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/Basic/SourceManager.h"
#include "llvm/Support/Streams.h"

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

// 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;
};

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


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) {
    
    VarDecl* VD = cast<VarDecl>(const_cast<ScopedDecl*>(I->first));
    
    if (VD->hasGlobalStorage() || isa<ParmVarDecl>(VD)) {
      RVal X = RVal::GetSymbolValue(SymMgr, VD);
      StateMgr.BindVar(StateImpl, VD, 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, false);
}

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));
    }
  }
}

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

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);
}

/// 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;
  
  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)
        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) {
  
  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()) {
      Nodify(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)
      Nodify(Dst, B, Pred, SetBlkExprRVal(NewState, B, MakeConstantVal(1U, B)));
      
    isFeasible = false;
    NewState = Assume(St, X, false, isFeasible);
    
    if (isFeasible)
      Nodify(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);
    Nodify(Dst, B, Pred, SetBlkExprRVal(St, B, X));
  }
}
 

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

  Builder = &builder;
  StmtEntryNode = builder.getLastNode();
  CurrentStmt = S;
  NodeSet Dst;
  
  // Create the cleaned state.

  CleanedState = StateMgr.RemoveDeadBindings(StmtEntryNode->getState(),
                                             CurrentStmt, Liveness);
  
  Builder->SetCleanedState(CleanedState);

  // Visit the statement.

  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)
    builder.generateNode(S, GetState(StmtEntryNode), StmtEntryNode);
  
  // NULL out these variables to cleanup.
  
  CurrentStmt = NULL;
  StmtEntryNode = NULL;
  Builder = NULL;
  CleanedState = NULL;
}

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.
  
  ValueState* St = GetState(Pred);  
  RVal X = RVal::MakeVal(BasicVals, D);
  RVal Y = isa<lval::DeclVal>(X) ? GetRVal(St, cast<lval::DeclVal>(X)) : X;
  Nodify(Dst, D, Pred, SetBlkExprRVal(St, D, Y));
}

void GRExprEngine::VisitCall(CallExpr* CE, NodeTy* Pred,
                             CallExpr::arg_iterator AI,
                             CallExpr::arg_iterator AE,
                             NodeSet& Dst) {
  
  // Process the arguments.
  
  if (AI != AE) {
    
    NodeSet DstTmp;      
    Visit(*AI, Pred, DstTmp);    
    ++AI;
    
    for (NodeSet::iterator DI=DstTmp.begin(), DE=DstTmp.end(); DI != DE; ++DI)
      VisitCall(CE, *DI, AI, AE, Dst);
    
    return;
  }

  // If we reach here we have processed all of the arguments.  Evaluate
  // the callee expression.
  
  NodeSet DstTmp;    
  Expr* Callee = CE->getCallee()->IgnoreParenCasts();

  VisitLVal(Callee, Pred, DstTmp);
  
  if (DstTmp.empty())
    DstTmp.Add(Pred);
  
  // Finally, evaluate the function call.
  for (NodeSet::iterator DI = DstTmp.begin(), DE = DstTmp.end(); DI!=DE; ++DI) {

    ValueState* St = GetState(*DI);
    RVal L = GetLVal(St, Callee);

    // FIXME: Add support for symbolic function calls (calls involving
    //  function pointer values that are symbolic).
    
    // Check for undefined control-flow or calls to NULL.
    
    if (L.isUndef() || isa<lval::ConcreteInt>(L)) {      
      NodeTy* N = Builder->generateNode(CE, St, *DI);
      
      if (N) {
        N->markAsSink();
        BadCalls.insert(N);
      }
      
      continue;
    }
    
    // Check for the "noreturn" attribute.
    
    SaveAndRestore<bool> OldSink(Builder->BuildSinks);
    
    if (isa<lval::FuncVal>(L)) {      
      
      FunctionDecl* FD = cast<lval::FuncVal>(L).getDecl();
      
      if (FD->getAttr<NoReturnAttr>())
        Builder->BuildSinks = true;
      else {
        // HACK: Some functions are not marked noreturn, and don't return.
        //  Here are a few hardwired ones.  If this takes too long, we can
        //  potentially cache these results.
        const char* s = FD->getIdentifier()->getName();
        unsigned n = strlen(s);
        
        switch (n) {
          default:
            break;
            
          case 4:
            if (!memcmp(s, "exit", 4)) Builder->BuildSinks = true;
            break;

          case 5:
            if (!memcmp(s, "panic", 5)) Builder->BuildSinks = true;
            break;
        }
      }
    }
    
    // Evaluate the call.
    
    
    bool invalidateArgs = false;
    
    if (L.isUnknown()) {
      // Check for an "unknown" callee.      
      invalidateArgs = true;
    }
    else if (isa<lval::FuncVal>(L)) {
      
