GRExprEngine.cpp

//=-- GRExprEngine.cpp - Path-Sensitive Expression-Level Dataflow ---*- C++ -*-=
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file defines a meta-engine for path-sensitive dataflow analysis that
//  is built on GREngine, but provides the boilerplate to execute transfer
//  functions and build the ExplodedGraph at the expression level.
//
//===----------------------------------------------------------------------===//

#include "clang/Analysis/PathSensitive/GRExprEngine.h"
#include "clang/Analysis/PathSensitive/BugReporter.h"
#include "clang/Basic/SourceManager.h"
#include "llvm/Support/Streams.h"

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

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

//===----------------------------------------------------------------------===//
// Engine construction and deletion.
//===----------------------------------------------------------------------===//

static inline Selector GetNullarySelector(const char* name, ASTContext& Ctx) {
  IdentifierInfo* II = &Ctx.Idents.get(name);
  return Ctx.Selectors.getSelector(0, &II);
}


GRExprEngine::GRExprEngine(CFG& cfg, Decl& CD, ASTContext& Ctx)
  : CoreEngine(cfg, CD, Ctx, *this), 
    G(CoreEngine.getGraph()),
    Liveness(G.getCFG()),
    Builder(NULL),
    StateMgr(G.getContext(), G.getAllocator()),
    BasicVals(StateMgr.getBasicValueFactory()),
    TF(NULL), // FIXME
    SymMgr(StateMgr.getSymbolManager()),
    CurrentStmt(NULL),
  NSExceptionII(NULL), NSExceptionInstanceRaiseSelectors(NULL),
  RaiseSel(GetNullarySelector("raise", G.getContext())) {
  
  // Compute liveness information.
  Liveness.runOnCFG(G.getCFG());
  Liveness.runOnAllBlocks(G.getCFG(), NULL, true);
}

GRExprEngine::~GRExprEngine() {
  for (BugTypeSet::iterator I = BugTypes.begin(), E = BugTypes.end(); I!=E; ++I)
    delete *I;
    
  for (SimpleChecksTy::iterator I = CallChecks.begin(), E = CallChecks.end();
       I != E; ++I)
    delete *I;
  
  for (SimpleChecksTy::iterator I=MsgExprChecks.begin(), E=MsgExprChecks.end();
       I != E; ++I)
    delete *I;  
  
  delete [] NSExceptionInstanceRaiseSelectors;
}

//===----------------------------------------------------------------------===//
// Utility methods.
//===----------------------------------------------------------------------===//

// SaveAndRestore - A utility class that uses RIIA to save and restore
//  the value of a variable.
template<typename T>
struct VISIBILITY_HIDDEN SaveAndRestore {
  SaveAndRestore(T& x) : X(x), old_value(x) {}
  ~SaveAndRestore() { X = old_value; }
  T get() { return old_value; }
  
  T& X;
  T old_value;
};

// SaveOr - Similar to SaveAndRestore.  Operates only on bools; the old
//  value of a variable is saved, and during the dstor the old value is
//  or'ed with the new value.
struct VISIBILITY_HIDDEN SaveOr {
  SaveOr(bool& x) : X(x), old_value(x) { x = false; }
  ~SaveOr() { X |= old_value; }
  
  bool& X;
  bool old_value;
};


void GRExprEngine::EmitWarnings(Diagnostic& Diag, PathDiagnosticClient* PD) {
  for (bug_type_iterator I = bug_types_begin(), E = bug_types_end(); I!=E; ++I){
    BugReporter BR(Diag, PD, getContext(), *this);
    (*I)->EmitWarnings(BR);
  }
  
  for (SimpleChecksTy::iterator I = CallChecks.begin(), E = CallChecks.end();
       I != E; ++I) {
    BugReporter BR(Diag, PD, getContext(), *this);
    (*I)->EmitWarnings(BR);
  }
  
  for (SimpleChecksTy::iterator I=MsgExprChecks.begin(), E=MsgExprChecks.end();
       I != E; ++I) {
    BugReporter BR(Diag, PD, getContext(), *this);
    (*I)->EmitWarnings(BR);
  }
}

void GRExprEngine::setTransferFunctions(GRTransferFuncs* tf) {
  TF = tf;
  TF->RegisterChecks(*this);
}

void GRExprEngine::AddCallCheck(GRSimpleAPICheck* A) {
  CallChecks.push_back(A);
}

void GRExprEngine::AddObjCMessageExprCheck(GRSimpleAPICheck* A) {
  MsgExprChecks.push_back(A);
}

