Changes from Curtis Dunham implementing lazy cycle detection algorithm.

Changes from me implementing different way of representing points-to anything.
Changes from me that improve slightly on LCD.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44895 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Daniel Berlin 2007-12-12 00:37:04 +00:00
parent 39c883cfc5
commit 3a3f163ba6

View File

@ -71,12 +71,20 @@
#include <list>
#include <stack>
#include <vector>
#include <queue>
// Determining the actual set of nodes the universal set can consist of is very
// expensive because it means propagating around very large sets. We rely on
// other analysis being able to determine which nodes can never be pointed to in
// order to disambiguate further than "points-to anything".
#define FULL_UNIVERSAL 0
using namespace llvm;
STATISTIC(NumIters , "Number of iterations to reach convergence");
STATISTIC(NumConstraints, "Number of constraints");
STATISTIC(NumNodes , "Number of nodes");
STATISTIC(NumUnified , "Number of variables unified");
STATISTIC(NumErased , "Number of redundant constraints erased");
namespace {
const unsigned SelfRep = (unsigned)-1;
@ -157,6 +165,24 @@ namespace {
}
};
// Information DenseSet requires implemented in order to be able to do
// it's thing
struct PairKeyInfo {
static inline std::pair<unsigned, unsigned> getEmptyKey() {
return std::make_pair(~0UL, ~0UL);
}
static inline std::pair<unsigned, unsigned> getTombstoneKey() {
return std::make_pair(~0UL - 1, ~0UL - 1);
}
static unsigned getHashValue(const std::pair<unsigned, unsigned> &P) {
return P.first ^ P.second;
}
static unsigned isEqual(const std::pair<unsigned, unsigned> &LHS,
const std::pair<unsigned, unsigned> &RHS) {
return LHS == RHS;
}
};
struct ConstraintKeyInfo {
static inline Constraint getEmptyKey() {
return Constraint(Constraint::Copy, ~0UL, ~0UL, ~0UL);
@ -180,11 +206,14 @@ namespace {
// artificial Node's that represent the set of pointed-to variables shared
// for each location equivalent Node.
struct Node {
private:
static unsigned Counter;
public:
Value *Val;
SparseBitVector<> *Edges;
SparseBitVector<> *PointsTo;
SparseBitVector<> *OldPointsTo;
bool Changed;
std::list<Constraint> Constraints;
// Pointer and location equivalence labels
@ -212,14 +241,17 @@ namespace {
// standard union-find representation with path compression. NodeRep
// gives the index into GraphNodes for the representative Node.
unsigned NodeRep;
public:
// Modification timestamp. Assigned from Counter.
// Used for work list prioritization.
unsigned Timestamp;
Node(bool direct = true) :
Val(0), Edges(0), PointsTo(0), OldPointsTo(0), Changed(false),
Val(0), Edges(0), PointsTo(0), OldPointsTo(0),
PointerEquivLabel(0), LocationEquivLabel(0), PredEdges(0),
ImplicitPredEdges(0), PointedToBy(0), NumInEdges(0),
StoredInHash(false), Direct(direct), AddressTaken(false),
NodeRep(SelfRep) { }
NodeRep(SelfRep), Timestamp(0) { }
Node *setValue(Value *V) {
assert(Val == 0 && "Value already set for this node!");
@ -246,6 +278,60 @@ namespace {
/// intersects with the points-to set of the specified node on any nodes
/// except for the specified node to ignore.
bool intersectsIgnoring(Node *N, unsigned) const;
// Timestamp a node (used for work list prioritization)
void Stamp() {
Timestamp = Counter++;
}
bool isRep() {
return( (int) NodeRep < 0 );
}
};
struct WorkListElement {
Node* node;
unsigned Timestamp;
WorkListElement(Node* n, unsigned t) : node(n), Timestamp(t) {}
// Note that we reverse the sense of the comparison because we
// actually want to give low timestamps the priority over high,
// whereas priority is typically interpreted as a greater value is
// given high priority.
bool operator<(const WorkListElement& that) const {
return( this->Timestamp > that.Timestamp );
}
};
// Priority-queue based work list specialized for Nodes.
class WorkList {
std::priority_queue<WorkListElement> Q;
public:
void insert(Node* n) {
Q.push( WorkListElement(n, n->Timestamp) );
}
// We automatically discard non-representative nodes and nodes
// that were in the work list twice (we keep a copy of the
// timestamp in the work list so we can detect this situation by
// comparing against the node's current timestamp).
