GRExprEngine.cpp

//===-- GRExprEngine.cpp - Simple, Path-Sens. Constant Prop. -----*- C++ -*-==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//               Constant Propagation via Graph Reachability
//
//  This files defines a simple analysis that performs path-sensitive
//  constant propagation within a function.  An example use of this analysis
//  is to perform simple checks for NULL dereferences.
//
//===----------------------------------------------------------------------===//

#include "ValueState.h"

#include "clang/Analysis/PathSensitive/GRCoreEngine.h"
#include "clang/Analysis/PathSensitive/GRTransferFuncs.h"
#include "GRSimpleVals.h"

#include "clang/AST/Expr.h"
#include "clang/AST/ASTContext.h"
#include "clang/Analysis/Analyses/LiveVariables.h"
#include "clang/Basic/Diagnostic.h"

#include "llvm/Support/Casting.h"
#include "llvm/Support/DataTypes.h"
#include "llvm/ADT/APSInt.h"
#include "llvm/ADT/FoldingSet.h"
#include "llvm/ADT/ImmutableMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Streams.h"

#include <functional>

#ifndef NDEBUG
#include "llvm/Support/GraphWriter.h"
#include <sstream>
#endif

using namespace clang;
using llvm::dyn_cast;
using llvm::cast;
using llvm::APSInt;

//===----------------------------------------------------------------------===//
// The Checker.
//
//  FIXME: This checker logic should be eventually broken into two components.
//         The first is the "meta"-level checking logic; the code that
//         does the Stmt visitation, fetching values from the map, etc.
//         The second part does the actual state manipulation.  This way we
//         get more of a separate of concerns of these two pieces, with the
//         latter potentially being refactored back into the main checking
//         logic.
//===----------------------------------------------------------------------===//

namespace {
  
class VISIBILITY_HIDDEN GRExprEngine {
    
public:
  typedef ValueStateManager::StateTy StateTy;
  typedef ExplodedGraph<GRExprEngine> GraphTy;
  typedef GraphTy::NodeTy NodeTy;
  
  // Builders.
  typedef GRStmtNodeBuilder<GRExprEngine> StmtNodeBuilder;
  typedef GRBranchNodeBuilder<GRExprEngine> BranchNodeBuilder;
  typedef GRIndirectGotoNodeBuilder<GRExprEngine> IndirectGotoNodeBuilder;
  typedef GRSwitchNodeBuilder<GRExprEngine> SwitchNodeBuilder;
  
  class NodeSet {
    typedef llvm::SmallVector<NodeTy*,3> ImplTy;
    ImplTy Impl;
  public:
    
    NodeSet() {}
    NodeSet(NodeTy* N) { assert (N && !N->isSink()); Impl.push_back(N); }
    
    void Add(NodeTy* N) { if (N && !N->isSink()) Impl.push_back(N); }
    
    typedef ImplTy::iterator       iterator;
    typedef ImplTy::const_iterator const_iterator;
        
    unsigned size() const { return Impl.size(); }
    bool empty() const { return Impl.empty(); }
    
    iterator begin() { return Impl.begin(); }
    iterator end()   { return Impl.end(); }

    const_iterator begin() const { return Impl.begin(); }
    const_iterator end() const { return Impl.end(); }
  };
                                                                
protected:
  /// G - the simulation graph.
  GraphTy& G;
  
  /// Liveness - live-variables information the ValueDecl* and block-level
  ///  Expr* in the CFG.  Used to prune out dead state.
  LiveVariables Liveness;

  /// Builder - The current GRStmtNodeBuilder which is used when building the
  ///  nodes for a given statement.
  StmtNodeBuilder* Builder;
  
  /// StateMgr - Object that manages the data for all created states.
  ValueStateManager StateMgr;
  
  /// ValueMgr - Object that manages the data for all created RValues.
  ValueManager& ValMgr;
  
  /// TF - Object that represents a bundle of transfer functions
  ///  for manipulating and creating RValues.
  GRTransferFuncs& TF;
  
  /// SymMgr - Object that manages the symbol information.
  SymbolManager& SymMgr;
  
  /// StmtEntryNode - The immediate predecessor node.
  NodeTy* StmtEntryNode;
  
  /// CurrentStmt - The current block-level statement.
  Stmt* CurrentStmt;
  
  /// UninitBranches - Nodes in the ExplodedGraph that result from
  ///  taking a branch based on an uninitialized value.
  typedef llvm::SmallPtrSet<NodeTy*,5> UninitBranchesTy;
  UninitBranchesTy UninitBranches;
  
