llvm-6502/include/llvm/ADT/SCCIterator.h
Duncan P. N. Exon Smith efe40a5a1d SCCIterator: Merge MinVisitNumStack and VisitStack
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
2014-02-13 18:26:15 +00:00

247 lines
8.4 KiB
C++

//===---- ADT/SCCIterator.h - Strongly Connected Comp. Iter. ----*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This builds on the llvm/ADT/GraphTraits.h file to find the strongly
/// connected components (SCCs) of a graph in O(N+E) time using Tarjan's DFS
/// algorithm.
///
/// The SCC iterator has the important property that if a node in SCC S1 has an
/// edge to a node in SCC S2, then it visits S1 *after* S2.
///
/// To visit S1 *before* S2, use the scc_iterator on the Inverse graph. (NOTE:
/// This requires some simple wrappers and is not supported yet.)
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_ADT_SCCITERATOR_H
#define LLVM_ADT_SCCITERATOR_H
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/GraphTraits.h"
#include <vector>
namespace llvm {
/// \brief Enumerate the SCCs of a directed graph in reverse topological order
/// of the SCC DAG.
///
/// This is implemented using Tarjan's DFS algorithm using an internal stack to
/// build up a vector of nodes in a particular SCC. Note that it is a forward
/// iterator and thus you cannot backtrack or re-visit nodes.
template <class GraphT, class GT = GraphTraits<GraphT> >
class scc_iterator
: public std::iterator<std::forward_iterator_tag,
std::vector<typename GT::NodeType>, ptrdiff_t> {
typedef typename GT::NodeType NodeType;
typedef typename GT::ChildIteratorType ChildItTy;
typedef std::vector<NodeType *> SccTy;
typedef std::iterator<std::forward_iterator_tag,
std::vector<typename GT::NodeType>, ptrdiff_t> super;
typedef typename super::reference reference;
typedef typename super::pointer pointer;
// Element of VisitStack during DFS.
struct StackElement {
NodeType *Node; ///< The current node pointer.
ChildItTy NextChild; ///< The next child, modified inplace during DFS.
unsigned MinVisited; ///< Minimum uplink value of all children of Node.
StackElement(NodeType *Node, const ChildItTy &Child, unsigned Min)
: Node(Node), NextChild(Child), MinVisited(Min) {}
bool operator==(const StackElement &Other) const {
return Node == Other.Node &&
NextChild == Other.NextChild &&
MinVisited == Other.MinVisited;
}
};
// The visit counters used to detect when a complete SCC is on the stack.
// visitNum is the global counter.
// nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
unsigned visitNum;
DenseMap<NodeType *, unsigned> nodeVisitNumbers;
// Stack holding nodes of the SCC.
std::vector<NodeType *> SCCNodeStack;
// The current SCC, retrieved using operator*().
SccTy CurrentSCC;
// DFS stack, Used to maintain the ordering. The top contains the current
// node, the next child to visit, and the minimum uplink value of all child
std::vector<StackElement> VisitStack;
// A single "visit" within the non-recursive DFS traversal.
void DFSVisitOne(NodeType *N) {
++visitNum;
nodeVisitNumbers[N] = visitNum;
SCCNodeStack.push_back(N);
VisitStack.push_back(StackElement(N, GT::child_begin(N), visitNum));
#if 0 // Enable if needed when debugging.
dbgs() << "TarjanSCC: Node " << N <<
" : visitNum = " << visitNum << "\n";
#endif
}
// The stack-based DFS traversal; defined below.
void DFSVisitChildren() {
assert(!VisitStack.empty());
while (VisitStack.back().NextChild !=
GT::child_end(VisitStack.back().Node)) {
// TOS has at least one more child so continue DFS
NodeType *childN = *VisitStack.back().NextChild++;
typename DenseMap<NodeType *, unsigned>::iterator Visited =
nodeVisitNumbers.find(childN);
if (Visited == nodeVisitNumbers.end()) {
// this node has never been seen.
DFSVisitOne(childN);
continue;
}
unsigned childNum = Visited->second;
if (VisitStack.back().MinVisited > childNum)
VisitStack.back().MinVisited = childNum;
}
}
// Compute the next SCC using the DFS traversal.
void GetNextSCC() {
CurrentSCC.clear(); // Prepare to compute the next SCC
while (!VisitStack.empty()) {
DFSVisitChildren();
// Pop the leaf on top of the VisitStack.