ValueState* GRExprEngine::getInitialState() {

  // The LiveVariables information already has a compilation of all VarDecls
  // used in the function.  Iterate through this set, and "symbolicate"
  // any VarDecl whose value originally comes from outside the function.
  
  typedef LiveVariables::AnalysisDataTy LVDataTy;
  LVDataTy& D = Liveness.getAnalysisData();
  
  ValueState StateImpl = *StateMgr.getInitialState();
  
  for (LVDataTy::decl_iterator I=D.begin_decl(), E=D.end_decl(); I != E; ++I) {
    
    ScopedDecl *SD = const_cast<ScopedDecl*>(I->first);
    if (VarDecl* VD = dyn_cast<VarDecl>(SD)) {
      if (VD->hasGlobalStorage() || isa<ParmVarDecl>(VD)) {
        RVal X = RVal::GetSymbolValue(SymMgr, VD);
        StateMgr.BindVar(StateImpl, VD, X);
      }
    } else if (ImplicitParamDecl *IPD = dyn_cast<ImplicitParamDecl>(SD)) {
        RVal X = RVal::GetSymbolValue(SymMgr, IPD);
        StateMgr.BindVar(StateImpl, IPD, X);
    }
      

  }
  
  return StateMgr.getPersistentState(StateImpl);
}      
      
ValueState* GRExprEngine::SetRVal(ValueState* St, Expr* Ex, RVal V) {

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

  return StateMgr.SetRVal(St, Ex, V, isBlkExpr, true);
}

//===----------------------------------------------------------------------===//
// Top-level transfer function logic (Dispatcher).
//===----------------------------------------------------------------------===//

void GRExprEngine::ProcessStmt(Stmt* S, StmtNodeBuilder& builder) {
  
  Builder = &builder;
  EntryNode = builder.getLastNode();
  CurrentStmt = S;
  
  // Set up our simple checks.
  
  // FIXME: This can probably be installed directly in GRCoreEngine, obviating
  //  the need to do a copy every time we hit a block-level statement.
  
  if (!MsgExprChecks.empty())
    Builder->setObjCMsgExprAuditors((GRAuditor<ValueState>**) &MsgExprChecks[0],
         (GRAuditor<ValueState>**) (&MsgExprChecks[0] + MsgExprChecks.size()));
  
  
  if (!CallChecks.empty())
    Builder->setCallExprAuditors((GRAuditor<ValueState>**) &CallChecks[0],
         (GRAuditor<ValueState>**) (&CallChecks[0] + CallChecks.size()));
  
  // Create the cleaned state.
  
  CleanedState = StateMgr.RemoveDeadBindings(EntryNode->getState(), CurrentStmt,
                                             Liveness, DeadSymbols);
  
  // Process any special transfer function for dead symbols.
  
  NodeSet Tmp;
  
  if (DeadSymbols.empty())
    Tmp.Add(EntryNode);
  else {
    SaveAndRestore<bool> OldSink(Builder->BuildSinks);
    SaveOr OldHasGen(Builder->HasGeneratedNode);

    SaveAndRestore<bool> OldPurgeDeadSymbols(Builder->PurgingDeadSymbols);
    Builder->PurgingDeadSymbols = true;
    