Node* pop() {
while( !Q.empty() ) {
WorkListElement x = Q.top(); Q.pop();
Node* INode = x.node;
if( INode->isRep() &&
INode->Timestamp == x.Timestamp ) {
return(x.node);
}
}
return(0);
}
bool empty() {
return Q.empty();
}
};
/// GraphNodes - This vector is populated as part of the object
@ -290,17 +376,20 @@ namespace {
};
// Stack for Tarjan's
std::stack<unsigned> SCCStack;
// Topological Index -> Graph node
std::vector<unsigned> Topo2Node;
// Graph Node -> Topological Index;
std::vector<unsigned> Node2Topo;
// Map from Graph Node to DFS number
std::vector<unsigned> Node2DFS;
// Map from Graph Node to Deleted from graph.
std::vector<bool> Node2Deleted;
// Current DFS and RPO numbers
// Same as Node Maps, but implemented as std::map because it is faster to
// clear
std::map<unsigned, unsigned> Tarjan2DFS;
std::map<unsigned, bool> Tarjan2Deleted;
// Current DFS number
unsigned DFSNumber;
unsigned RPONumber;
// Work lists.
WorkList w1, w2;
WorkList *CurrWL, *NextWL; // "current" and "next" work lists
// Offline variable substitution related things
@ -443,7 +532,8 @@ namespace {
return Index;
}
unsigned UniteNodes(unsigned First, unsigned Second);
unsigned UniteNodes(unsigned First, unsigned Second,
bool UnionByRank = true);
unsigned FindNode(unsigned Node);
void IdentifyObjects(Module &M);
@ -458,7 +548,7 @@ namespace {
void HVN();
void UnitePointerEquivalences();
void SolveConstraints();
void QueryNode(unsigned Node);
bool QueryNode(unsigned Node);
void Condense(unsigned Node);
void HUValNum(unsigned Node);
void HVNValNum(unsigned Node);
@ -503,6 +593,9 @@ namespace {
RegisterPass<Andersens> X("anders-aa",
"Andersen's Interprocedural Alias Analysis");
RegisterAnalysisGroup<AliasAnalysis> Y(X);
// Initialize Timestamp Counter (static).
unsigned Andersens::Node::Counter = 0;
}
ModulePass *llvm::createAndersensPass() { return new Andersens(); }
@ -981,9 +1074,15 @@ void Andersens::CollectConstraints(Module &M) {
UniversalSet));
// Memory objects passed into external function calls can have the
// universal set point to them.
#if FULL_UNIVERSAL
Constraints.push_back(Constraint(Constraint::Copy,
UniversalSet,
getNode(I)));
#else
Constraints.push_back(Constraint(Constraint::Copy,
getNode(I),
UniversalSet));
#endif
}
// If this is an external varargs function, it can also store pointers
@ -1139,9 +1238,17 @@ void Andersens::AddConstraintsForCall(CallSite CS, Function *F) {
UniversalSet));
}
} else if (F && isa<PointerType>(F->getFunctionType()->getReturnType())) {
#if FULL_UNIVERSAL
Constraints.push_back(Constraint(Constraint::Copy,
UniversalSet,
getNode(CallValue) + CallReturnPos));
#else
Constraints.push_back(Constraint(Constraint::Copy,
getNode(CallValue) + CallReturnPos,
UniversalSet));
#endif
}
CallSite::arg_iterator ArgI = CS.arg_begin(), ArgE = CS.arg_end();
@ -1159,9 +1266,15 @@ void Andersens::AddConstraintsForCall(CallSite CS, Function *F) {
UniversalSet));
}
} else if (isa<PointerType>((*ArgI)->getType())) {
#if FULL_UNIVERSAL
Constraints.push_back(Constraint(Constraint::Copy,
UniversalSet,
getNode(*ArgI)));
#else
Constraints.push_back(Constraint(Constraint::Copy,
getNode(*ArgI),
UniversalSet));
#endif
}
} else {
//Indirect Call
@ -1837,7 +1950,9 @@ unsigned Andersens::FindEquivalentNode(unsigned NodeIndex,
if (!GraphNodes[NodeIndex].AddressTaken) {
if (PEClass2Node[NodeLabel] != -1) {
// We found an existing node with the same pointer label, so unify them.