NodeType *visitingN = VisitStack.back().Node;
unsigned minVisitNum = VisitStack.back().MinVisited;
assert(VisitStack.back().NextChild == GT::child_end(visitingN));
VisitStack.pop_back();
// Propagate MinVisitNum to parent so we can detect the SCC starting node.
if (!VisitStack.empty() && VisitStack.back().MinVisited > minVisitNum)
VisitStack.back().MinVisited = minVisitNum;
#if 0 // Enable if needed when debugging.
dbgs() << "TarjanSCC: Popped node " << visitingN <<
" : minVisitNum = " << minVisitNum << "; Node visit num = " <<
nodeVisitNumbers[visitingN] << "\n";
#endif
if (minVisitNum != nodeVisitNumbers[visitingN])
continue;
// A full SCC is on the SCCNodeStack! It includes all nodes below
// visitingN on the stack. Copy those nodes to CurrentSCC,
// reset their minVisit values, and return (this suspends
// the DFS traversal till the next ++).
do {
CurrentSCC.push_back(SCCNodeStack.back());
SCCNodeStack.pop_back();
nodeVisitNumbers[CurrentSCC.back()] = ~0U;
} while (CurrentSCC.back() != visitingN);
return;
}
}
inline scc_iterator(NodeType *entryN) : visitNum(0) {
DFSVisitOne(entryN);
GetNextSCC();
}
// End is when the DFS stack is empty.
inline scc_iterator() {}
public:
static inline scc_iterator begin(const GraphT &G) {
return scc_iterator(GT::getEntryNode(G));
}
static inline scc_iterator end(const GraphT &) { return scc_iterator(); }
/// \brief Direct loop termination test which is more efficient than
/// comparison with \c end().
inline bool isAtEnd() const {
assert(!CurrentSCC.empty() || VisitStack.empty());
return CurrentSCC.empty();
}
inline bool operator==(const scc_iterator &x) const {
return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
}
inline bool operator!=(const scc_iterator &x) const { return !operator==(x); }
inline scc_iterator &operator++() {
GetNextSCC();
return *this;
}
inline scc_iterator operator++(int) {
scc_iterator tmp = *this;
++*this;
return tmp;
}
inline const SccTy &operator*() const {
assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
return CurrentSCC;
}
inline SccTy &operator*() {
assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
return CurrentSCC;
}
/// \brief Test if the current SCC has a loop.
///
/// If the SCC has more than one node, this is trivially true. If not, it may
/// still contain a loop if the node has an edge back to itself.
bool hasLoop() const {
assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
if (CurrentSCC.size() > 1)
return true;
NodeType *N = CurrentSCC.front();
for (ChildItTy CI = GT::child_begin(N), CE = GT::child_end(N); CI != CE;
++CI)
if (*CI == N)
return true;
return false;
}
/// This informs the \c scc_iterator that the specified \c Old node
/// has been deleted, and \c New is to be used in its place.
void ReplaceNode(NodeType *Old, NodeType *New) {
assert(nodeVisitNumbers.count(Old) && "Old not in scc_iterator?");
nodeVisitNumbers[New] = nodeVisitNumbers[Old];
nodeVisitNumbers.erase(Old);
}
};
/// \brief Construct the begin iterator for a deduced graph type T.
template <class T> scc_iterator<T> scc_begin(const T &G) {
return scc_iterator<T>::begin(G);
}
/// \brief Construct the end iterator for a deduced graph type T.
template <class T> scc_iterator<T> scc_end(const T &G) {
return scc_iterator<T>::end(G);
}
/// \brief Construct the begin iterator for a deduced graph type T's Inverse<T>.
template <class T> scc_iterator<Inverse<T> > scc_begin(const Inverse<T> &G) {
return scc_iterator<Inverse<T> >::begin(G);
}
/// \brief Construct the end iterator for a deduced graph type T's Inverse<T>.
template <class T> scc_iterator<Inverse<T> > scc_end(const Inverse<T> &G) {
return scc_iterator<Inverse<T> >::end(G);
}
} // End llvm namespace
#endif