  /// ImplicitNullDeref - Nodes in the ExplodedGraph that result from
  ///  taking a dereference on a symbolic pointer that may be NULL.
  typedef llvm::SmallPtrSet<NodeTy*,5> NullDerefTy;
  NullDerefTy ImplicitNullDeref;
  NullDerefTy ExplicitNullDeref;
  
  
  bool StateCleaned;
  
public:
  GRExprEngine(GraphTy& g) : 
      G(g), Liveness(G.getCFG(), G.getFunctionDecl()),
      Builder(NULL),
      StateMgr(G.getContext(), G.getAllocator()),
      ValMgr(StateMgr.getValueManager()),
      TF(*(new GRSimpleVals())), // FIXME.
      SymMgr(StateMgr.getSymbolManager()),
      StmtEntryNode(NULL), CurrentStmt(NULL) {
    
    // Compute liveness information.
    Liveness.runOnCFG(G.getCFG());
    Liveness.runOnAllBlocks(G.getCFG(), NULL, true);
  }
  
  /// getContext - Return the ASTContext associated with this analysis.
  ASTContext& getContext() const { return G.getContext(); }
  
  /// getCFG - Returns the CFG associated with this analysis.
  CFG& getCFG() { return G.getCFG(); }
  
  /// getInitialState - Return the initial state used for the root vertex
  ///  in the ExplodedGraph.
  StateTy getInitialState() {
    StateTy St = StateMgr.getInitialState();
    
    // Iterate the parameters.
    FunctionDecl& F = G.getFunctionDecl();
    
    for (FunctionDecl::param_iterator I=F.param_begin(), E=F.param_end(); 
          I!=E; ++I)
      St = SetValue(St, lval::DeclVal(*I), RValue::GetSymbolValue(SymMgr, *I));
    
    return St;
  }
  
  bool isUninitControlFlow(const NodeTy* N) const {
    return N->isSink() && UninitBranches.count(const_cast<NodeTy*>(N)) != 0;
  }
  
  bool isImplicitNullDeref(const NodeTy* N) const {
    return N->isSink() && ImplicitNullDeref.count(const_cast<NodeTy*>(N)) != 0;
  }
  
  bool isExplicitNullDeref(const NodeTy* N) const {
    return N->isSink() && ExplicitNullDeref.count(const_cast<NodeTy*>(N)) != 0;
  }
  
  typedef NullDerefTy::iterator null_iterator;
  null_iterator null_begin() { return ExplicitNullDeref.begin(); }
  null_iterator null_end() { return ExplicitNullDeref.end(); }

  /// ProcessStmt - Called by GRCoreEngine. Used to generate new successor
  ///  nodes by processing the 'effects' of a block-level statement.
  void ProcessStmt(Stmt* S, StmtNodeBuilder& builder);    
  
  /// ProcessBranch - Called by GRCoreEngine.  Used to generate successor
  ///  nodes by processing the 'effects' of a branch condition.
  void ProcessBranch(Expr* Condition, Stmt* Term, BranchNodeBuilder& builder);

  /// ProcessIndirectGoto - Called by GRCoreEngine.  Used to generate successor
  ///  nodes by processing the 'effects' of a computed goto jump.
  void ProcessIndirectGoto(IndirectGotoNodeBuilder& builder);
  
  /// ProcessSwitch - Called by GRCoreEngine.  Used to generate successor
  ///  nodes by processing the 'effects' of a switch statement.
  void ProcessSwitch(SwitchNodeBuilder& builder);  
  
  /// RemoveDeadBindings - Return a new state that is the same as 'St' except
  ///  that all subexpression mappings are removed and that any
  ///  block-level expressions that are not live at 'S' also have their
  ///  mappings removed.
  inline StateTy RemoveDeadBindings(Stmt* S, StateTy St) {
    return StateMgr.RemoveDeadBindings(St, S, Liveness);
  }

  StateTy SetValue(StateTy St, Expr* S, const RValue& V);

  StateTy SetValue(StateTy St, const Expr* S, const RValue& V) {
    return SetValue(St, const_cast<Expr*>(S), V);
  }
  
  /// SetValue - This version of SetValue is used to batch process a set
  ///  of different possible RValues and return a set of different states.
  const StateTy::BufferTy& SetValue(StateTy St, Expr* S,
                                    const RValue::BufferTy& V,
                                    StateTy::BufferTy& RetBuf);
  
