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This patch merges MinVisitNumStack with VisitStack using a StackElement struct. Patch by Mehdi Amini! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201353 91177308-0d34-0410-b5e6-96231b3b80d8
247 lines
8.4 KiB
C++
247 lines
8.4 KiB
C++
//===---- ADT/SCCIterator.h - Strongly Connected Comp. Iter. ----*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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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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//===----------------------------------------------------------------------===//
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/// \file
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///
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/// This builds on the llvm/ADT/GraphTraits.h file to find the strongly
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/// connected components (SCCs) of a graph in O(N+E) time using Tarjan's DFS
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/// algorithm.
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///
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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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///
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/// To visit S1 *before* S2, use the scc_iterator on the Inverse graph. (NOTE:
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/// This requires some simple wrappers and is not supported yet.)
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///
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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/DenseMap.h"
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#include "llvm/ADT/GraphTraits.h"
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#include <vector>
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namespace llvm {
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/// \brief Enumerate the SCCs of a directed graph in reverse topological order
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/// of the SCC DAG.
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///
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/// This is implemented using Tarjan's DFS algorithm using an internal stack to
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/// build up a vector of nodes in a particular SCC. Note that it is a forward
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/// iterator and thus you cannot backtrack or re-visit nodes.
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template <class GraphT, class GT = GraphTraits<GraphT> >
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class scc_iterator
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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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// Element of VisitStack during DFS.
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struct StackElement {
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NodeType *Node; ///< The current node pointer.
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ChildItTy NextChild; ///< The next child, modified inplace during DFS.
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unsigned MinVisited; ///< Minimum uplink value of all children of Node.
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StackElement(NodeType *Node, const ChildItTy &Child, unsigned Min)
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: Node(Node), NextChild(Child), MinVisited(Min) {}
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bool operator==(const StackElement &Other) const {
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return Node == Other.Node &&
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NextChild == Other.NextChild &&
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MinVisited == Other.MinVisited;
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}
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};
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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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unsigned visitNum;
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DenseMap<NodeType *, unsigned> nodeVisitNumbers;
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// Stack holding nodes of the SCC.
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std::vector<NodeType *> SCCNodeStack;
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// The current SCC, retrieved using operator*().
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SccTy CurrentSCC;
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// DFS stack, Used to maintain the ordering. The top contains the current
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// node, the next child to visit, and the minimum uplink value of all child
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std::vector<StackElement> VisitStack;
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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;
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nodeVisitNumbers[N] = visitNum;
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SCCNodeStack.push_back(N);
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VisitStack.push_back(StackElement(N, GT::child_begin(N), visitNum));
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#if 0 // Enable if needed when debugging.
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dbgs() << "TarjanSCC: Node " << N <<
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" : visitNum = " << visitNum << "\n";
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#endif
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}
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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().NextChild !=
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GT::child_end(VisitStack.back().Node)) {
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// TOS has at least one more child so continue DFS
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NodeType *childN = *VisitStack.back().NextChild++;
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typename DenseMap<NodeType *, unsigned>::iterator Visited =
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nodeVisitNumbers.find(childN);
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if (Visited == nodeVisitNumbers.end()) {
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// this node has never been seen.
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DFSVisitOne(childN);
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continue;
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}
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unsigned childNum = Visited->second;
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if (VisitStack.back().MinVisited > childNum)
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VisitStack.back().MinVisited = childNum;
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}
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}
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// Compute the next SCC using the DFS traversal.
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void GetNextSCC() {
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CurrentSCC.clear(); // Prepare to compute the next SCC
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while (!VisitStack.empty()) {
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DFSVisitChildren();
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// Pop the leaf on top of the VisitStack.
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NodeType *visitingN = VisitStack.back().Node;
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unsigned minVisitNum = VisitStack.back().MinVisited;
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assert(VisitStack.back().NextChild == GT::child_end(visitingN));
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VisitStack.pop_back();
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// Propagate MinVisitNum to parent so we can detect the SCC starting node.
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if (!VisitStack.empty() && VisitStack.back().MinVisited > minVisitNum)
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VisitStack.back().MinVisited = minVisitNum;
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#if 0 // Enable if needed when debugging.
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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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#endif
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if (minVisitNum != nodeVisitNumbers[visitingN])
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continue;
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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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do {
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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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return;
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}
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}
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inline scc_iterator(NodeType *entryN) : visitNum(0) {
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DFSVisitOne(entryN);
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GetNextSCC();
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}
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// End is when the DFS stack is empty.
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inline scc_iterator() {}
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public:
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static inline scc_iterator begin(const GraphT &G) {
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return scc_iterator(GT::getEntryNode(G));
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}
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static inline scc_iterator end(const GraphT &) { return scc_iterator(); }
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/// \brief Direct loop termination test which is more efficient than
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/// comparison with \c 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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}
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inline bool operator==(const scc_iterator &x) const {
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return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
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}
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inline bool operator!=(const scc_iterator &x) const { return !operator==(x); }
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inline scc_iterator &operator++() {
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GetNextSCC();
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return *this;
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}
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inline scc_iterator operator++(int) {
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scc_iterator tmp = *this;
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++*this;
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return tmp;
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}
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inline const SccTy &operator*() const {
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assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
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return CurrentSCC;
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}
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inline SccTy &operator*() {
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assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
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return CurrentSCC;
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}
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/// \brief Test if the current SCC has a loop.
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///
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/// If the SCC has more than one node, this is trivially true. If not, it may
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/// still contain a loop if the 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)
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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;
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++CI)
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if (*CI == N)
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return true;
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return false;
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}
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/// This informs the \c scc_iterator that the specified \c Old node
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/// has been deleted, and \c 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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}
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};
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/// \brief Construct the begin iterator for a deduced graph type T.
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template <class T> scc_iterator<T> scc_begin(const T &G) {
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return scc_iterator<T>::begin(G);
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}
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/// \brief Construct the end iterator for a deduced graph type T.
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template <class T> scc_iterator<T> scc_end(const T &G) {
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return scc_iterator<T>::end(G);
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}
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/// \brief Construct the begin iterator for a deduced graph type T's Inverse<T>.
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template <class T> 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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}
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/// \brief Construct the end iterator for a deduced graph type T's Inverse<T>.
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template <class T> 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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}
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} // End llvm namespace
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#endif
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