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/Optional.h" 21 #include "llvm/ADT/SmallPtrSet.h" 22 #include "llvm/ADT/SmallVector.h" 23 #include "llvm/ADT/iterator_range.h" 24 #include <iterator> 25 #include <set> 26 #include <utility> 27 #include <vector> 28 29 namespace llvm { 30 31 // The po_iterator_storage template provides access to the set of already 32 // visited nodes during the po_iterator's depth-first traversal. 33 // 34 // The default implementation simply contains a set of visited nodes, while 35 // the External=true version uses a reference to an external set. 36 // 37 // It is possible to prune the depth-first traversal in several ways: 38 // 39 // - When providing an external set that already contains some graph nodes, 40 // those nodes won't be visited again. This is useful for restarting a 41 // post-order traversal on a graph with nodes that aren't dominated by a 42 // single node. 43 // 44 // - By providing a custom SetType class, unwanted graph nodes can be excluded 45 // by having the insert() function return false. This could for example 46 // confine a CFG traversal to blocks in a specific loop. 47 // 48 // - Finally, by specializing the po_iterator_storage template itself, graph 49 // edges can be pruned by returning false in the insertEdge() function. This 50 // could be used to remove loop back-edges from the CFG seen by po_iterator. 51 // 52 // A specialized po_iterator_storage class can observe both the pre-order and 53 // the post-order. The insertEdge() function is called in a pre-order, while 54 // the finishPostorder() function is called just before the po_iterator moves 55 // on to the next node. 56 57 /// Default po_iterator_storage implementation with an internal set object. 58 template<class SetType, bool External> 59 class po_iterator_storage { 60 SetType Visited; 61 62 public: 63 // Return true if edge destination should be visited. 64 template <typename NodeRef> 65 bool insertEdge(Optional<NodeRef> From, NodeRef To) { 66 return Visited.insert(To).second; 67 } 68 69 // Called after all children of BB have been visited. 70 template <typename NodeRef> void finishPostorder(NodeRef BB) {} 71 }; 72 73 /// Specialization of po_iterator_storage that references an external set. 74 template<class SetType> 75 class po_iterator_storage<SetType, true> { 76 SetType &Visited; 77 78 public: 79 po_iterator_storage(SetType &VSet) : Visited(VSet) {} 80 po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {} 81 82 // Return true if edge destination should be visited, called with From = 0 for 83 // the root node. 84 // Graph edges can be pruned by specializing this function. 85 template <class NodeRef> bool insertEdge(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 93 template <class GraphT, 94 class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>, 95 bool ExtStorage = false, class GT = GraphTraits<GraphT>> 96 class po_iterator : public po_iterator_storage<SetType, ExtStorage> { 97 public: 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 = value_type &; 103 104 private: 105 using NodeRef = typename GT::NodeRef; 106 using ChildItTy = typename GT::ChildIteratorType; 107 108 // VisitStack - Used to maintain the ordering. Top = current block 109 // First element is basic block pointer, second is the 'next child' to visit 110 SmallVector<std::pair<NodeRef, ChildItTy>, 8> VisitStack; 111 112 po_iterator(NodeRef BB) { 113 this->insertEdge(Optional<NodeRef>(), BB); 114 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); 115 traverseChild(); 116 } 117 118 po_iterator() = default; // End is when stack is empty. 119 120 po_iterator(NodeRef BB, SetType &S) 121 : po_iterator_storage<SetType, ExtStorage>(S) { 122 if (this->insertEdge(Optional<NodeRef>(), BB)) { 123 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); 124 traverseChild(); 125 } 126 } 127 128 po_iterator(SetType &S) 129 : po_iterator_storage<SetType, ExtStorage>(S) { 130 } // End is when stack is empty. 131 132 void traverseChild() { 133 while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) { 134 NodeRef BB = *VisitStack.back().second++; 135 if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) { 136 // If the block is not visited... 137 VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB))); 138 } 139 } 140 } 141 142 public: 143 // Provide static "constructors"... 144 static po_iterator begin(const GraphT &G) { 145 return po_iterator(GT::getEntryNode(G)); 146 } 147 static po_iterator end(const GraphT &G) { return po_iterator(); } 148 149 static po_iterator begin(const GraphT &G, SetType &S) { 150 return po_iterator(GT::getEntryNode(G), S); 151 } 152 static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); } 153 154 bool operator==(const po_iterator &x) const { 155 return VisitStack == x.VisitStack; 156 } 157 bool operator!=(const po_iterator &x) const { return !(*this == x); } 158 159 const NodeRef &operator*() const { return VisitStack.back().first; } 160 161 // This is a nonstandard operator-> that dereferences the pointer an extra 162 // time... so that you can actually call methods ON the BasicBlock, because 163 // the contained type is a pointer. This allows BBIt->getTerminator() f.e. 164 // 165 NodeRef operator->() const { return **this; } 166 167 po_iterator &operator++() { // Preincrement 168 this->finishPostorder(VisitStack.back().first); 169 VisitStack.pop_back(); 170 if (!VisitStack.empty()) 171 traverseChild(); 172 return *this; 173 } 174 175 po_iterator operator++(int) { // Postincrement 176 po_iterator tmp = *this; 177 ++*this; 178 return tmp; 179 } 180 }; 181 182 // Provide global constructors that automatically figure out correct types... 