  StateTy SetValue(StateTy St, const LValue& LV, const RValue& V);
  
  inline RValue GetValue(const StateTy& St, Expr* S) {
    return StateMgr.GetValue(St, S);
  }
  
  inline RValue GetValue(const StateTy& St, Expr* S, bool& hasVal) {
    return StateMgr.GetValue(St, S, &hasVal);
  }
  
  inline RValue GetValue(const StateTy& St, const Expr* S) {
    return GetValue(St, const_cast<Expr*>(S));
  }
  
  inline RValue GetValue(const StateTy& St, const LValue& LV,
                         QualType* T = NULL) {
    
    return StateMgr.GetValue(St, LV, T);
  }
  
  inline LValue GetLValue(const StateTy& St, Expr* S) {
    return StateMgr.GetLValue(St, S);
  }
  
  inline NonLValue GetRValueConstant(uint64_t X, Expr* E) {
    return NonLValue::GetValue(ValMgr, X, E->getType(), E->getLocStart());
  }
    
  /// Assume - Create new state by assuming that a given expression
  ///  is true or false.
  inline StateTy Assume(StateTy St, RValue Cond, bool Assumption, 
                        bool& isFeasible) {
    if (isa<LValue>(Cond))
      return Assume(St, cast<LValue>(Cond), Assumption, isFeasible);
    else
      return Assume(St, cast<NonLValue>(Cond), Assumption, isFeasible);
  }
  
  StateTy Assume(StateTy St, LValue Cond, bool Assumption, bool& isFeasible);
  StateTy Assume(StateTy St, NonLValue Cond, bool Assumption, bool& isFeasible);
  
  StateTy AssumeSymNE(StateTy St, SymbolID sym, const llvm::APSInt& V,
                      bool& isFeasible);

  StateTy AssumeSymEQ(StateTy St, SymbolID sym, const llvm::APSInt& V,
                      bool& isFeasible);
  
  StateTy AssumeSymInt(StateTy St, bool Assumption, const SymIntConstraint& C,
                       bool& isFeasible);
  
  NodeTy* Nodify(NodeSet& Dst, Stmt* S, NodeTy* Pred, StateTy St);
  
  /// Nodify - This version of Nodify is used to batch process a set of states.
  ///  The states are not guaranteed to be unique.
  void Nodify(NodeSet& Dst, Stmt* S, NodeTy* Pred, const StateTy::BufferTy& SB);
  
  /// Visit - Transfer function logic for all statements.  Dispatches to
  ///  other functions that handle specific kinds of statements.
  void Visit(Stmt* S, NodeTy* Pred, NodeSet& Dst);

  /// VisitBinaryOperator - Transfer function logic for binary operators.
  void VisitBinaryOperator(BinaryOperator* B, NodeTy* Pred, NodeSet& Dst);
  
  void VisitAssignmentLHS(Expr* E, NodeTy* Pred, NodeSet& Dst);

  /// VisitCast - Transfer function logic for all casts (implicit and explicit).
  void VisitCast(Expr* CastE, Expr* E, NodeTy* Pred, NodeSet& Dst);  
  
  /// VisitDeclRefExpr - Transfer function logic for DeclRefExprs.
  void VisitDeclRefExpr(DeclRefExpr* DR, NodeTy* Pred, NodeSet& Dst); 

  /// VisitDeclStmt - Transfer function logic for DeclStmts.
  void VisitDeclStmt(DeclStmt* DS, NodeTy* Pred, NodeSet& Dst); 
  
  /// VisitGuardedExpr - Transfer function logic for ?, __builtin_choose
  void VisitGuardedExpr(Expr* S, Expr* LHS, Expr* RHS,
                        NodeTy* Pred, NodeSet& Dst);
  
  /// VisitLogicalExpr - Transfer function logic for '&&', '||'
  void VisitLogicalExpr(BinaryOperator* B, NodeTy* Pred, NodeSet& Dst);
  
  /// VisitSizeOfAlignOfTypeExpr - Transfer function for sizeof(type).
  void VisitSizeOfAlignOfTypeExpr(SizeOfAlignOfTypeExpr* S, NodeTy* Pred,
                                  NodeSet& Dst);
  
  /// VisitUnaryOperator - Transfer function logic for unary operators.
  void VisitUnaryOperator(UnaryOperator* B, NodeTy* Pred, NodeSet& Dst);
  
  
  inline RValue EvalCast(ValueManager& ValMgr, RValue R, Expr* CastExpr) {
    return TF.EvalCast(ValMgr, R, CastExpr);
  }
  
  inline NonLValue EvalMinus(ValueManager& ValMgr, UnaryOperator* U,
                              NonLValue X) {
    return TF.EvalMinus(ValMgr, U, X);    
  }
  
  inline NonLValue EvalComplement(ValueManager& ValMgr, NonLValue X) {
    return TF.EvalComplement(ValMgr, X);
  }
  
  inline NonLValue EvalBinaryOp(ValueManager& ValMgr, BinaryOperator::Opcode Op,
                                NonLValue LHS, NonLValue RHS) {
    return TF.EvalBinaryOp(ValMgr, Op, LHS, RHS);
  }    
  
  inline RValue EvalBinaryOp(ValueManager& ValMgr, BinaryOperator::Opcode Op,
                             LValue LHS, LValue RHS) {
    return TF.EvalBinaryOp(ValMgr, Op, LHS, RHS);
  }
};
} // end anonymous namespace