return UniteNodes(PEClass2Node[NodeLabel], NodeIndex);
// We specifically request that Union-By-Rank not be used so that
// PEClass2Node[NodeLabel] U= NodeIndex and not the other way around.
return UniteNodes(PEClass2Node[NodeLabel], NodeIndex, false);
} else {
PEClass2Node[NodeLabel] = NodeIndex;
PENLEClass2Node[NodeLabel] = NodeIndex;
@ -1952,7 +2067,7 @@ void Andersens::OptimizeConstraints() {
void Andersens::UnitePointerEquivalences() {
DOUT << "Uniting remaining pointer equivalences\n";
for (unsigned i = 0; i < GraphNodes.size(); ++i) {
if (GraphNodes[i].AddressTaken && GraphNodes[i].NodeRep == SelfRep) {
if (GraphNodes[i].AddressTaken && GraphNodes[i].isRep()) {
unsigned Label = GraphNodes[i].PointerEquivLabel;
if (Label && PENLEClass2Node[Label] != -1)
@ -1982,37 +2097,43 @@ void Andersens::CreateConstraintGraph() {
}
}
// Perform cycle detection, DFS, and RPO finding.
void Andersens::QueryNode(unsigned Node) {
assert(GraphNodes[Node].NodeRep == SelfRep && "Querying a non-rep node");
// Perform DFS and cycle detection.
bool Andersens::QueryNode(unsigned Node) {
assert(GraphNodes[Node].isRep() && "Querying a non-rep node");
unsigned OurDFS = ++DFSNumber;
SparseBitVector<> ToErase;
SparseBitVector<> NewEdges;
Node2DFS[Node] = OurDFS;
Tarjan2DFS[Node] = OurDFS;
// Changed denotes a change from a recursive call that we will bubble up.
// Merged is set if we actually merge a node ourselves.
bool Changed = false, Merged = false;
for (SparseBitVector<>::iterator bi = GraphNodes[Node].Edges->begin();
bi != GraphNodes[Node].Edges->end();
++bi) {
unsigned RepNode = FindNode(*bi);
// If we are going to add an edge to repnode, we have no need for the edge
// to e anymore.
// If this edge points to a non-representative node but we are
// already planning to add an edge to its representative, we have no
// need for this edge anymore.
if (RepNode != *bi && NewEdges.test(RepNode)){
ToErase.set(*bi);
continue;
}
// Continue about our DFS.
if (!Node2Deleted[RepNode]){
if (Node2DFS[RepNode] == 0) {
QueryNode(RepNode);
// May have been changed by query
if (!Tarjan2Deleted[RepNode]){
if (Tarjan2DFS[RepNode] == 0) {
Changed |= QueryNode(RepNode);
// May have been changed by QueryNode
RepNode = FindNode(RepNode);
}
if (Node2DFS[RepNode] < Node2DFS[Node])
Node2DFS[Node] = Node2DFS[RepNode];
if (Tarjan2DFS[RepNode] < Tarjan2DFS[Node])
Tarjan2DFS[Node] = Tarjan2DFS[RepNode];
}
// We may have just discovered that e belongs to a cycle, in which case we
// can also erase it.
// We may have just discovered that this node is part of a cycle, in
// which case we can also erase it.
if (RepNode != *bi) {
ToErase.set(*bi);
NewEdges.set(RepNode);
@ -2022,36 +2143,46 @@ void Andersens::QueryNode(unsigned Node) {
GraphNodes[Node].Edges->intersectWithComplement(ToErase);
GraphNodes[Node].Edges |= NewEdges;
// If this node is a root of a non-trivial SCC, place it on our worklist to be
// processed
if (OurDFS == Node2DFS[Node]) {
bool Changed = false;
while (!SCCStack.empty() && Node2DFS[SCCStack.top()] >= OurDFS) {
Node = UniteNodes(Node, FindNode(SCCStack.top()));
// If this node is a root of a non-trivial SCC, place it on our
// worklist to be processed.
