src/llvm/include/llvm/ADT/SCCIterator.h

   1 //===---- ADT/SCCIterator.h - Strongly Connected Comp. Iter. ----*- C++ -*-===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // 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/DenseMap.h"
  25 #include "llvm/ADT/GraphTraits.h"
  26 #include <vector>
  27
  28 namespace llvm {
  29
  30 //===----------------------------------------------------------------------===//
  31 ///
  32 /// scc_iterator - Enumerate the SCCs of a directed graph, in
  33 /// reverse topological order of the SCC DAG.
  34 ///
  35 template<class GraphT, class GT = GraphTraits<GraphT> >
  36 class scc_iterator
  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;
  46
  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.
  50   unsigned visitNum;
  51   DenseMap<NodeType *, unsigned> nodeVisitNumbers;
  52
  53   // SCCNodeStack - Stack holding nodes of the SCC.
  54   std::vector<NodeType *> SCCNodeStack;
  55
  56   // CurrentSCC - The current SCC, retrieved using operator*().
  57   SccTy CurrentSCC;
  58
  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;
  62
  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;
  67
  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";
  77   }
  78
  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.
  87         DFSVisitOne(childN);
  88         continue;
  89       }
  90
  91       unsigned childNum = nodeVisitNumbers[childN];
  92       if (MinVisitNumStack.back() > childNum)
  93         MinVisitNumStack.back() = childNum;
  94     }
  95   }
  96
  97   // Compute the next SCC using the DFS traversal.
  98   void GetNextSCC() {
  99     assert(VisitStack.size() == MinVisitNumStack.size());
 100     CurrentSCC.clear();                 // Prepare to compute the next SCC
 101     while (!VisitStack.empty()) {
 102       DFSVisitChildren();
 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;
 110
 111       //dbgs() << "TarjanSCC: Popped node " << visitingN <<
 112       //      " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
 113       //      nodeVisitNumbers[visitingN] << "\n";
 114
 115       if (minVisitNum != nodeVisitNumbers[visitingN])
 116         continue;
 117
 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 ++).
 122       do {
 123         CurrentSCC.push_back(SCCNodeStack.back());
 124         SCCNodeStack.pop_back();
 125         nodeVisitNumbers[CurrentSCC.back()] = ~0U;
 126       } while (CurrentSCC.back() != visitingN);
 127       return;
 128     }
 129   }
 130
 131   inline scc_iterator(NodeType *entryN) : visitNum(0) {
 132     DFSVisitOne(entryN);
 133     GetNextSCC();
 134   }
 135   inline scc_iterator() { /* End is when DFS stack is empty */ }
 136
 137 public:
 138   typedef scc_iterator<GraphT, GT> _Self;
 139
 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(); }
 143
 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();
 148   }
 149
 150   inline bool operator==(const _Self& x) const {
 151     return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
 152   }
 153   inline bool operator!=(const _Self& x) const { return !operator==(x); }
 154
 155   // Iterator traversal: forward iteration only
 156   inline _Self& operator++() {          // Preincrement
 157     GetNextSCC();
 158     return *this;
 159   }
 160   inline _Self operator++(int) {        // Postincrement
 161     _Self tmp = *this; ++*this; return tmp;
 162   }
 163
 164   // Retrieve a reference to the current SCC
 165   inline const SccTy &operator*() const {
 166     assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
 167     return CurrentSCC;
 168   }
 169   inline SccTy &operator*() {
 170     assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
 171     return CurrentSCC;
 172   }
 173
 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)
 182       if (*CI == N)
 183         return true;
 184     return false;
 185   }
 186
 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);
 193   }
 194 };
 195
 196
 197 // Global constructor for the SCC iterator.
 198 template <class T>
 199 scc_iterator<T> scc_begin(const T &G) {
 200   return scc_iterator<T>::begin(G);
 201 }
 202
 203 template <class T>
 204 scc_iterator<T> scc_end(const T &G) {
 205   return scc_iterator<T>::end(G);
 206 }
 207
 208 template <class T>
 209 scc_iterator<Inverse<T> > scc_begin(const Inverse<T> &G) {
 210   return scc_iterator<Inverse<T> >::begin(G);
 211 }
 212
 213 template <class T>
 214 scc_iterator<Inverse<T> > scc_end(const Inverse<T> &G) {
 215   return scc_iterator<Inverse<T> >::end(G);
 216 }
 217
 218 } // End llvm namespace
 219
 220 #endif