      IdentifierInfo* Info = cast<lval::FuncVal>(L).getDecl()->getIdentifier();
      
      if (unsigned id = Info->getBuiltinID()) {
        switch (id) {
          case Builtin::BI__builtin_expect: {
            // For __builtin_expect, just return the value of the subexpression.
            assert (CE->arg_begin() != CE->arg_end());            
            RVal X = GetRVal(St, *(CE->arg_begin()));
            Nodify(Dst, CE, *DI, SetRVal(St, CE, X));
            continue;            
          }
            
          default:
            invalidateArgs = true;
            break;
        }
      }
    }
        
    if (invalidateArgs) {
      // Invalidate all arguments passed in by reference (LVals).
      for (CallExpr::arg_iterator I = CE->arg_begin(), E = CE->arg_end();
                                                       I != E; ++I) {
        RVal V = GetRVal(St, *I);

        if (isa<LVal>(V))
          St = SetRVal(St, cast<LVal>(V), UnknownVal());
      }
      
      Nodify(Dst, CE, *DI, St);
    }
    else {

      // Check any arguments passed-by-value against being undefined.

      bool badArg = false;
      
      for (CallExpr::arg_iterator I = CE->arg_begin(), E = CE->arg_end();
           I != E; ++I) {

        if (GetRVal(GetState(*DI), *I).isUndef()) {        
          NodeTy* N = Builder->generateNode(CE, GetState(*DI), *DI);
        
          if (N) {
            N->markAsSink();
            UndefArgs[N] = *I;
          }
          
          badArg = true;
          break;
        }
      }
        
      if (badArg)
        continue;        
      
      // Dispatch to the plug-in transfer function.      
      
      unsigned size = Dst.size();
      
      EvalCall(Dst, CE, cast<LVal>(L), *DI);
      
      if (!Builder->BuildSinks && Dst.size() == size)
        Nodify(Dst, CE, *DI, St);
    }
  }
}

void GRExprEngine::VisitCast(Expr* CastE, Expr* Ex, NodeTy* Pred, NodeSet& Dst){
  
  NodeSet S1;
  QualType T = CastE->getType();
  
  if (T->isReferenceType())
    VisitLVal(Ex, Pred, S1);
  else
    Visit(Ex, Pred, S1);
  
  // Check for redundant casts or casting to "void"
  if (T->isVoidType() ||
      Ex->getType() == T || 
      (T->isPointerType() && Ex->getType()->isFunctionType())) {
    
    for (NodeSet::iterator I1 = S1.begin(), E1 = S1.end(); I1 != E1; ++I1)
      Dst.Add(*I1);

    return;
  }
  
  for (NodeSet::iterator I1 = S1.begin(), E1 = S1.end(); I1 != E1; ++I1) {
    NodeTy* N = *I1;
    ValueState* St = GetState(N);
    
    RVal V = T->isReferenceType() ? GetLVal(St, Ex) : GetRVal(St, Ex);
    
    Nodify(Dst, CastE, N, SetRVal(St, CastE, EvalCast(V, CastE->getType())));
  }
}

void GRExprEngine::VisitDeclStmt(DeclStmt* DS, GRExprEngine::NodeTy* Pred,
                                 GRExprEngine::NodeSet& Dst) {
  
  ValueState* St = GetState(Pred);
  
  for (const ScopedDecl* D = DS->getDecl(); D; D = D->getNextDeclarator())
    if (const VarDecl* VD = dyn_cast<VarDecl>(D)) {
      
      // FIXME: Add support for local arrays.
      if (VD->getType()->isArrayType())
        continue;
      
      const Expr* Ex = VD->getInit();
      
      if (!VD->hasGlobalStorage() || VD->getStorageClass() == VarDecl::Static) {
        
        // In this context, Static => Local variable.
        
        assert (!VD->getStorageClass() == VarDecl::Static ||
                !isa<FileVarDecl>(VD));
        
        // If there is no initializer, set the value of the
        // variable to "Undefined".
        //
        // FIXME: static variables may have an initializer, but the second
        //  time a function is called those values may not be current.

        QualType T = VD->getType();
        
        if ( VD->getStorageClass() == VarDecl::Static) {
          
          // C99: 6.7.8 Initialization
          //  If an object that has static storage duration is not initialized
          //  explicitly, then: 
          //   —if it has pointer type, it is initialized to a null pointer; 
          //   —if it has arithmetic type, it is initialized to (positive or 
          //     unsigned) zero; 
          
          // FIXME: Handle structs.  Now we treat their values as unknown.
          
          if (T->isPointerType()) {
            
            St = SetRVal(St, lval::DeclVal(VD),
                         lval::ConcreteInt(BasicVals.getValue(0, T)));
          }
          else if (T->isIntegerType()) {
            
            St = SetRVal(St, lval::DeclVal(VD),
                         nonlval::ConcreteInt(BasicVals.getValue(0, T)));
          }
          

        }
        else {
          
          // FIXME: Handle structs.  Now we treat them as unknown.  What
          //  we need to do is treat their members as unknown.
          
          if (T->isPointerType() || T->isIntegerType())
            St = SetRVal(St, lval::DeclVal(VD),
                         Ex ? GetRVal(St, Ex) : UndefinedVal());
        }
      }
    }