if (OurDFS == Tarjan2DFS[Node]) {
while (!SCCStack.empty() && Tarjan2DFS[SCCStack.top()] >= OurDFS) {
Node = UniteNodes(Node, SCCStack.top());
SCCStack.pop();
Changed = true;
Merged = true;
}
Node2Deleted[Node] = true;
RPONumber++;
Tarjan2Deleted[Node] = true;
Topo2Node.at(GraphNodes.size() - RPONumber) = Node;
Node2Topo[Node] = GraphNodes.size() - RPONumber;
if (Changed)
GraphNodes[Node].Changed = true;
if (Merged)
NextWL->insert(&GraphNodes[Node]);
} else {
SCCStack.push(Node);
}
}
return(Changed | Merged);
}
/// SolveConstraints - This stage iteratively processes the constraints list
/// propagating constraints (adding edges to the Nodes in the points-to graph)
/// until a fixed point is reached.
///
/// We use a variant of the technique called "Lazy Cycle Detection", which is
/// described in "The Ant and the Grasshopper: Fast and Accurate Pointer
/// Analysis for Millions of Lines of Code. In Programming Language Design and
/// Implementation (PLDI), June 2007."
/// The paper describes performing cycle detection one node at a time, which can
/// be expensive if there are no cycles, but there are long chains of nodes that
/// it heuristically believes are cycles (because it will DFS from each node
/// without state from previous nodes).
/// Instead, we use the heuristic to build a worklist of nodes to check, then
/// cycle detect them all at the same time to do this more cheaply. This
/// catches cycles slightly later than the original technique did, but does it
/// make significantly cheaper.
void Andersens::SolveConstraints() {
bool Changed = true;
unsigned Iteration = 0;
CurrWL = &w1;
NextWL = &w2;
OptimizeConstraints();
#undef DEBUG_TYPE
@ -2069,55 +2200,66 @@ void Andersens::SolveConstraints() {
CreateConstraintGraph();
UnitePointerEquivalences();
assert(SCCStack.empty() && "SCC Stack should be empty by now!");
Topo2Node.insert(Topo2Node.begin(), GraphNodes.size(), Unvisited);
Node2Topo.insert(Node2Topo.begin(), GraphNodes.size(), Unvisited);
Node2DFS.clear();
Node2Deleted.clear();
Node2DFS.insert(Node2DFS.begin(), GraphNodes.size(), 0);
Node2Deleted.insert(Node2Deleted.begin(), GraphNodes.size(), false);
DFSNumber = 0;
RPONumber = 0;
// Order graph and mark starting nodes as changed.
DenseSet<Constraint, ConstraintKeyInfo> Seen;
DenseSet<std::pair<unsigned,unsigned>, PairKeyInfo> EdgesChecked;
// Order graph and add initial nodes to work list.
for (unsigned i = 0; i < GraphNodes.size(); ++i) {
unsigned N = FindNode(i);
Node *INode = &GraphNodes[i];
if (Node2DFS[N] == 0) {
QueryNode(N);
// Mark as changed if it's a representation and can contribute to the
// calculation right now.
if (INode->NodeRep == SelfRep && !INode->PointsTo->empty()
&& (!INode->Edges->empty() || !INode->Constraints.empty()))
INode->Changed = true;
// Add to work list if it's a representative and can contribute to the
// calculation right now.
if (INode->isRep() && !INode->PointsTo->empty()
&& (!INode->Edges->empty() || !INode->Constraints.empty())) {
INode->Stamp();
CurrWL->insert(INode);
}
}
std::queue<unsigned int> TarjanWL;
while( !CurrWL->empty() ) {
DOUT << "Starting iteration #" << ++NumIters << "\n";
do {
Changed = false;
++NumIters;
DOUT << "Starting iteration #" << Iteration++ << "\n";
// TODO: In the microoptimization category, we could just make Topo2Node
// a fast map and thus only contain the visited nodes.
for (unsigned i = 0; i < GraphNodes.size(); ++i) {
unsigned CurrNodeIndex = Topo2Node[i];
Node *CurrNode;
// We may not revisit all nodes on every iteration
if (CurrNodeIndex == Unvisited)
continue;
CurrNode = &GraphNodes[CurrNodeIndex];
// See if this is a node we need to process on this iteration
if (!CurrNode->Changed || CurrNode->NodeRep != SelfRep)
continue;
CurrNode->Changed = false;
Node* CurrNode;
unsigned CurrNodeIndex;
// Actual cycle checking code. We cycle check all of the lazy cycle
// candidates from the last iteration in one go.
