SCCIterator.h revision 360784
1//===- ADT/SCCIterator.h - Strongly Connected Comp. Iter. -------*- 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/// \file 9/// 10/// This builds on the llvm/ADT/GraphTraits.h file to find the strongly 11/// connected components (SCCs) of a graph in O(N+E) time using Tarjan's DFS 12/// algorithm. 13/// 14/// The SCC iterator has the important property that if a node in SCC S1 has an 15/// edge to a node in SCC S2, then it visits S1 *after* S2. 16/// 17/// To visit S1 *before* S2, use the scc_iterator on the Inverse graph. (NOTE: 18/// This requires some simple wrappers and is not supported yet.) 19/// 20//===----------------------------------------------------------------------===// 21 22#ifndef LLVM_ADT_SCCITERATOR_H 23#define LLVM_ADT_SCCITERATOR_H 24 25#include "llvm/ADT/DenseMap.h" 26#include "llvm/ADT/GraphTraits.h" 27#include "llvm/ADT/iterator.h" 28#include <cassert> 29#include <cstddef> 30#include <iterator> 31#include <vector> 32 33namespace llvm { 34 35/// Enumerate the SCCs of a directed graph in reverse topological order 36/// of the SCC DAG. 37/// 38/// This is implemented using Tarjan's DFS algorithm using an internal stack to 39/// build up a vector of nodes in a particular SCC. Note that it is a forward 40/// iterator and thus you cannot backtrack or re-visit nodes. 41template <class GraphT, class GT = GraphTraits<GraphT>> 42class scc_iterator : public iterator_facade_base< 43 scc_iterator<GraphT, GT>, std::forward_iterator_tag, 44 const std::vector<typename GT::NodeRef>, ptrdiff_t> { 45 using NodeRef = typename GT::NodeRef; 46 using ChildItTy = typename GT::ChildIteratorType; 47 using SccTy = std::vector<NodeRef>; 48 using reference = typename scc_iterator::reference; 49 50 /// Element of VisitStack during DFS. 51 struct StackElement { 52 NodeRef Node; ///< The current node pointer. 53 ChildItTy NextChild; ///< The next child, modified inplace during DFS. 54 unsigned MinVisited; ///< Minimum uplink value of all children of Node. 55 56 StackElement(NodeRef Node, const ChildItTy &Child, unsigned Min) 57 : Node(Node), NextChild(Child), MinVisited(Min) {} 58 59 bool operator==(const StackElement &Other) const { 60 return Node == Other.Node && 61 NextChild == Other.NextChild && 62 MinVisited == Other.MinVisited; 63 } 64 }; 65 66 /// The visit counters used to detect when a complete SCC is on the stack. 67 /// visitNum is the global counter. 68 /// 69 /// nodeVisitNumbers are per-node visit numbers, also used as DFS flags. 70 unsigned visitNum; 71 DenseMap<NodeRef, unsigned> nodeVisitNumbers; 72 73 /// Stack holding nodes of the SCC. 74 std::vector<NodeRef> SCCNodeStack; 75 76 /// The current SCC, retrieved using operator*(). 77 SccTy CurrentSCC; 78 79 /// DFS stack, Used to maintain the ordering. The top contains the current 80 /// node, the next child to visit, and the minimum uplink value of all child 81 std::vector<StackElement> VisitStack; 82 83 /// A single "visit" within the non-recursive DFS traversal. 84 void DFSVisitOne(NodeRef N); 85 86 /// The stack-based DFS traversal; defined below. 87 void DFSVisitChildren(); 88 89 /// Compute the next SCC using the DFS traversal. 90 void GetNextSCC(); 91 92 scc_iterator(NodeRef entryN) : visitNum(0) { 93 DFSVisitOne(entryN); 94 GetNextSCC(); 95 } 96 97 /// End is when the DFS stack is empty. 98 scc_iterator() = default; 99 100public: 101 static scc_iterator begin(const GraphT &G) { 102 return scc_iterator(GT::getEntryNode(G)); 103 } 104 static scc_iterator end(const GraphT &) { return scc_iterator(); } 105 106 /// Direct loop termination test which is more efficient than 107 /// comparison with \c end(). 108 bool isAtEnd() const { 109 assert(!CurrentSCC.empty() || VisitStack.empty()); 110 return CurrentSCC.empty(); 111 } 112 113 bool operator==(const scc_iterator &x) const { 114 return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC; 115 } 116 117 scc_iterator &operator++() { 118 GetNextSCC(); 119 return *this; 120 } 121 122 reference operator*() const { 123 assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!"); 124 return CurrentSCC; 125 } 126 127 /// Test if the current SCC has a loop. 128 /// 129 /// If the SCC has more than one node, this is trivially true. If not, it may 130 /// still contain a loop if the node has an edge back to itself. 