183 // 184 template <class T> 185 po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); } 186 template <class T> 187 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); } 188 189 template <class T> iterator_range<po_iterator<T>> post_order(const T &G) { 190 return make_range(po_begin(G), po_end(G)); 191 } 192 193 // Provide global definitions of external postorder iterators... 194 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 195 struct po_ext_iterator : public po_iterator<T, SetType, true> { 196 po_ext_iterator(const po_iterator<T, SetType, true> &V) : 197 po_iterator<T, SetType, true>(V) {} 198 }; 199 200 template<class T, class SetType> 201 po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) { 202 return po_ext_iterator<T, SetType>::begin(G, S); 203 } 204 205 template<class T, class SetType> 206 po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) { 207 return po_ext_iterator<T, SetType>::end(G, S); 208 } 209 210 template <class T, class SetType> 211 iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) { 212 return make_range(po_ext_begin(G, S), po_ext_end(G, S)); 213 } 214 215 // Provide global definitions of inverse post order iterators... 216 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>, 217 bool External = false> 218 struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> { 219 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) : 220 po_iterator<Inverse<T>, SetType, External> (V) {} 221 }; 222 223 template <class T> 224 ipo_iterator<T> ipo_begin(const T &G) { 225 return ipo_iterator<T>::begin(G); 226 } 227 228 template <class T> 229 ipo_iterator<T> ipo_end(const T &G){ 230 return ipo_iterator<T>::end(G); 231 } 232 233 template <class T> 234 iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) { 235 return make_range(ipo_begin(G), ipo_end(G)); 236 } 237 238 // Provide global definitions of external inverse postorder iterators... 239 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 240 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> { 241 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) : 242 ipo_iterator<T, SetType, true>(V) {} 243 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) : 244 ipo_iterator<T, SetType, true>(V) {} 245 }; 246 247 template <class T, class SetType> 248 ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) { 249 return ipo_ext_iterator<T, SetType>::begin(G, S); 250 } 251 252 template <class T, class SetType> 253 ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) { 254 return ipo_ext_iterator<T, SetType>::end(G, S); 255 } 256 257 template <class T, class SetType> 258 iterator_range<ipo_ext_iterator<T, SetType>> 259 inverse_post_order_ext(const T &G, SetType &S) { 260 return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S)); 261 } 262 263 //===--------------------------------------------------------------------===// 264 // Reverse Post Order CFG iterator code 265 //===--------------------------------------------------------------------===// 266 // 267 // This is used to visit basic blocks in a method in reverse post order. This 268 // class is awkward to use because I don't know a good incremental algorithm to 269 // computer RPO from a graph. Because of this, the construction of the 270 // ReversePostOrderTraversal object is expensive (it must walk the entire graph 271 // with a postorder iterator to build the data structures). The moral of this 272 // story is: Don't create more ReversePostOrderTraversal classes than necessary. 273 // 274 // Because it does the traversal in its constructor, it won't invalidate when 275 // BasicBlocks are removed, *but* it may contain erased blocks. Some places 276 // rely on this behavior (i.e. GVN). 277 // 278 // This class should be used like this: 279 // { 280 // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create 281 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { 282 // ... 283 // } 284 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { 285 // ... 286 // } 287 // } 288 // 289 290 template<class GraphT, class GT = GraphTraits<GraphT>> 291 class ReversePostOrderTraversal { 292 using NodeRef = typename GT::NodeRef; 293 294 std::vector<NodeRef> Blocks; // Block list in normal PO order 295 296 void Initialize(const GraphT &G) { 297 std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks)); 298 } 299 300 public: 301 using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator; 302 using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator; 303 304 ReversePostOrderTraversal(const GraphT &G) { Initialize(G); } 305 306 // Because we want a reverse post order, use reverse iterators from the vector 307 rpo_iterator begin() { return Blocks.rbegin(); } 308 const_rpo_iterator begin() const { return Blocks.crbegin(); } 309 rpo_iterator end() { return Blocks.rend(); } 310 const_rpo_iterator end() const { return Blocks.crend(); } 311 }; 312 313 } // end namespace llvm 314 315 #endif // LLVM_ADT_POSTORDERITERATOR_H 316