SCCIterator.h revision 551ccae044b0ff658fe629dd67edd5ffe75d10e8 
1//===-- Support/SCCIterator.h - Strongly Connected Comp. Iter. --*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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.
12//
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.
15//
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.)
18//
19//===----------------------------------------------------------------------===//
20
21#ifndef LLVM_ADT_SCCITERATOR_H
22#define LLVM_ADT_SCCITERATOR_H
23
24#include "llvm/ADT/GraphTraits.h"
25#include "llvm/ADT/iterator"
26#include <vector>
27#include <map>
28
29namespace llvm {
30
31//===----------------------------------------------------------------------===//
32///
33/// scc_iterator - Enumerate the SCCs of a directed graph, in
34/// reverse topological order of the SCC DAG.
35///
36template<class GraphT, class GT = GraphTraits<GraphT> >
37class scc_iterator
38  : public forward_iterator<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 forward_iterator<SccTy, ptrdiff_t> super;
43  typedef typename super::reference reference;
44  typedef typename super::pointer pointer;
45
46  // The visit counters used to detect when a complete SCC is on the stack.
47  // visitNum is the global counter.
48  // nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
49  unsigned visitNum;
50  std::map<NodeType *, unsigned> nodeVisitNumbers;
51
52  // SCCNodeStack - Stack holding nodes of the SCC.
53  std::vector<NodeType *> SCCNodeStack;
54
55  // CurrentSCC - The current SCC, retrieved using operator*().
56  SccTy CurrentSCC;
57
58  // VisitStack - Used to maintain the ordering.  Top = current block
59  // First element is basic block pointer, second is the 'next child' to visit
60  std::vector<std::pair<NodeType *, ChildItTy> > VisitStack;
61
62  // MinVistNumStack - Stack holding the "min" values for each node in the DFS.
63  // This is used to track the minimum uplink values for all children of
64  // the corresponding node on the VisitStack.
65  std::vector<unsigned> MinVisitNumStack;
66
67  // A single "visit" within the non-recursive DFS traversal.
68  void DFSVisitOne(NodeType* N) {
69    ++visitNum;                         // Global counter for the visit order
70    nodeVisitNumbers[N] = visitNum;
71    SCCNodeStack.push_back(N);
72    MinVisitNumStack.push_back(visitNum);
73    VisitStack.push_back(std::make_pair(N, GT::child_begin(N)));
74    //DEBUG(std::cerr << "TarjanSCC: Node " << N <<
75    //      " : visitNum = " << visitNum << "\n");
76  }
77
78  // The stack-based DFS traversal; defined below.
79  void DFSVisitChildren() {
80    assert(!VisitStack.empty());
81    while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
82      // TOS has at least one more child so continue DFS
83      NodeType *childN = *VisitStack.back().second++;
84      if (!nodeVisitNumbers.count(childN)) {
85        // this node has never been seen
86        DFSVisitOne(childN);
87      } else {
88        unsigned childNum = nodeVisitNumbers[childN];
89        if (MinVisitNumStack.back() > childNum)
90          MinVisitNumStack.back() = childNum;
91      }
92    }
93  }
94
95  // Compute the next SCC using the DFS traversal.
96  void GetNextSCC() {
97    assert(VisitStack.size() == MinVisitNumStack.size());
98    CurrentSCC.clear();                 // Prepare to compute the next SCC
99    while (!VisitStack.empty()) {
100      DFSVisitChildren();
101      assert(VisitStack.back().second ==GT::child_end(VisitStack.back().first));
102      NodeType* visitingN = VisitStack.back().first;
103      unsigned minVisitNum = MinVisitNumStack.back();
104      VisitStack.pop_back();
105      MinVisitNumStack.pop_back();
106      if (!MinVisitNumStack.empty() && MinVisitNumStack.back() > minVisitNum)
107        MinVisitNumStack.back() = minVisitNum;
108
109      //DEBUG(std::cerr << "TarjanSCC: Popped node " << visitingN <<
110      //      " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
111      //      nodeVisitNumbers[visitingN] << "\n");
112
113      if (minVisitNum == nodeVisitNumbers[visitingN]) {
114        // A full SCC is on the SCCNodeStack!  It includes all nodes below
115          // visitingN on the stack.  Copy those nodes to CurrentSCC,
116          // reset their minVisit values, and return (this suspends
117          // the DFS traversal till the next ++).
118          do {
119            CurrentSCC.push_back(SCCNodeStack.back());
120            SCCNodeStack.pop_back();
121            nodeVisitNumbers[CurrentSCC.back()] = ~0UL;
122          } while (CurrentSCC.back() != visitingN);
123          return;
124        }
125    }
126  }
127
128  inline scc_iterator(NodeType *entryN) : visitNum(0) {
129    DFSVisitOne(entryN);
130    GetNextSCC();
131  }
132  inline scc_iterator() { /* End is when DFS stack is empty */ }
133
134public:
135  typedef scc_iterator<GraphT, GT> _Self;
136
137  // Provide static "constructors"...
138  static inline _Self begin(GraphT& G) { return _Self(GT::getEntryNode(G)); }
139  static inline _Self end  (GraphT& G) { return _Self(); }
140
141  // Direct loop termination test (I.fini() is more efficient than I == end())
142  inline bool fini() const {
143    assert(!CurrentSCC.empty() || VisitStack.empty());
144    return CurrentSCC.empty();
145  }
146
147  inline bool operator==(const _Self& x) const {
148    return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
149  }
150  inline bool operator!=(const _Self& x) const { return !operator==(x); }
151
152  // Iterator traversal: forward iteration only
153  inline _Self& operator++() {          // Preincrement
154    GetNextSCC();
155    return *this;
156  }
157  inline _Self operator++(int) {        // Postincrement
158    _Self tmp = *this; ++*this; return tmp;
159  }
160
161  // Retrieve a reference to the current SCC
162  inline const SccTy &operator*() const {
163    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
164    return CurrentSCC;
165  }
166  inline SccTy &operator*() {
167    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
168    return CurrentSCC;
169  }
170
171  // hasLoop() -- Test if the current SCC has a loop.  If it has more than one
172  // node, this is trivially true.  If not, it may still contain a loop if the
173  // node has an edge back to itself.
174  bool hasLoop() const {
175    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
176    if (CurrentSCC.size() > 1) return true;
177    NodeType *N = CurrentSCC.front();
178    for (ChildItTy CI = GT::child_begin(N), CE=GT::child_end(N); CI != CE; ++CI)
179      if (*CI == N)
180        return true;
181    return false;
182  }
183};
184
185
186// Global constructor for the SCC iterator.
187template <class T>
188scc_iterator<T> scc_begin(T G) {
189  return scc_iterator<T>::begin(G);
190}
191
192template <class T>
193scc_iterator<T> scc_end(T G) {
194  return scc_iterator<T>::end(G);
195}
196
197} // End llvm namespace
198
199#endif
200