1 //===---- ADT/SCCIterator.h - Strongly Connected Comp. Iter. ----*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This builds on the llvm/ADT/GraphTraits.h file to find the strongly connected
11 // components (SCCs) of a graph in O(N+E) time using Tarjan's DFS algorithm.
13 // The SCC iterator has the important property that if a node in SCC S1 has an
14 // edge to a node in SCC S2, then it visits S1 *after* S2.
16 // To visit S1 *before* S2, use the scc_iterator on the Inverse graph.
17 // (NOTE: This requires some simple wrappers and is not supported yet.)
19 //===----------------------------------------------------------------------===//
21 #ifndef LLVM_ADT_SCCITERATOR_H
22 #define LLVM_ADT_SCCITERATOR_H
24 #include "llvm/ADT/DenseMap.h"
25 #include "llvm/ADT/GraphTraits.h"
30 //===----------------------------------------------------------------------===//
32 /// scc_iterator - Enumerate the SCCs of a directed graph, in
33 /// reverse topological order of the SCC DAG.
35 template<class GraphT, class GT = GraphTraits<GraphT> >
37 : public std::iterator<std::forward_iterator_tag,
38 std::vector<typename GT::NodeType>, ptrdiff_t> {
39 typedef typename GT::NodeType NodeType;
40 typedef typename GT::ChildIteratorType ChildItTy;
41 typedef std::vector<NodeType*> SccTy;
42 typedef std::iterator<std::forward_iterator_tag,
43 std::vector<typename GT::NodeType>, ptrdiff_t> super;
44 typedef typename super::reference reference;
45 typedef typename super::pointer pointer;
47 // The visit counters used to detect when a complete SCC is on the stack.
48 // visitNum is the global counter.
49 // nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
51 DenseMap<NodeType *, unsigned> nodeVisitNumbers;
53 // SCCNodeStack - Stack holding nodes of the SCC.
54 std::vector<NodeType *> SCCNodeStack;
56 // CurrentSCC - The current SCC, retrieved using operator*().
59 // VisitStack - Used to maintain the ordering. Top = current block
60 // First element is basic block pointer, second is the 'next child' to visit
61 std::vector<std::pair<NodeType *, ChildItTy> > VisitStack;
63 // MinVisitNumStack - Stack holding the "min" values for each node in the DFS.
64 // This is used to track the minimum uplink values for all children of
65 // the corresponding node on the VisitStack.
66 std::vector<unsigned> MinVisitNumStack;
68 // A single "visit" within the non-recursive DFS traversal.
69 void DFSVisitOne(NodeType *N) {
70 ++visitNum; // Global counter for the visit order
71 nodeVisitNumbers[N] = visitNum;
72 SCCNodeStack.push_back(N);
73 MinVisitNumStack.push_back(visitNum);
74 VisitStack.push_back(std::make_pair(N, GT::child_begin(N)));
75 //dbgs() << "TarjanSCC: Node " << N <<
76 // " : visitNum = " << visitNum << "\n";
79 // The stack-based DFS traversal; defined below.
80 void DFSVisitChildren() {
81 assert(!VisitStack.empty());
82 while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
83 // TOS has at least one more child so continue DFS
84 NodeType *childN = *VisitStack.back().second++;
85 if (!nodeVisitNumbers.count(childN)) {
86 // this node has never been seen.
91 unsigned childNum = nodeVisitNumbers[childN];
92 if (MinVisitNumStack.back() > childNum)
93 MinVisitNumStack.back() = childNum;
97 // Compute the next SCC using the DFS traversal.
99 assert(VisitStack.size() == MinVisitNumStack.size());
100 CurrentSCC.clear(); // Prepare to compute the next SCC
101 while (!VisitStack.empty()) {
103 assert(VisitStack.back().second ==GT::child_end(VisitStack.back().first));
104 NodeType *visitingN = VisitStack.back().first;
105 unsigned minVisitNum = MinVisitNumStack.back();
106 VisitStack.pop_back();
107 MinVisitNumStack.pop_back();
108 if (!MinVisitNumStack.empty() && MinVisitNumStack.back() > minVisitNum)
109 MinVisitNumStack.back() = minVisitNum;
111 //dbgs() << "TarjanSCC: Popped node " << visitingN <<
112 // " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
113 // nodeVisitNumbers[visitingN] << "\n";
115 if (minVisitNum != nodeVisitNumbers[visitingN])
118 // A full SCC is on the SCCNodeStack! It includes all nodes below
119 // visitingN on the stack. Copy those nodes to CurrentSCC,
120 // reset their minVisit values, and return (this suspends
121 // the DFS traversal till the next ++).
123 CurrentSCC.push_back(SCCNodeStack.back());
124 SCCNodeStack.pop_back();
125 nodeVisitNumbers[CurrentSCC.back()] = ~0U;
126 } while (CurrentSCC.back() != visitingN);
131 inline scc_iterator(NodeType *entryN) : visitNum(0) {
135 inline scc_iterator() { /* End is when DFS stack is empty */ }
138 typedef scc_iterator<GraphT, GT> _Self;
140 // Provide static "constructors"...
141 static inline _Self begin(const GraphT &G){return _Self(GT::getEntryNode(G));}
142 static inline _Self end (const GraphT &) { return _Self(); }
144 // Direct loop termination test: I.isAtEnd() is more efficient than I == end()
145 inline bool isAtEnd() const {
146 assert(!CurrentSCC.empty() || VisitStack.empty());
147 return CurrentSCC.empty();
150 inline bool operator==(const _Self& x) const {
151 return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
153 inline bool operator!=(const _Self& x) const { return !operator==(x); }
155 // Iterator traversal: forward iteration only
156 inline _Self& operator++() { // Preincrement
160 inline _Self operator++(int) { // Postincrement
161 _Self tmp = *this; ++*this; return tmp;
164 // Retrieve a reference to the current SCC
165 inline const SccTy &operator*() const {
166 assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
169 inline SccTy &operator*() {
170 assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
174 // hasLoop() -- Test if the current SCC has a loop. If it has more than one
175 // node, this is trivially true. If not, it may still contain a loop if the
176 // node has an edge back to itself.
177 bool hasLoop() const {
178 assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
179 if (CurrentSCC.size() > 1) return true;
180 NodeType *N = CurrentSCC.front();
181 for (ChildItTy CI = GT::child_begin(N), CE=GT::child_end(N); CI != CE; ++CI)
187 /// ReplaceNode - This informs the scc_iterator that the specified Old node
188 /// has been deleted, and New is to be used in its place.
189 void ReplaceNode(NodeType *Old, NodeType *New) {
190 assert(nodeVisitNumbers.count(Old) && "Old not in scc_iterator?");
191 nodeVisitNumbers[New] = nodeVisitNumbers[Old];
192 nodeVisitNumbers.erase(Old);
197 // Global constructor for the SCC iterator.
199 scc_iterator<T> scc_begin(const T &G) {
200 return scc_iterator<T>::begin(G);
204 scc_iterator<T> scc_end(const T &G) {
205 return scc_iterator<T>::end(G);
209 scc_iterator<Inverse<T> > scc_begin(const Inverse<T> &G) {
210 return scc_iterator<Inverse<T> >::begin(G);
214 scc_iterator<Inverse<T> > scc_end(const Inverse<T> &G) {
215 return scc_iterator<Inverse<T> >::end(G);
218 } // End llvm namespace