    TF->EvalDeadSymbols(Tmp, *this, *Builder, EntryNode, S, 
                        CleanedState, DeadSymbols);

    if (!Builder->BuildSinks && !Builder->HasGeneratedNode)
      Tmp.Add(EntryNode);
  }
  
  bool HasAutoGenerated = false;

  for (NodeSet::iterator I=Tmp.begin(), E=Tmp.end(); I!=E; ++I) {

    NodeSet Dst;
    
    // Set the cleaned state.  
    Builder->SetCleanedState(*I == EntryNode ? CleanedState : GetState(*I));
    
    // Visit the statement.  
    Visit(S, *I, Dst);

    // Do we need to auto-generate a node?  We only need to do this to generate
    // a node with a "cleaned" state; GRCoreEngine will actually handle
    // auto-transitions for other cases.    
    if (Dst.size() == 1 && *Dst.begin() == EntryNode
        && !Builder->HasGeneratedNode && !HasAutoGenerated) {
      HasAutoGenerated = true;
      builder.generateNode(S, GetState(EntryNode), *I);
    }
  }
  
  // NULL out these variables to cleanup.
  CleanedState = NULL;
  EntryNode = NULL;
  CurrentStmt = NULL;
  Builder = NULL;
}

void GRExprEngine::Visit(Stmt* S, NodeTy* Pred, NodeSet& Dst) {
  
  // FIXME: add metadata to the CFG so that we can disable
  //  this check when we KNOW that there is no block-level subexpression.
  //  The motivation is that this check requires a hashtable lookup.
  
  if (S != CurrentStmt && getCFG().isBlkExpr(S)) {
    Dst.Add(Pred);
    return;
  }
  
  switch (S->getStmtClass()) {
      
    default:
      // Cases we intentionally have "default" handle:
      //   AddrLabelExpr, IntegerLiteral, CharacterLiteral
      
      Dst.Add(Pred); // No-op. Simply propagate the current state unchanged.
      break;
    
    case Stmt::ArraySubscriptExprClass:
      VisitArraySubscriptExpr(cast<ArraySubscriptExpr>(S), Pred, Dst, false);
      break;
      
    case Stmt::AsmStmtClass:
      VisitAsmStmt(cast<AsmStmt>(S), Pred, Dst);
      break;
      
    case Stmt::BinaryOperatorClass: {
      BinaryOperator* B = cast<BinaryOperator>(S);
      
      if (B->isLogicalOp()) {
        VisitLogicalExpr(B, Pred, Dst);
        break;
      }
      else if (B->getOpcode() == BinaryOperator::Comma) {
        ValueState* St = GetState(Pred);
        MakeNode(Dst, B, Pred, SetRVal(St, B, GetRVal(St, B->getRHS())));
        break;
      }
      
      VisitBinaryOperator(cast<BinaryOperator>(S), Pred, Dst);
      break;
    }
      
    case Stmt::CallExprClass: {
      CallExpr* C = cast<CallExpr>(S);
      VisitCall(C, Pred, C->arg_begin(), C->arg_end(), Dst);
      break;      
    }
      
    case Stmt::CastExprClass: {
      CastExpr* C = cast<CastExpr>(S);
      VisitCast(C, C->getSubExpr(), Pred, Dst);
      break;
    }
      
      // FIXME: ChooseExpr is really a constant.  We need to fix
      //        the CFG do not model them as explicit control-flow.
      
    case Stmt::ChooseExprClass: { // __builtin_choose_expr
      ChooseExpr* C = cast<ChooseExpr>(S);
      VisitGuardedExpr(C, C->getLHS(), C->getRHS(), Pred, Dst);
      break;
    }
      
    case Stmt::CompoundAssignOperatorClass:
      VisitBinaryOperator(cast<BinaryOperator>(S), Pred, Dst);
      break;
      
    case Stmt::ConditionalOperatorClass: { // '?' operator
      ConditionalOperator* C = cast<ConditionalOperator>(S);
      VisitGuardedExpr(C, C->getLHS(), C->getRHS(), Pred, Dst);
      break;
    }
      
    case Stmt::DeclRefExprClass:
      VisitDeclRefExpr(cast<DeclRefExpr>(S), Pred, Dst, false);
      break;
      