131 bool hasLoop() const; 132 133 /// This informs the \c scc_iterator that the specified \c Old node 134 /// has been deleted, and \c New is to be used in its place. 135 void ReplaceNode(NodeRef Old, NodeRef New) { 136 assert(nodeVisitNumbers.count(Old) && "Old not in scc_iterator?"); 137 // Do the assignment in two steps, in case 'New' is not yet in the map, and 138 // inserting it causes the map to grow. 139 auto tempVal = nodeVisitNumbers[Old]; 140 nodeVisitNumbers[New] = tempVal; 141 nodeVisitNumbers.erase(Old); 142 } 143}; 144 145template <class GraphT, class GT> 146void scc_iterator<GraphT, GT>::DFSVisitOne(NodeRef N) { 147 ++visitNum; 148 nodeVisitNumbers[N] = visitNum; 149 SCCNodeStack.push_back(N); 150 VisitStack.push_back(StackElement(N, GT::child_begin(N), visitNum)); 151#if 0 // Enable if needed when debugging. 152 dbgs() << "TarjanSCC: Node " << N << 153 " : visitNum = " << visitNum << "\n"; 154#endif 155} 156 157template <class GraphT, class GT> 158void scc_iterator<GraphT, GT>::DFSVisitChildren() { 159 assert(!VisitStack.empty()); 160 while (VisitStack.back().NextChild != GT::child_end(VisitStack.back().Node)) { 161 // TOS has at least one more child so continue DFS 162 NodeRef childN = *VisitStack.back().NextChild++; 163 typename DenseMap<NodeRef, unsigned>::iterator Visited = 164 nodeVisitNumbers.find(childN); 165 if (Visited == nodeVisitNumbers.end()) { 166 // this node has never been seen. 167 DFSVisitOne(childN); 168 continue; 169 } 170 171 unsigned childNum = Visited->second; 172 if (VisitStack.back().MinVisited > childNum) 173 VisitStack.back().MinVisited = childNum; 174 } 175} 176 177template <class GraphT, class GT> void scc_iterator<GraphT, GT>::GetNextSCC() { 178 CurrentSCC.clear(); // Prepare to compute the next SCC 179 while (!VisitStack.empty()) { 180 DFSVisitChildren(); 181 182 // Pop the leaf on top of the VisitStack. 183 NodeRef visitingN = VisitStack.back().Node; 184 unsigned minVisitNum = VisitStack.back().MinVisited; 185 assert(VisitStack.back().NextChild == GT::child_end(visitingN)); 186 VisitStack.pop_back(); 187 188 // Propagate MinVisitNum to parent so we can detect the SCC starting node. 189 if (!VisitStack.empty() && VisitStack.back().MinVisited > minVisitNum) 190 VisitStack.back().MinVisited = minVisitNum; 191 192#if 0 // Enable if needed when debugging. 193 dbgs() << "TarjanSCC: Popped node " << visitingN << 194 " : minVisitNum = " << minVisitNum << "; Node visit num = " << 195 nodeVisitNumbers[visitingN] << "\n"; 196#endif 197 198 if (minVisitNum != nodeVisitNumbers[visitingN]) 199 continue; 200 201 // A full SCC is on the SCCNodeStack! It includes all nodes below 202 // visitingN on the stack. Copy those nodes to CurrentSCC, 203 // reset their minVisit values, and return (this suspends 204 // the DFS traversal till the next ++). 205 do { 206 CurrentSCC.push_back(SCCNodeStack.back()); 207 SCCNodeStack.pop_back(); 208 nodeVisitNumbers[CurrentSCC.back()] = ~0U; 209 } while (CurrentSCC.back() != visitingN); 210 return; 211 } 212} 213 214template <class GraphT, class GT> 215bool scc_iterator<GraphT, GT>::hasLoop() const { 216 assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!"); 217 if (CurrentSCC.size() > 1) 218 return true; 219 NodeRef N = CurrentSCC.front(); 220 for (ChildItTy CI = GT::child_begin(N), CE = GT::child_end(N); CI != CE; 221 ++CI) 222 if (*CI == N) 223 return true; 224 return false; 225 } 226 227/// Construct the begin iterator for a deduced graph type T. 228template <class T> scc_iterator<T> scc_begin(const T &G) { 229 return scc_iterator<T>::begin(G); 230} 231 232/// Construct the end iterator for a deduced graph type T. 233template <class T> scc_iterator<T> scc_end(const T &G) { 234 return scc_iterator<T>::end(G); 235} 236 237} // end namespace llvm 238 239#endif // LLVM_ADT_SCCITERATOR_H 240