GRExprEngine::StateTy
GRExprEngine::SetValue(StateTy St, Expr* S, const RValue& V) {

  if (!StateCleaned) {
    St = RemoveDeadBindings(CurrentStmt, St);
    StateCleaned = true;
  }

  bool isBlkExpr = false;
    
  if (S == CurrentStmt) {
    isBlkExpr = getCFG().isBlkExpr(S);
    
    if (!isBlkExpr)
      return St;
  }

  return StateMgr.SetValue(St, S, isBlkExpr, V);
}

const GRExprEngine::StateTy::BufferTy&
GRExprEngine::SetValue(StateTy St, Expr* S, const RValue::BufferTy& RB,
                      StateTy::BufferTy& RetBuf) {
  
  assert (RetBuf.empty());
  
  for (RValue::BufferTy::const_iterator I=RB.begin(), E=RB.end(); I!=E; ++I)
    RetBuf.push_back(SetValue(St, S, *I));
                     
  return RetBuf;
}

GRExprEngine::StateTy
GRExprEngine::SetValue(StateTy St, const LValue& LV, const RValue& V) {
  
  if (LV.isUnknown())
    return St;
  
  if (!StateCleaned) {
    St = RemoveDeadBindings(CurrentStmt, St);
    StateCleaned = true;
  }
  
  return StateMgr.SetValue(St, LV, V);
}

void GRExprEngine::ProcessBranch(Expr* Condition, Stmt* Term,
                                BranchNodeBuilder& builder) {

  // Remove old bindings for subexpressions.
  StateTy PrevState = StateMgr.RemoveSubExprBindings(builder.getState());
  
  RValue V = GetValue(PrevState, Condition);
  
  switch (V.getBaseKind()) {
    default:
      break;

    case RValue::UnknownKind:
      builder.generateNode(PrevState, true);
      builder.generateNode(PrevState, false);
      return;
      
    case RValue::UninitializedKind: {      
      NodeTy* N = builder.generateNode(PrevState, true);

      if (N) {
        N->markAsSink();
        UninitBranches.insert(N);
      }
      
      builder.markInfeasible(false);
      return;
    }      
  }

  // Get the current block counter.
  GRBlockCounter BC = builder.getBlockCounter();
    
  unsigned BlockID = builder.getTargetBlock(true)->getBlockID();
  unsigned NumVisited = BC.getNumVisited(BlockID);
  
  if (isa<nonlval::ConcreteInt>(V) || 
      BC.getNumVisited(builder.getTargetBlock(true)->getBlockID()) < 1) {
    
    // Process the true branch.

    bool isFeasible = true;
    
    StateTy St = Assume(PrevState, V, true, isFeasible);

    if (isFeasible)
      builder.generateNode(St, true);
    else
      builder.markInfeasible(true);
  }
  else
    builder.markInfeasible(true);
  
  BlockID = builder.getTargetBlock(false)->getBlockID();
  NumVisited = BC.getNumVisited(BlockID);
  
  if (isa<nonlval::ConcreteInt>(V) || 
      BC.getNumVisited(builder.getTargetBlock(false)->getBlockID()) < 1) {
    
    // Process the false branch.  
    
    bool isFeasible = false;
    
    StateTy St = Assume(PrevState, V, false, isFeasible);
    
    if (isFeasible)
      builder.generateNode(St, false);
    else
      builder.markInfeasible(false);
  }
  else
    builder.markInfeasible(false);
}

/// ProcessIndirectGoto - Called by GRCoreEngine.  Used to generate successor
///  nodes by processing the 'effects' of a computed goto jump.
void GRExprEngine::ProcessIndirectGoto(IndirectGotoNodeBuilder& builder) {

  StateTy St = builder.getState();  
  LValue V = cast<LValue>(GetValue(St, builder.getTarget()));
  