    case Stmt::DeclStmtClass:
      VisitDeclStmt(cast<DeclStmt>(S), Pred, Dst);
      break;
      
    case Stmt::ImplicitCastExprClass: {
      ImplicitCastExpr* C = cast<ImplicitCastExpr>(S);
      VisitCast(C, C->getSubExpr(), Pred, Dst);
      break;
    }
      
    case Stmt::MemberExprClass: {
      VisitMemberExpr(cast<MemberExpr>(S), Pred, Dst, false);
      break;
    }
      
    case Stmt::ObjCMessageExprClass: {
      VisitObjCMessageExpr(cast<ObjCMessageExpr>(S), Pred, Dst);
      break;
    }
      
    case Stmt::ParenExprClass:
      Visit(cast<ParenExpr>(S)->getSubExpr()->IgnoreParens(), Pred, Dst);
      break;
      
    case Stmt::ReturnStmtClass:
      VisitReturnStmt(cast<ReturnStmt>(S), Pred, Dst);
      break;
      
    case Stmt::SizeOfAlignOfTypeExprClass:
      VisitSizeOfAlignOfTypeExpr(cast<SizeOfAlignOfTypeExpr>(S), Pred, Dst);
      break;
      
    case Stmt::StmtExprClass: {
      StmtExpr* SE = cast<StmtExpr>(S);
      
      ValueState* St = GetState(Pred);
      
      // FIXME: Not certain if we can have empty StmtExprs.  If so, we should
      // probably just remove these from the CFG.
      assert (!SE->getSubStmt()->body_empty());
      
      if (Expr* LastExpr = dyn_cast<Expr>(*SE->getSubStmt()->body_rbegin()))
        MakeNode(Dst, SE, Pred, SetRVal(St, SE, GetRVal(St, LastExpr)));
      else
        Dst.Add(Pred);
      
      break;
    }
      
    case Stmt::UnaryOperatorClass:
      VisitUnaryOperator(cast<UnaryOperator>(S), Pred, Dst, false);
      break;
  }
}

void GRExprEngine::VisitLVal(Expr* Ex, NodeTy* Pred, NodeSet& Dst) {
  
  Ex = Ex->IgnoreParens();
  
  if (Ex != CurrentStmt && getCFG().isBlkExpr(Ex)) {
    Dst.Add(Pred);
    return;
  }
  
  switch (Ex->getStmtClass()) {
    default:
      Visit(Ex, Pred, Dst);
      return;
      
    case Stmt::ArraySubscriptExprClass:
      VisitArraySubscriptExpr(cast<ArraySubscriptExpr>(Ex), Pred, Dst, true);
      return;
      
    case Stmt::DeclRefExprClass:
      VisitDeclRefExpr(cast<DeclRefExpr>(Ex), Pred, Dst, true);
      return;
      
    case Stmt::UnaryOperatorClass:
      VisitUnaryOperator(cast<UnaryOperator>(Ex), Pred, Dst, true);
      return;
      
    case Stmt::MemberExprClass:
      VisitMemberExpr(cast<MemberExpr>(Ex), Pred, Dst, true);
      return;
  }
}

//===----------------------------------------------------------------------===//
// Block entrance.  (Update counters).
//===----------------------------------------------------------------------===//

bool GRExprEngine::ProcessBlockEntrance(CFGBlock* B, ValueState*,
                                        GRBlockCounter BC) {
  
  return BC.getNumVisited(B->getBlockID()) < 3;
}

//===----------------------------------------------------------------------===//
// Branch processing.
//===----------------------------------------------------------------------===//

ValueState* GRExprEngine::MarkBranch(ValueState* St, Stmt* Terminator,
                                     bool branchTaken) {
  
  switch (Terminator->getStmtClass()) {
    default:
      return St;
      
    case Stmt::BinaryOperatorClass: { // '&&' and '||'
      
      BinaryOperator* B = cast<BinaryOperator>(Terminator);
      BinaryOperator::Opcode Op = B->getOpcode();
      
      assert (Op == BinaryOperator::LAnd || Op == BinaryOperator::LOr);
      
      // For &&, if we take the true branch, then the value of the whole
      // expression is that of the RHS expression.
      //
      // For ||, if we take the false branch, then the value of the whole
      // expression is that of the RHS expression.
      
