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