if (!TarjanWL.empty()) {
DFSNumber = 0;
Tarjan2DFS.clear();
Tarjan2Deleted.clear();
while (!TarjanWL.empty()) {
unsigned int ToTarjan = TarjanWL.front();
TarjanWL.pop();
if (!Tarjan2Deleted[ToTarjan]
&& GraphNodes[ToTarjan].isRep()
&& Tarjan2DFS[ToTarjan] == 0)
QueryNode(ToTarjan);
}
}
// Add to work list if it's a representative and can contribute to the
// calculation right now.
while( (CurrNode = CurrWL->pop()) != NULL ) {
CurrNodeIndex = CurrNode - &GraphNodes[0];
CurrNode->Stamp();
// Figure out the changed points to bits
SparseBitVector<> CurrPointsTo;
CurrPointsTo.intersectWithComplement(CurrNode->PointsTo,
CurrNode->OldPointsTo);
if (CurrPointsTo.empty()){
if (CurrPointsTo.empty())
continue;
}
*(CurrNode->OldPointsTo) |= CurrPointsTo;
Seen.clear();
/* Now process the constraints for this node. */
for (std::list<Constraint>::iterator li = CurrNode->Constraints.begin();
@ -2125,7 +2267,16 @@ void Andersens::SolveConstraints() {
li->Src = FindNode(li->Src);
li->Dest = FindNode(li->Dest);
// TODO: We could delete redundant constraints here.
// Delete redundant constraints
if( Seen.count(*li) ) {
std::list<Constraint>::iterator lk = li; li++;
CurrNode->Constraints.erase(lk);
++NumErased;
continue;
}
Seen.insert(*li);
// Src and Dest will be the vars we are going to process.
// This may look a bit ugly, but what it does is allow us to process
// both store and load constraints with the same code.
@ -2173,15 +2324,14 @@ void Andersens::SolveConstraints() {
// Add an edge to the graph, so we can just do regular bitmap ior next
// time. It may also let us notice a cycle.
if (GraphNodes[*Src].Edges->test_and_set(*Dest)) {
if (GraphNodes[*Dest].PointsTo |= *(GraphNodes[*Src].PointsTo)) {
GraphNodes[*Dest].Changed = true;
// If we changed a node we've already processed, we need another
// iteration.
if (Node2Topo[*Dest] <= i)
Changed = true;
}
}
#if !FULL_UNIVERSAL
if (*Dest < NumberSpecialNodes)
continue;
#endif
if (GraphNodes[*Src].Edges->test_and_set(*Dest))
if (GraphNodes[*Dest].PointsTo |= *(GraphNodes[*Src].PointsTo))
NextWL->insert(&GraphNodes[*Dest]);
}
li++;
}
@ -2190,8 +2340,6 @@ void Andersens::SolveConstraints() {
// Now all we have left to do is propagate points-to info along the
// edges, erasing the redundant edges.
for (SparseBitVector<>::iterator bi = CurrNode->Edges->begin();
bi != CurrNode->Edges->end();
++bi) {
@ -2199,18 +2347,31 @@ void Andersens::SolveConstraints() {
unsigned DestVar = *bi;
unsigned Rep = FindNode(DestVar);
// If we ended up with this node as our destination, or we've already
// got an edge for the representative, delete the current edge.
if (Rep == CurrNodeIndex ||
(Rep != DestVar && NewEdges.test(Rep))) {
ToErase.set(DestVar);
continue;
// If we ended up with this node as our destination, or we've already
// got an edge for the representative, delete the current edge.
if (Rep == CurrNodeIndex ||
(Rep != DestVar && NewEdges.test(Rep))) {
ToErase.set(DestVar);
continue;
}
std::pair<unsigned,unsigned> edge(CurrNodeIndex,Rep);
// This is where we do lazy cycle detection.