      Expr* Ex = (Op == BinaryOperator::LAnd && branchTaken) ||
                 (Op == BinaryOperator::LOr && !branchTaken)  
               ? B->getRHS() : B->getLHS();
        
      return SetBlkExprRVal(St, B, UndefinedVal(Ex));
    }
      
    case Stmt::ConditionalOperatorClass: { // ?:
      
      ConditionalOperator* C = cast<ConditionalOperator>(Terminator);
      
      // For ?, if branchTaken == true then the value is either the LHS or
      // the condition itself. (GNU extension).
      
      Expr* Ex;      
      
      if (branchTaken)
        Ex = C->getLHS() ? C->getLHS() : C->getCond();        
      else
        Ex = C->getRHS();
      
      return SetBlkExprRVal(St, C, UndefinedVal(Ex));
    }
      
    case Stmt::ChooseExprClass: { // ?:
      
      ChooseExpr* C = cast<ChooseExpr>(Terminator);
      
      Expr* Ex = branchTaken ? C->getLHS() : C->getRHS();      
      return SetBlkExprRVal(St, C, UndefinedVal(Ex));
    }
  }
}

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

  // Remove old bindings for subexpressions.
  ValueState* PrevState = StateMgr.RemoveSubExprBindings(builder.getState());
  
  // Check for NULL conditions; e.g. "for(;;)"
  if (!Condition) { 
    builder.markInfeasible(false);
    return;
  }
  
  RVal V = GetRVal(PrevState, Condition);
  
  switch (V.getBaseKind()) {
    default:
      break;

    case RVal::UnknownKind:
      builder.generateNode(MarkBranch(PrevState, Term, true), true);
      builder.generateNode(MarkBranch(PrevState, Term, false), false);
      return;
      
    case RVal::UndefinedKind: {      
      NodeTy* N = builder.generateNode(PrevState, true);

      if (N) {
        N->markAsSink();
        UndefBranches.insert(N);
      }
      
      builder.markInfeasible(false);
      return;
    }      
  }
    
  // Process the true branch.

  bool isFeasible = false;  
  ValueState* St = Assume(PrevState, V, true, isFeasible);

  if (isFeasible)
    builder.generateNode(MarkBranch(St, Term, true), true);
  else
    builder.markInfeasible(true);
      
  // Process the false branch.  
  
  isFeasible = false;
  St = Assume(PrevState, V, false, isFeasible);
  
  if (isFeasible)
    builder.generateNode(MarkBranch(St, Term, false), 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) {

  ValueState* St = builder.getState();  
  RVal V = GetRVal(St, builder.getTarget());
  
  // Three possibilities:
  //
  //   (1) We know the computed label.
  //   (2) The label is NULL (or some other constant), or Undefined.
  //   (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<UndefinedVal>(V)) {
    // Dispatch to the first target and mark it as a sink.
    NodeTy* N = builder.generateNode(builder.begin(), St, true);
    UndefBranches.insert(N);
    return;
  }
  
  // This is really a catch-all.  We don't support symbolics yet.
  
  assert (V.isUnknown());
  
  for (iterator I=builder.begin(), E=builder.end(); I != E; ++I)
    builder.generateNode(I, St);
}


void GRExprEngine::VisitGuardedExpr(Expr* Ex, Expr* L, Expr* R,
                                    NodeTy* Pred, NodeSet& Dst) {
  
  assert (Ex == CurrentStmt && getCFG().isBlkExpr(Ex));
  
  ValueState* St = GetState(Pred);
  RVal X = GetBlkExprRVal(St, Ex);
  
  assert (X.isUndef());
  
