){ 227 return ipo_iterator<T>::end(G); 228 } 229 230 template <class T> 231 iterator_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... 236 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 237 struct 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 }; 243 244 template <class T, class SetType> 245 ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) { 246 return ipo_ext_iterator<T, SetType>::begin(G, S); 247 } 248 249 template <class T, class SetType> 250 ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) { 251 return ipo_ext_iterator<T, SetType>::end(G, S); 252 } 253 254 template <class T, class SetType> 255 iterator_range<ipo_ext_iterator<T, SetType>> 256 inverse_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 // 286 287 template<class GraphT, class GT = GraphTraits<GraphT>> 288 class 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 } 296 297 public: 298 using rpo_iterator = typename std::vector<NodeRef>::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 rpo_iterator end() { return Blocks.rend(); } 305 }; 306 307 } // end namespace llvm 308 309 #endif // LLVM_ADT_POSTORDERITERATOR_H 310