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//===-- Support/SCCIterator.h - Strongly Connected Comp. Iter. --*- C++ -*-===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This builds on the llvm/ADT/GraphTraits.h file to find the strongly connected
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// components (SCCs) of a graph in O(N+E) time using Tarjan's DFS algorithm.
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// The SCC iterator has the important property that if a node in SCC S1 has an
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// edge to a node in SCC S2, then it visits S1 *after* S2.
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// To visit S1 *before* S2, use the scc_iterator on the Inverse graph.
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// (NOTE: This requires some simple wrappers and is not supported yet.)
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_ADT_SCCITERATOR_H
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#define LLVM_ADT_SCCITERATOR_H
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#include "llvm/ADT/GraphTraits.h"
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#include "llvm/ADT/DenseMap.h"
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//===----------------------------------------------------------------------===//
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/// scc_iterator - Enumerate the SCCs of a directed graph, in
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/// reverse topological order of the SCC DAG.
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template<class GraphT, class GT = GraphTraits<GraphT> >
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: public std::iterator<std::forward_iterator_tag,
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std::vector<typename GT::NodeType>, ptrdiff_t> {
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typedef typename GT::NodeType NodeType;
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typedef typename GT::ChildIteratorType ChildItTy;
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typedef std::vector<NodeType*> SccTy;
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typedef std::iterator<std::forward_iterator_tag,
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std::vector<typename GT::NodeType>, ptrdiff_t> super;
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typedef typename super::reference reference;
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typedef typename super::pointer pointer;
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// The visit counters used to detect when a complete SCC is on the stack.
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// visitNum is the global counter.
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// nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
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DenseMap<NodeType *, unsigned> nodeVisitNumbers;
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// SCCNodeStack - Stack holding nodes of the SCC.
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std::vector<NodeType *> SCCNodeStack;
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// CurrentSCC - The current SCC, retrieved using operator*().
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// VisitStack - Used to maintain the ordering. Top = current block
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// First element is basic block pointer, second is the 'next child' to visit
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std::vector<std::pair<NodeType *, ChildItTy> > VisitStack;
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// MinVistNumStack - Stack holding the "min" values for each node in the DFS.
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// This is used to track the minimum uplink values for all children of
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// the corresponding node on the VisitStack.
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std::vector<unsigned> MinVisitNumStack;
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// A single "visit" within the non-recursive DFS traversal.
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void DFSVisitOne(NodeType *N) {
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++visitNum; // Global counter for the visit order
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nodeVisitNumbers[N] = visitNum;
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SCCNodeStack.push_back(N);
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MinVisitNumStack.push_back(visitNum);
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VisitStack.push_back(std::make_pair(N, GT::child_begin(N)));
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//dbgs() << "TarjanSCC: Node " << N <<
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// " : visitNum = " << visitNum << "\n";
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// The stack-based DFS traversal; defined below.
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void DFSVisitChildren() {
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assert(!VisitStack.empty());
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while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
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// TOS has at least one more child so continue DFS
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NodeType *childN = *VisitStack.back().second++;
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if (!nodeVisitNumbers.count(childN)) {
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// this node has never been seen.
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unsigned childNum = nodeVisitNumbers[childN];
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if (MinVisitNumStack.back() > childNum)
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MinVisitNumStack.back() = childNum;
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// Compute the next SCC using the DFS traversal.
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assert(VisitStack.size() == MinVisitNumStack.size());
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CurrentSCC.clear(); // Prepare to compute the next SCC
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while (!VisitStack.empty()) {
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assert(VisitStack.back().second ==GT::child_end(VisitStack.back().first));
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NodeType *visitingN = VisitStack.back().first;
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unsigned minVisitNum = MinVisitNumStack.back();
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VisitStack.pop_back();
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MinVisitNumStack.pop_back();
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if (!MinVisitNumStack.empty() && MinVisitNumStack.back() > minVisitNum)
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MinVisitNumStack.back() = minVisitNum;
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//dbgs() << "TarjanSCC: Popped node " << visitingN <<
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// " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
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// nodeVisitNumbers[visitingN] << "\n";
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if (minVisitNum != nodeVisitNumbers[visitingN])
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// A full SCC is on the SCCNodeStack! It includes all nodes below
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// visitingN on the stack. Copy those nodes to CurrentSCC,
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// reset their minVisit values, and return (this suspends
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// the DFS traversal till the next ++).
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CurrentSCC.push_back(SCCNodeStack.back());
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SCCNodeStack.pop_back();
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nodeVisitNumbers[CurrentSCC.back()] = ~0U;
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} while (CurrentSCC.back() != visitingN);
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inline scc_iterator(NodeType *entryN) : visitNum(0) {
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inline scc_iterator() { /* End is when DFS stack is empty */ }
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typedef scc_iterator<GraphT, GT> _Self;
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// Provide static "constructors"...
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static inline _Self begin(const GraphT &G){return _Self(GT::getEntryNode(G));}
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static inline _Self end (const GraphT &G) { return _Self(); }
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// Direct loop termination test: I.isAtEnd() is more efficient than I == end()
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inline bool isAtEnd() const {
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assert(!CurrentSCC.empty() || VisitStack.empty());
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return CurrentSCC.empty();
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inline bool operator==(const _Self& x) const {
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return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
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inline bool operator!=(const _Self& x) const { return !operator==(x); }
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// Iterator traversal: forward iteration only
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inline _Self& operator++() { // Preincrement
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inline _Self operator++(int) { // Postincrement
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_Self tmp = *this; ++*this; return tmp;
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// Retrieve a reference to the current SCC
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inline const SccTy &operator*() const {
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assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
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inline SccTy &operator*() {
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assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
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// hasLoop() -- Test if the current SCC has a loop. If it has more than one
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// node, this is trivially true. If not, it may still contain a loop if the
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// node has an edge back to itself.
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bool hasLoop() const {
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assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
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if (CurrentSCC.size() > 1) return true;
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NodeType *N = CurrentSCC.front();
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for (ChildItTy CI = GT::child_begin(N), CE=GT::child_end(N); CI != CE; ++CI)
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/// ReplaceNode - This informs the scc_iterator that the specified Old node
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/// has been deleted, and New is to be used in its place.
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void ReplaceNode(NodeType *Old, NodeType *New) {
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assert(nodeVisitNumbers.count(Old) && "Old not in scc_iterator?");
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nodeVisitNumbers[New] = nodeVisitNumbers[Old];
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nodeVisitNumbers.erase(Old);
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// Global constructor for the SCC iterator.
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scc_iterator<T> scc_begin(const T &G) {
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return scc_iterator<T>::begin(G);
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scc_iterator<T> scc_end(const T &G) {
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return scc_iterator<T>::end(G);
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scc_iterator<Inverse<T> > scc_begin(const Inverse<T> &G) {
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return scc_iterator<Inverse<T> >::begin(G);
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scc_iterator<Inverse<T> > scc_end(const Inverse<T> &G) {
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return scc_iterator<Inverse<T> >::end(G);
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} // End llvm namespace