  Expr* SE = (Expr*) cast<UndefinedVal>(X).getData();
  
  assert (SE);
  
  X = GetBlkExprRVal(St, SE);
  
  // Make sure that we invalidate the previous binding.
  MakeNode(Dst, Ex, Pred, StateMgr.SetRVal(St, Ex, X, true, true));
}

/// 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;
  
  ValueState* St = builder.getState();  
  Expr* CondE = builder.getCondition();
  RVal  CondV = GetRVal(St, CondE);

  if (CondV.isUndef()) {
    NodeTy* N = builder.generateDefaultCaseNode(St, true);
    UndefBranches.insert(N);
    return;
  }
  
  ValueState*  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(CondE->getType());

  APSInt V1(bits, false);
  APSInt V2 = V1;
  bool DefaultFeasible = false;
  
  for (iterator I = builder.begin(), EI = builder.end(); I != EI; ++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 NonLVals.
        
    do {
      nonlval::ConcreteInt CaseVal(BasicVals.getValue(V1));
      
      RVal Res = EvalBinOp(BinaryOperator::EQ, CondV, CaseVal);
      
      // Now "assume" that the case matches.
      
      bool isFeasible = false;      
      ValueState* 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).
      
      isFeasible = false;
      StNew = Assume(DefaultSt, Res, false, isFeasible);
      
      if (isFeasible) {
        DefaultFeasible = true;
        DefaultSt = StNew;
      }

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

//===----------------------------------------------------------------------===//
// Transfer functions: logical operations ('&&', '||').
//===----------------------------------------------------------------------===//

void GRExprEngine::VisitLogicalExpr(BinaryOperator* B, NodeTy* Pred,
                                    NodeSet& Dst) {
  
  assert (B->getOpcode() == BinaryOperator::LAnd ||
          B->getOpcode() == BinaryOperator::LOr);
  
  assert (B == CurrentStmt && getCFG().isBlkExpr(B));
  
  ValueState* St = GetState(Pred);
  RVal X = GetBlkExprRVal(St, B);
  
  assert (X.isUndef());
  
  Expr* Ex = (Expr*) cast<UndefinedVal>(X).getData();
  
  assert (Ex);
  
  if (Ex == B->getRHS()) {
    
    X = GetBlkExprRVal(St, Ex);
    
    // Handle undefined values.
    
    if (X.isUndef()) {
      MakeNode(Dst, B, Pred, SetBlkExprRVal(St, B, X));
      return;
    }
    
    // We took the RHS.  Because the value of the '&&' or '||' expression must
    // evaluate to 0 or 1, we must assume the value of the RHS evaluates to 0
    // or 1.  Alternatively, we could take a lazy approach, and calculate this
    // value later when necessary.  We don't have the machinery in place for
    // this right now, and since most logical expressions are used for branches,
    // the payoff is not likely to be large.  Instead, we do eager evaluation.
        
    bool isFeasible = false;
    ValueState* NewState = Assume(St, X, true, isFeasible);
    
    if (isFeasible)
      MakeNode(Dst, B, Pred,
               SetBlkExprRVal(NewState, B, MakeConstantVal(1U, B)));
      
    isFeasible = false;
    NewState = Assume(St, X, false, isFeasible);
    
    if (isFeasible)
      MakeNode(Dst, B, Pred,
               SetBlkExprRVal(NewState, B, MakeConstantVal(0U, B)));
  }
  else {
    // We took the LHS expression.  Depending on whether we are '&&' or
    // '||' we know what the value of the expression is via properties of
    // the short-circuiting.
    