  // Three possibilities:
  //
  //   (1) We know the computed label.
  //   (2) The label is NULL (or some other constant), or Uninitialized.
  //   (3) We have no clue about the label.  Dispatch to all targets.
  //
  
  typedef IndirectGotoNodeBuilder::iterator iterator;

  if (isa<lval::GotoLabel>(V)) {
    LabelStmt* L = cast<lval::GotoLabel>(V).getLabel();
    
    for (iterator I=builder.begin(), E=builder.end(); I != E; ++I) {
      if (I.getLabel() == L) {
        builder.generateNode(I, St);
        return;
      }
    }
    
    assert (false && "No block with label.");
    return;
  }

  if (isa<lval::ConcreteInt>(V) || isa<UninitializedVal>(V)) {
    // Dispatch to the first target and mark it as a sink.
    NodeTy* N = builder.generateNode(builder.begin(), St, true);
    UninitBranches.insert(N);
    return;
  }
  
  // This is really a catch-all.  We don't support symbolics yet.
  
  assert (isa<UnknownVal>(V));
  
  for (iterator I=builder.begin(), E=builder.end(); I != E; ++I)
    builder.generateNode(I, St);
}

/// ProcessSwitch - Called by GRCoreEngine.  Used to generate successor
///  nodes by processing the 'effects' of a switch statement.
void GRExprEngine::ProcessSwitch(SwitchNodeBuilder& builder) {
  
  typedef SwitchNodeBuilder::iterator iterator;
  
  StateTy St = builder.getState();  
  NonLValue CondV = cast<NonLValue>(GetValue(St, builder.getCondition()));

  if (isa<UninitializedVal>(CondV)) {
    NodeTy* N = builder.generateDefaultCaseNode(St, true);
    UninitBranches.insert(N);
    return;
  }
  
  StateTy  DefaultSt = St;
  
  // While most of this can be assumed (such as the signedness), having it
  // just computed makes sure everything makes the same assumptions end-to-end.
  unsigned bits = getContext().getTypeSize(getContext().IntTy,SourceLocation());
  APSInt V1(bits, false);
  APSInt V2 = V1;
  
  for (iterator I=builder.begin(), E=builder.end(); I!=E; ++I) {

    CaseStmt* Case = cast<CaseStmt>(I.getCase());
    
    // Evaluate the case.
    if (!Case->getLHS()->isIntegerConstantExpr(V1, getContext(), 0, true)) {
      assert (false && "Case condition must evaluate to an integer constant.");
      return;
    }
    
    // Get the RHS of the case, if it exists.
    
    if (Expr* E = Case->getRHS()) {
      if (!E->isIntegerConstantExpr(V2, getContext(), 0, true)) {
        assert (false &&
                "Case condition (RHS) must evaluate to an integer constant.");
        return ;
      }
      
      assert (V1 <= V2);
    }
    else V2 = V1;
    
    // FIXME: Eventually we should replace the logic below with a range
    //  comparison, rather than concretize the values within the range.
    //  This should be easy once we have "ranges" for NonLValues.
        
    do {      
      nonlval::ConcreteInt CaseVal(ValMgr.getValue(V1));
      
      NonLValue Res = EvalBinaryOp(ValMgr, BinaryOperator::EQ, CondV, CaseVal);
      
      // Now "assume" that the case matches.
      bool isFeasible;
      
      StateTy StNew = Assume(St, Res, true, isFeasible);
      
      if (isFeasible) {
        builder.generateCaseStmtNode(I, StNew);
       
        // If CondV evaluates to a constant, then we know that this
        // is the *only* case that we can take, so stop evaluating the
        // others.
        if (isa<nonlval::ConcreteInt>(CondV))
          return;
      }
      
      // Now "assume" that the case doesn't match.  Add this state
      // to the default state (if it is feasible).
      
      StNew = Assume(DefaultSt, Res, false, isFeasible);
      
      if (isFeasible)
        DefaultSt = StNew;

      // Concretize the next value in the range.      
      ++V1;
      
    } while (V1 < V2);
  }
  
  // If we reach here, than we know that the default branch is
  // possible.  
  builder.generateDefaultCaseNode(DefaultSt);
}


void GRExprEngine::VisitLogicalExpr(BinaryOperator* B, NodeTy* Pred,
                                   NodeSet& Dst) {

  bool hasR2;
  StateTy PrevState = Pred->getState();

  RValue R1 = GetValue(PrevState, B->getLHS());
  RValue R2 = GetValue(PrevState, B->getRHS(), hasR2);
    
  if (isa<UnknownVal>(R1) && 
       (isa<UnknownVal>(R2) ||
        isa<UninitializedVal>(R2))) {    

    Nodify(Dst, B, Pred, SetValue(PrevState, B, R2));
    return;
  }    
  else if (isa<UninitializedVal>(R1)) {
    Nodify(Dst, B, Pred, SetValue(PrevState, B, R1));
    return;
  }

