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 <type_traits> 27 #include <utility> 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(std::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> 86 bool insertEdge(std::optional<NodeRef> From, NodeRef To) { 87 return Visited.insert(To).second; 88 } 89 90 // Called after all children of BB have been visited. 91 template <class NodeRef> void finishPostorder(NodeRef BB) {} 92 }; 93 94 template <class GraphT, 95 class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>, 96 bool ExtStorage = false, class GT = GraphTraits<GraphT>> 97 class po_iterator : public po_iterator_storage<SetType, ExtStorage> { 98 public: 99 // When External storage is used we are not multi-pass safe. 100 using iterator_category = 101 std::conditional_t<ExtStorage, std::input_iterator_tag, 102 std::forward_iterator_tag>; 103 using value_type = typename GT::NodeRef; 104 using difference_type = std::ptrdiff_t; 105 using pointer = value_type *; 106 using reference = const value_type &; 107 108 private: 109 using NodeRef = typename GT::NodeRef; 110 using ChildItTy = typename GT::ChildIteratorType; 111 112 /// Used to maintain the ordering. 113 /// First element is basic block pointer, second is iterator for the next 114 /// child to visit, third is the end iterator. 115 SmallVector<std::tuple<NodeRef, ChildItTy, ChildItTy>, 8> VisitStack; 116 117 po_iterator(NodeRef BB) { 118 this->insertEdge(std::optional<NodeRef>(), BB); 119 VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); 120 traverseChild(); 121 } 122 123 po_iterator() = default; // End is when stack is empty. 124 125 po_iterator(NodeRef BB, SetType &S) 126 : po_iterator_storage<SetType, ExtStorage>(S) { 127 if (this->insertEdge(std::optional<NodeRef>(), BB)) { 128 VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); 129 traverseChild(); 130 } 131 } 132 133 po_iterator(SetType &S) 134 : po_iterator_storage<SetType, ExtStorage>(S) { 135 } // End is when stack is empty. 136 137 void traverseChild() { 138 while (true) { 139 auto &Entry = VisitStack.back(); 140 if (std::get<1>(Entry) == std::get<2>(Entry)) 141 break; 142 NodeRef BB = *std::get<1>(Entry)++; 143 if (this->insertEdge(std::optional<NodeRef>(std::get<0>(Entry)), BB)) { 144 // If the block is not visited... 145 VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB)); 146 } 147 } 148 } 149 150 public: 151 // Provide static "constructors"... 152 static po_iterator begin(const GraphT &G) { 153 return po_iterator(GT::getEntryNode(G)); 154 } 155 static po_iterator end(const GraphT &G) { return po_iterator(); } 156 157 static po_iterator begin(const GraphT &G, SetType &S) { 158 return po_iterator(GT::getEntryNode(G), S); 159 } 160 static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); } 161 162 bool operator==(const po_iterator &x) const { 163 return VisitStack == x.VisitStack; 164 } 165 bool operator!=(const po_iterator &x) const { return !(*this == x); } 166 167 reference operator*() const { return std::get<0>(VisitStack.back()); } 168 169 // This is a nonstandard operator-> that dereferences the pointer an extra 170 // time... so that you can actually call methods ON the BasicBlock, because 171 // the contained type is a pointer. This allows BBIt->getTerminator() f.e. 172 // 173 NodeRef operator->() const { return **this; } 174 175 po_iterator &operator++() { // Preincrement 176 this->finishPostorder(std::get<0>(VisitStack.back())); 177 VisitStack.pop_back(); 178 if (!VisitStack.empty()) 179 traverseChild(); 180 return *this; 181 } 182 183 po_iterator operator++(int) { // Postincrement 184 po_iterator tmp = *this; 185 ++*this; 186 return tmp; 187 } 188 }; 189 190 // Provide global constructors that automatically figure out correct types... 191 // 192 template <class T> 193 po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); } 194 template <class T> 195 po_iterator<T> po_end (const T &G) { return po_iterator<T>::end(G); } 196 197 template <class T> iterator_range<po_iterator<T>> post_order(const T &G) { 198 return make_range(po_begin(G), po_end(G)); 199 } 200 201 // Provide global definitions of external postorder iterators... 