    X = MakeConstantVal( B->getOpcode() == BinaryOperator::LAnd ? 0U : 1U, B);
    MakeNode(Dst, B, Pred, SetBlkExprRVal(St, B, X));
  }
}
 
//===----------------------------------------------------------------------===//
// Transfer functions: Loads and stores.
//===----------------------------------------------------------------------===//

void GRExprEngine::VisitDeclRefExpr(DeclRefExpr* D, NodeTy* Pred, NodeSet& Dst,
                                    bool asLVal) {
  
  ValueState* St = GetState(Pred);
  RVal X = RVal::MakeVal(BasicVals, D);
  
  if (asLVal)
    MakeNode(Dst, D, Pred, SetRVal(St, D, cast<LVal>(X)));
  else {
    RVal V = isa<lval::DeclVal>(X) ? GetRVal(St, cast<LVal>(X)) : X;
    MakeNode(Dst, D, Pred, SetRVal(St, D, V));
  }
}

/// VisitArraySubscriptExpr - Transfer function for array accesses
void GRExprEngine::VisitArraySubscriptExpr(ArraySubscriptExpr* A, NodeTy* Pred,
                                           NodeSet& Dst, bool asLVal) {
  
  Expr* Base = A->getBase()->IgnoreParens();
  Expr* Idx  = A->getIdx()->IgnoreParens();
  
  // Always visit the base as an LVal expression.  This computes the
  // abstract address of the base object.
  NodeSet Tmp;
  
  if (LVal::IsLValType(Base->getType())) // Base always is an LVal.
    Visit(Base, Pred, Tmp);
  else  
    VisitLVal(Base, Pred, Tmp);
  
  for (NodeSet::iterator I1=Tmp.begin(), E1=Tmp.end(); I1!=E1; ++I1) {
    
    // Evaluate the index.

    NodeSet Tmp2;
    Visit(Idx, *I1, Tmp2);
      
    for (NodeSet::iterator I2=Tmp2.begin(), E2=Tmp2.end(); I2!=E2; ++I2) {

      ValueState* St = GetState(*I2);
      RVal BaseV = GetRVal(St, Base);
      RVal IdxV  = GetRVal(St, Idx);      
      
      // If IdxV is 0, return just BaseV.
      
      bool useBase = false;
      
      if (nonlval::ConcreteInt* IdxInt = dyn_cast<nonlval::ConcreteInt>(&IdxV))        
        useBase = IdxInt->getValue() == 0;
      
      RVal V = useBase ? BaseV : lval::ArrayOffset::Make(BasicVals, BaseV,IdxV);

      if (asLVal)
        MakeNode(Dst, A, *I2, SetRVal(St, A, V));
      else
        EvalLoad(Dst, A, *I2, St, V);
    }
  }
}

/// VisitMemberExpr - Transfer function for member expressions.
void GRExprEngine::VisitMemberExpr(MemberExpr* M, NodeTy* Pred,
                                   NodeSet& Dst, bool asLVal) {
  
  Expr* Base = M->getBase()->IgnoreParens();

  // Always visit the base as an LVal expression.  This computes the
  // abstract address of the base object.
  NodeSet Tmp;
  
  if (asLVal) {
      
    if (LVal::IsLValType(Base->getType())) // Base always is an LVal.
      Visit(Base, Pred, Tmp);
    else  
      VisitLVal(Base, Pred, Tmp);
  
    for (NodeSet::iterator I=Tmp.begin(), E=Tmp.end(); I!=E; ++I) {
      ValueState* St = GetState(*I);
      RVal BaseV = GetRVal(St, Base);      
      
      RVal V = lval::FieldOffset::Make(BasicVals, GetRVal(St, Base),
                                       M->getMemberDecl());
      
      MakeNode(Dst, M, *I, SetRVal(St, M, V));
    }
    
    return;
  }

  // Evaluate the base.  Can be an LVal or NonLVal (depends on whether
  //  or not isArrow() is true).
  Visit(Base, Pred, Tmp);
  
  for (NodeSet::iterator I=Tmp.begin(), E=Tmp.end(); I!=E; ++I) {

    ValueState* St = GetState(*I);
    RVal BaseV = GetRVal(St, Base);
    
    if (LVal::IsLValType(Base->getType())) {
    
      assert (M->isArrow());
      