  // R1 is an expression that can evaluate to either 'true' or 'false'.
  if (B->getOpcode() == BinaryOperator::LAnd) {
    // hasR2 == 'false' means that LHS evaluated to 'false' and that
    // we short-circuited, leading to a value of '0' for the '&&' expression.
    if (hasR2 == false) { 
      Nodify(Dst, B, Pred, SetValue(PrevState, B, GetRValueConstant(0U, B)));
      return;
    }
  }
  else {
    assert (B->getOpcode() == BinaryOperator::LOr);
    // hasR2 == 'false' means that the LHS evaluate to 'true' and that
    //  we short-circuited, leading to a value of '1' for the '||' expression.
    if (hasR2 == false) {
      Nodify(Dst, B, Pred, SetValue(PrevState, B, GetRValueConstant(1U, B)));
      return;      
    }
  }
    
  // If we reach here we did not short-circuit.  Assume R2 == true and
  // R2 == false.
    
  bool isFeasible;
  StateTy St = Assume(PrevState, R2, true, isFeasible);
  
  if (isFeasible)
    Nodify(Dst, B, Pred, SetValue(PrevState, B, GetRValueConstant(1U, B)));

  St = Assume(PrevState, R2, false, isFeasible);
  
  if (isFeasible)
    Nodify(Dst, B, Pred, SetValue(PrevState, B, GetRValueConstant(0U, B)));  
}



void GRExprEngine::ProcessStmt(Stmt* S, StmtNodeBuilder& builder) {
  Builder = &builder;

  StmtEntryNode = builder.getLastNode();
  CurrentStmt = S;
  NodeSet Dst;
  StateCleaned = false;

  Visit(S, StmtEntryNode, Dst);

  // If no nodes were generated, generate a new node that has all the
  // dead mappings removed.
  if (Dst.size() == 1 && *Dst.begin() == StmtEntryNode) {
    StateTy St = RemoveDeadBindings(S, StmtEntryNode->getState());
    builder.generateNode(S, St, StmtEntryNode);
  }
  
  CurrentStmt = NULL;
  StmtEntryNode = NULL;
  Builder = NULL;
}

GRExprEngine::NodeTy*
GRExprEngine::Nodify(NodeSet& Dst, Stmt* S, NodeTy* Pred, StateTy St) {
 
  // If the state hasn't changed, don't generate a new node.
  if (St == Pred->getState())
    return NULL;
  
  NodeTy* N = Builder->generateNode(S, St, Pred);
  Dst.Add(N);
  return N;
}

void GRExprEngine::Nodify(NodeSet& Dst, Stmt* S, NodeTy* Pred,
                         const StateTy::BufferTy& SB) {
  
  for (StateTy::BufferTy::const_iterator I=SB.begin(), E=SB.end(); I!=E; ++I)
    Nodify(Dst, S, Pred, *I);
}

void GRExprEngine::VisitDeclRefExpr(DeclRefExpr* D, NodeTy* Pred, NodeSet& Dst){
  if (D != CurrentStmt) {
    Dst.Add(Pred); // No-op. Simply propagate the current state unchanged.
    return;
  }
  
  // If we are here, we are loading the value of the decl and binding
  // it to the block-level expression.
  
  StateTy St = Pred->getState();
  
  Nodify(Dst, D, Pred,
         SetValue(St, D, GetValue(St, lval::DeclVal(D->getDecl()))));
}

void GRExprEngine::VisitCast(Expr* CastE, Expr* E, NodeTy* Pred, NodeSet& Dst) {
  
  QualType T = CastE->getType();

  // Check for redundant casts.
  if (E->getType() == T) {
    Dst.Add(Pred);
    return;
  }
  
  NodeSet S1;
  Visit(E, Pred, S1);
  
  for (NodeSet::iterator I1=S1.begin(), E1=S1.end(); I1 != E1; ++I1) {
    NodeTy* N = *I1;
    StateTy St = N->getState();
    const RValue& V = GetValue(St, E);
    Nodify(Dst, CastE, N, SetValue(St, CastE, EvalCast(ValMgr, V, CastE)));
  }
}

void GRExprEngine::VisitDeclStmt(DeclStmt* DS, GRExprEngine::NodeTy* Pred,
                                GRExprEngine::NodeSet& Dst) {
  
  StateTy St = Pred->getState();
  
  for (const ScopedDecl* D = DS->getDecl(); D; D = D->getNextDeclarator())
    if (const VarDecl* VD = dyn_cast<VarDecl>(D)) {
      const Expr* E = VD->getInit();      
      St = SetValue(St, lval::DeclVal(VD),
                    E ? GetValue(St, E) : UninitializedVal());
    }