202 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 203 struct po_ext_iterator : public po_iterator<T, SetType, true> { 204 po_ext_iterator(const po_iterator<T, SetType, true> &V) : 205 po_iterator<T, SetType, true>(V) {} 206 }; 207 208 template<class T, class SetType> 209 po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) { 210 return po_ext_iterator<T, SetType>::begin(G, S); 211 } 212 213 template<class T, class SetType> 214 po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) { 215 return po_ext_iterator<T, SetType>::end(G, S); 216 } 217 218 template <class T, class SetType> 219 iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) { 220 return make_range(po_ext_begin(G, S), po_ext_end(G, S)); 221 } 222 223 // Provide global definitions of inverse post order iterators... 224 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>, 225 bool External = false> 226 struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> { 227 ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) : 228 po_iterator<Inverse<T>, SetType, External> (V) {} 229 }; 230 231 template <class T> 232 ipo_iterator<T> ipo_begin(const T &G) { 233 return ipo_iterator<T>::begin(G); 234 } 235 236 template <class T> 237 ipo_iterator<T> ipo_end(const T &G){ 238 return ipo_iterator<T>::end(G); 239 } 240 241 template <class T> 242 iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) { 243 return make_range(ipo_begin(G), ipo_end(G)); 244 } 245 246 // Provide global definitions of external inverse postorder iterators... 247 template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>> 248 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> { 249 ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) : 250 ipo_iterator<T, SetType, true>(V) {} 251 ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) : 252 ipo_iterator<T, SetType, true>(V) {} 253 }; 254 255 template <class T, class SetType> 256 ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) { 257 return ipo_ext_iterator<T, SetType>::begin(G, S); 258 } 259 260 template <class T, class SetType> 261 ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) { 262 return ipo_ext_iterator<T, SetType>::end(G, S); 263 } 264 265 template <class T, class SetType> 266 iterator_range<ipo_ext_iterator<T, SetType>> 267 inverse_post_order_ext(const T &G, SetType &S) { 268 return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S)); 269 } 270 271 //===--------------------------------------------------------------------===// 272 // Reverse Post Order CFG iterator code 273 //===--------------------------------------------------------------------===// 274 // 275 // This is used to visit basic blocks in a method in reverse post order. This 276 // class is awkward to use because I don't know a good incremental algorithm to 277 // computer RPO from a graph. Because of this, the construction of the 278 // ReversePostOrderTraversal object is expensive (it must walk the entire graph 279 // with a postorder iterator to build the data structures). The moral of this 280 // story is: Don't create more ReversePostOrderTraversal classes than necessary. 281 // 282 // Because it does the traversal in its constructor, it won't invalidate when 283 // BasicBlocks are removed, *but* it may contain erased blocks. Some places 284 // rely on this behavior (i.e. GVN). 285 // 286 // This class should be used like this: 287 // { 288 // ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create 289 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { 290 // ... 291 // } 292 // for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) { 293 // ... 294 // } 295 // } 296 // 297 298 template<class GraphT, class GT = GraphTraits<GraphT>> 299 class ReversePostOrderTraversal { 300 using NodeRef = typename GT::NodeRef; 301 302 using VecTy = SmallVector<NodeRef, 8>; 303 VecTy Blocks; // Block list in normal PO order 304 305 void Initialize(const GraphT &G) { 306 std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks)); 307 } 308 309 public: 310 using rpo_iterator = typename VecTy::reverse_iterator; 311 using const_rpo_iterator = typename VecTy::const_reverse_iterator; 312 313 ReversePostOrderTraversal(const GraphT &G) { Initialize(G); } 314 315 // Because we want a reverse post order, use reverse iterators from the vector 316 rpo_iterator begin() { return Blocks.rbegin(); } 317 const_rpo_iterator begin() const { return Blocks.rbegin(); } 318 rpo_iterator end() { return Blocks.rend(); } 319 const_rpo_iterator end() const { return Blocks.rend(); } 320 }; 321 322 } // end namespace llvm 323 324 #endif // LLVM_ADT_POSTORDERITERATOR_H 325