      RVal V = lval::FieldOffset::Make(BasicVals, GetRVal(St, Base),
                                       M->getMemberDecl());
    
      EvalLoad(Dst, M, *I, St, V);
    }
    else {
      
      assert (!M->isArrow());
      
      if (BaseV.isUnknownOrUndef()) {
        MakeNode(Dst, M, *I, SetRVal(St, M, BaseV));
        continue;
      }

      // FIXME: Implement nonlval objects representing struct temporaries.
      assert (isa<NonLVal>(BaseV));
      MakeNode(Dst, M, *I, SetRVal(St, M, UnknownVal()));
    }
  }
}

void GRExprEngine::EvalStore(NodeSet& Dst, Expr* Ex, NodeTy* Pred,
                             ValueState* St, RVal location, RVal Val) {
  
  assert (Builder && "GRStmtNodeBuilder must be defined.");
  
  // Evaluate the location (checks for bad dereferences).
  St = EvalLocation(Ex, Pred, St, location);
  
  if (!St)
    return;
  
  // Proceed with the store.
  
  unsigned size = Dst.size();  

  SaveAndRestore<bool> OldSink(Builder->BuildSinks);
  SaveOr OldHasGen(Builder->HasGeneratedNode);

  assert (!location.isUndef());
  
  TF->EvalStore(Dst, *this, *Builder, Ex, Pred, St, location, Val);
  
  // Handle the case where no nodes where generated.  Auto-generate that
  // contains the updated state if we aren't generating sinks.
  
  if (!Builder->BuildSinks && Dst.size() == size && !Builder->HasGeneratedNode)
    TF->GRTransferFuncs::EvalStore(Dst, *this, *Builder, Ex, Pred, St,
                                   location, Val);
}

void GRExprEngine::EvalLoad(NodeSet& Dst, Expr* Ex, NodeTy* Pred,
                            ValueState* St, RVal location, bool CheckOnly) {

  // Evaluate the location (checks for bad dereferences).
  
  St = EvalLocation(Ex, Pred, St, location, true);
  
  if (!St)
    return;
  
  // Proceed with the load.

  // FIXME: Currently symbolic analysis "generates" new symbols
  //  for the contents of values.  We need a better approach.

  // FIXME: The "CheckOnly" option exists only because Array and Field
  //  loads aren't fully implemented.  Eventually this option will go away.
  
  if (CheckOnly)
    MakeNode(Dst, Ex, Pred, St);
  else if (location.isUnknown()) {
    // This is important.  We must nuke the old binding.
    MakeNode(Dst, Ex, Pred, SetRVal(St, Ex, UnknownVal()));
  }
  else    
    MakeNode(Dst, Ex, Pred, SetRVal(St, Ex, GetRVal(St, cast<LVal>(location),
                                                    Ex->getType())));  
}

ValueState* GRExprEngine::EvalLocation(Expr* Ex, NodeTy* Pred,
                                       ValueState* St, RVal location,
                                       bool isLoad) {
  
  // Check for loads/stores from/to undefined values.  
  if (location.isUndef()) {
    ProgramPoint::Kind K =
      isLoad ? ProgramPoint::PostLoadKind : ProgramPoint::PostStmtKind;
    
    if (NodeTy* Succ = Builder->generateNode(Ex, St, Pred, K)) {
      Succ->markAsSink();
      UndefDeref.insert(Succ);
    }
    
    return NULL;
  }
  
  // Check for loads/stores from/to unknown locations.  Treat as No-Ops.
  if (location.isUnknown())
    return St;
  
  // During a load, one of two possible situations arise:
  //  (1) A crash, because the location (pointer) was NULL.
  //  (2) The location (pointer) is not NULL, and the dereference works.
  // 
  // We add these assumptions.
  
  LVal LV = cast<LVal>(location);    
  
  // "Assume" that the pointer is not NULL.
  
  bool isFeasibleNotNull = false;
  ValueState* StNotNull = Assume(St, LV, true, isFeasibleNotNull);
  
  // "Assume" that the pointer is NULL.