  Nodify(Dst, DS, Pred, St);
  
  if (Dst.empty())
    Dst.Add(Pred);  
}


void GRExprEngine::VisitGuardedExpr(Expr* S, Expr* LHS, Expr* RHS,
                                   NodeTy* Pred, NodeSet& Dst) {
  
  StateTy St = Pred->getState();
  
  RValue R = GetValue(St, LHS);
  if (isa<UnknownVal>(R)) R = GetValue(St, RHS);
  
  Nodify(Dst, S, Pred, SetValue(St, S, R));
}

/// VisitSizeOfAlignOfTypeExpr - Transfer function for sizeof(type).
void GRExprEngine::VisitSizeOfAlignOfTypeExpr(SizeOfAlignOfTypeExpr* S,
                                             NodeTy* Pred,
                                             NodeSet& Dst) {
  
  // 6.5.3.4 sizeof: "The result type is an integer."
  
  QualType T = S->getArgumentType();
  
  // FIXME: Add support for VLAs.
  if (isa<VariableArrayType>(T.getTypePtr()))
    return;
  
  SourceLocation L = S->getExprLoc();
  uint64_t size = getContext().getTypeSize(T, L) / 8;
  
  Nodify(Dst, S, Pred,
         SetValue(Pred->getState(), S,
                  NonLValue::GetValue(ValMgr, size, getContext().IntTy, L)));
  
}

void GRExprEngine::VisitUnaryOperator(UnaryOperator* U,
                                     GRExprEngine::NodeTy* Pred,
                                     GRExprEngine::NodeSet& Dst) {
  
  NodeSet S1;
  UnaryOperator::Opcode Op = U->getOpcode();
  
  // FIXME: This is a hack so that for '*' and '&' we don't recurse
  //  on visiting the subexpression if it is a DeclRefExpr.  We should
  //  probably just handle AddrOf and Deref in their own methods to make
  //  this cleaner.
  if ((Op == UnaryOperator::Deref || Op == UnaryOperator::AddrOf) &&
      isa<DeclRefExpr>(U->getSubExpr()))
    S1.Add(Pred);
  else
    Visit(U->getSubExpr(), Pred, S1);
    
  for (NodeSet::iterator I1=S1.begin(), E1=S1.end(); I1 != E1; ++I1) {
    NodeTy* N1 = *I1;
    StateTy St = N1->getState();
    
    switch (U->getOpcode()) {
      case UnaryOperator::PostInc: {
        const LValue& L1 = GetLValue(St, U->getSubExpr());
        NonLValue R1 = cast<NonLValue>(GetValue(St, L1));
        
        NonLValue Result = EvalBinaryOp(ValMgr, BinaryOperator::Add,
                                        R1, GetRValueConstant(1U, U));
        
        Nodify(Dst, U, N1, SetValue(SetValue(St, U, R1), L1, Result));
        break;
      }
        
      case UnaryOperator::PostDec: {
        const LValue& L1 = GetLValue(St, U->getSubExpr());
        NonLValue R1 = cast<NonLValue>(GetValue(St, L1));
        
        NonLValue Result = EvalBinaryOp(ValMgr, BinaryOperator::Sub,
                                        R1, GetRValueConstant(1U, U));
        
        Nodify(Dst, U, N1, SetValue(SetValue(St, U, R1), L1, Result));
        break;
      }
        
      case UnaryOperator::PreInc: {
        const LValue& L1 = GetLValue(St, U->getSubExpr());
        NonLValue R1 = cast<NonLValue>(GetValue(St, L1));
        
        NonLValue Result = EvalBinaryOp(ValMgr, BinaryOperator::Add,
                                        R1, GetRValueConstant(1U, U));
        
        Nodify(Dst, U, N1, SetValue(SetValue(St, U, Result), L1, Result));
        break;
      }
        
      case UnaryOperator::PreDec: {
        const LValue& L1 = GetLValue(St, U->getSubExpr());
        NonLValue R1 = cast<NonLValue>(GetValue(St, L1));
        
        NonLValue Result = EvalBinaryOp(ValMgr, BinaryOperator::Sub,
                                        R1, GetRValueConstant(1U, U));

        Nodify(Dst, U, N1, SetValue(SetValue(St, U, Result), L1, Result));
        break;
      }
        
      case UnaryOperator::Minus: {
        const NonLValue& R1 = cast<NonLValue>(GetValue(St, U->getSubExpr()));
        Nodify(Dst, U, N1, SetValue(St, U, EvalMinus(ValMgr, U, R1)));
        break;
      }
        
      case UnaryOperator::Not: {
        const NonLValue& R1 = cast<NonLValue>(GetValue(St, U->getSubExpr()));
        Nodify(Dst, U, N1, SetValue(St, U, EvalComplement(ValMgr, R1)));
        break;
      }
        
      case UnaryOperator::LNot: {
        // C99 6.5.3.3: "The expression !E is equivalent to (0==E)."
        //
        //  Note: technically we do "E == 0", but this is the same in the
        //    transfer functions as "0 == E".
        