// If this is a cycle candidate (equal points-to sets and this
// particular edge has not been cycle-checked previously), add to the
// list to check for cycles on the next iteration.
if (!EdgesChecked.count(edge) &&
*(GraphNodes[Rep].PointsTo) == *(CurrNode->PointsTo)) {
EdgesChecked.insert(edge);
TarjanWL.push(Rep);
}
// Union the points-to sets into the dest
#if !FULL_UNIVERSAL
if (Rep >= NumberSpecialNodes)
#endif
if (GraphNodes[Rep].PointsTo |= CurrPointsTo) {
GraphNodes[Rep].Changed = true;
if (Node2Topo[Rep] <= i)
Changed = true;
NextWL->insert(&GraphNodes[Rep]);
}
// If this edge's destination was collapsed, rewrite the edge.
if (Rep != DestVar) {
@ -2221,28 +2382,12 @@ void Andersens::SolveConstraints() {
CurrNode->Edges->intersectWithComplement(ToErase);
CurrNode->Edges |= NewEdges;
}
if (Changed) {
DFSNumber = RPONumber = 0;
Node2Deleted.clear();
Topo2Node.clear();
Node2Topo.clear();
Node2DFS.clear();
Topo2Node.insert(Topo2Node.begin(), GraphNodes.size(), Unvisited);
Node2Topo.insert(Node2Topo.begin(), GraphNodes.size(), Unvisited);
Node2DFS.insert(Node2DFS.begin(), GraphNodes.size(), 0);
Node2Deleted.insert(Node2Deleted.begin(), GraphNodes.size(), false);
// Rediscover the DFS/Topo ordering, and cycle detect.
for (unsigned j = 0; j < GraphNodes.size(); j++) {
unsigned JRep = FindNode(j);
if (Node2DFS[JRep] == 0)
QueryNode(JRep);
}
}
} while (Changed);
// Switch to other work list.
WorkList* t = CurrWL; CurrWL = NextWL; NextWL = t;
}
Node2Topo.clear();
Topo2Node.clear();
Node2DFS.clear();
Node2Deleted.clear();
for (unsigned i = 0; i < GraphNodes.size(); ++i) {
@ -2258,25 +2403,42 @@ void Andersens::SolveConstraints() {
// Unite nodes First and Second, returning the one which is now the
// representative node. First and Second are indexes into GraphNodes
unsigned Andersens::UniteNodes(unsigned First, unsigned Second) {
unsigned Andersens::UniteNodes(unsigned First, unsigned Second,
bool UnionByRank) {
assert (First < GraphNodes.size() && Second < GraphNodes.size() &&
"Attempting to merge nodes that don't exist");
// TODO: implement union by rank
Node *FirstNode = &GraphNodes[First];
Node *SecondNode = &GraphNodes[Second];
assert (SecondNode->NodeRep == SelfRep && FirstNode->NodeRep == SelfRep &&
assert (SecondNode->isRep() && FirstNode->isRep() &&
"Trying to unite two non-representative nodes!");
if (First == Second)
return First;
if (UnionByRank) {
int RankFirst = (int) FirstNode ->NodeRep;
int RankSecond = (int) SecondNode->NodeRep;
// Rank starts at -1 and gets decremented as it increases.
// Translation: higher rank, lower NodeRep value, which is always negative.
if (RankFirst > RankSecond) {
unsigned t = First; First = Second; Second = t;
Node* tp = FirstNode; FirstNode = SecondNode; SecondNode = tp;
} else if (RankFirst == RankSecond) {
FirstNode->NodeRep = (unsigned) (RankFirst - 1);
}
}
SecondNode->NodeRep = First;
FirstNode->Changed |= SecondNode->Changed;
#if !FULL_UNIVERSAL
if (First >= NumberSpecialNodes)
#endif
if (FirstNode->PointsTo && SecondNode->PointsTo)
FirstNode->PointsTo |= *(SecondNode->PointsTo);
if (FirstNode->Edges && SecondNode->Edges)
FirstNode->Edges |= *(SecondNode->Edges);
if (!FirstNode->Constraints.empty() && !SecondNode->Constraints.empty())
if (!SecondNode->Constraints.empty())
FirstNode->Constraints.splice(FirstNode->Constraints.begin(),
SecondNode->Constraints);
if (FirstNode->OldPointsTo) {
@ -2309,7 +2471,7 @@ unsigned Andersens::FindNode(unsigned NodeIndex) {
assert (NodeIndex < GraphNodes.size()
&& "Attempting to find a node that can't exist");
Node *N = &GraphNodes[NodeIndex];
if (N->NodeRep == SelfRep)
if (N->isRep())
return NodeIndex;
else
return (N->NodeRep = FindNode(N->NodeRep));