        RValue V1 = GetValue(St, U->getSubExpr());
        
        if (isa<LValue>(V1)) {
          const LValue& L1 = cast<LValue>(V1);
          lval::ConcreteInt V2(ValMgr.getZeroWithPtrWidth());
          Nodify(Dst, U, N1,
                 SetValue(St, U, EvalBinaryOp(ValMgr, BinaryOperator::EQ,
                                              L1, V2)));
        }
        else {
          const NonLValue& R1 = cast<NonLValue>(V1);
          nonlval::ConcreteInt V2(ValMgr.getZeroWithPtrWidth());
          Nodify(Dst, U, N1,
                 SetValue(St, U, EvalBinaryOp(ValMgr, BinaryOperator::EQ,
                                              R1, V2)));
        }
        
        break;
      }
      
      case UnaryOperator::SizeOf: {
        // 6.5.3.4 sizeof: "The result type is an integer."
        
        QualType T = U->getSubExpr()->getType();
        
        // FIXME: Add support for VLAs.
        if (isa<VariableArrayType>(T.getTypePtr()))
          return;
        
        SourceLocation L = U->getExprLoc();
        uint64_t size = getContext().getTypeSize(T, L) / 8;
                
        Nodify(Dst, U, N1,
               SetValue(St, U, NonLValue::GetValue(ValMgr, size,
                                                   getContext().IntTy, L)));
        
        break;
      }
        
      case UnaryOperator::AddrOf: {
        const LValue& L1 = GetLValue(St, U->getSubExpr());
        Nodify(Dst, U, N1, SetValue(St, U, L1));
        break;
      }
        
      case UnaryOperator::Deref: {
        // FIXME: Stop when dereferencing an uninitialized value.
        // FIXME: Bifurcate when dereferencing a symbolic with no constraints?
        
        const RValue& V = GetValue(St, U->getSubExpr());
        const LValue& L1 = cast<LValue>(V);
        
        // After a dereference, one of two possible situations arise:
        //  (1) A crash, because the pointer was NULL.
        //  (2) The pointer is not NULL, and the dereference works.
        // 
        // We add these assumptions.
                
        bool isFeasibleNotNull;
       
        // "Assume" that the pointer is Not-NULL.
        StateTy StNotNull = Assume(St, L1, true, isFeasibleNotNull);
        
        if (isFeasibleNotNull) {
          QualType T = U->getType();
          Nodify(Dst, U, N1, SetValue(StNotNull, U,
                                      GetValue(StNotNull, L1, &T)));
        }
        
        bool isFeasibleNull;
        
        // "Assume" that the pointer is NULL.
        StateTy StNull = Assume(St, L1, false, isFeasibleNull);
        
        if (isFeasibleNull) {
          // We don't use "Nodify" here because the node will be a sink
          // and we have no intention of processing it later.
          NodeTy* NullNode = Builder->generateNode(U, StNull, N1);

          if (NullNode) {
            NullNode->markAsSink();
            
            if (isFeasibleNotNull)
              ImplicitNullDeref.insert(NullNode);
            else
              ExplicitNullDeref.insert(NullNode);
          }
        }
        
        break;
      }
        
      default: ;
        assert (false && "Not implemented.");
    }    
  }
}

void GRExprEngine::VisitAssignmentLHS(Expr* E, GRExprEngine::NodeTy* Pred,
                                     GRExprEngine::NodeSet& Dst) {

  if (isa<DeclRefExpr>(E)) {
    Dst.Add(Pred);
    return;
  }
  
  if (UnaryOperator* U = dyn_cast<UnaryOperator>(E)) {
    if (U->getOpcode() == UnaryOperator::Deref) {
      Visit(U->getSubExpr(), Pred, Dst);
      return;
    }
  }
  
  Visit(E, Pred, Dst);
}

void GRExprEngine::VisitBinaryOperator(BinaryOperator* B,
                                       GRExprEngine::NodeTy* Pred,
                                       GRExprEngine::NodeSet& Dst) {
  NodeSet S1;
  
  if (B->isAssignmentOp())
    VisitAssignmentLHS(B->getLHS(), Pred, S1);
  else
    Visit(B->getLHS(), Pred, S1);