  Nodify(Dst, DS, Pred, 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.
  Nodify(Dst, Ex, Pred, StateMgr.SetRVal(St, Ex, X, true, true));
}

/// VisitSizeOfAlignOfTypeExpr - Transfer function for sizeof(type).
void GRExprEngine::VisitSizeOfAlignOfTypeExpr(SizeOfAlignOfTypeExpr* Ex,
                                              NodeTy* Pred,
                                              NodeSet& Dst) {

  QualType T = Ex->getArgumentType();
  uint64_t amt;  
  
  if (Ex->isSizeOf()) {

    // FIXME: Add support for VLAs.
    if (!T.getTypePtr()->isConstantSizeType())
      return;
    
    amt = 1;  // Handle sizeof(void)
    
    if (T != getContext().VoidTy)
      amt = getContext().getTypeSize(T) / 8;
    
  }
  else  // Get alignment of the type.
    amt = getContext().getTypeAlign(T) / 8;
  
  Nodify(Dst, Ex, Pred,
         SetRVal(GetState(Pred), Ex,
                 NonLVal::MakeVal(BasicVals, amt, Ex->getType())));  
}

void GRExprEngine::VisitDeref(UnaryOperator* U, NodeTy* Pred,
                              NodeSet& Dst, bool GetLVal) {

  Expr* Ex = U->getSubExpr()->IgnoreParens();
    
  NodeSet DstTmp;
  
  if (isa<DeclRefExpr>(Ex))
    DstTmp.Add(Pred);
  else
    Visit(Ex, Pred, DstTmp);
  
  for (NodeSet::iterator I = DstTmp.begin(), DE = DstTmp.end(); I != DE; ++I) {

    NodeTy* N = *I;
    ValueState* St = GetState(N);
    
    // FIXME: Bifurcate when dereferencing a symbolic with no constraints?
    
    RVal V = GetRVal(St, Ex);
    
    // Check for dereferences of undefined values.
    
    if (V.isUndef()) {
      
      NodeTy* Succ = Builder->generateNode(U, St, N);
      
      if (Succ) {
        Succ->markAsSink();
        UndefDeref.insert(Succ);
      }
      
      continue;
    }
    
    // Check for dereferences of unknown values.  Treat as No-Ops.
    
    if (V.isUnknown()) {
      Dst.Add(N);
      continue;
    }
    
    // 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.
    
    LVal LV = cast<LVal>(V);    
    bool isFeasibleNotNull;
    
    // "Assume" that the pointer is Not-NULL.
    
    ValueState* StNotNull = Assume(St, LV, true, isFeasibleNotNull);
    
    if (isFeasibleNotNull) {

      if (GetLVal) Nodify(Dst, U, N, SetRVal(StNotNull, U, LV));
      else {
        
        // FIXME: Currently symbolic analysis "generates" new symbols
        //  for the contents of values.  We need a better approach.
      
        Nodify(Dst, U, N, SetRVal(StNotNull, U,
                                  GetRVal(StNotNull, LV, U->getType())));
      }
    }
    
    bool isFeasibleNull;
    
    // Now "assume" that the pointer is NULL.
    
    ValueState* StNull = Assume(St, LV, 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, N);
      
      if (NullNode) {

        NullNode->markAsSink();
        
        if (isFeasibleNotNull) ImplicitNullDeref.insert(NullNode);
        else ExplicitNullDeref.insert(NullNode);
      }
    }    
  }
}

void GRExprEngine::VisitUnaryOperator(UnaryOperator* U, NodeTy* Pred,
                                      NodeSet& Dst) {
  
  NodeSet S1;
  
  assert (U->getOpcode() != UnaryOperator::Deref);
  assert (U->getOpcode() != UnaryOperator::SizeOf);
  assert (U->getOpcode() != UnaryOperator::AlignOf);
  
  bool use_GetLVal = false;
  
  switch (U->getOpcode()) {
    case UnaryOperator::PostInc:
    case UnaryOperator::PostDec:
    case UnaryOperator::PreInc:
    case UnaryOperator::PreDec:
    case UnaryOperator::AddrOf:
      // Evalue subexpression as an LVal.
      use_GetLVal = true;
      VisitLVal(U->getSubExpr(), Pred, S1);
      break;
      
    default:
      Visit(U->getSubExpr(), Pred, S1);
      break;
  }

  for (NodeSet::iterator I1 = S1.begin(), E1 = S1.end(); I1 != E1; ++I1) {

    NodeTy* N1 = *I1;
    ValueState* St = GetState(N1);
        
    RVal SubV = use_GetLVal ? GetLVal(St, U->getSubExpr()) : 
                              GetRVal(St, U->getSubExpr());
    
    if (SubV.isUnknown()) {
      Dst.Add(N1);
      continue;
    }

    if (SubV.isUndef()) {
      Nodify(Dst, U, N1, SetRVal(St, U, SubV));
      continue;
    }
    
    if (U->isIncrementDecrementOp()) {
      
      // Handle ++ and -- (both pre- and post-increment).
      
      LVal SubLV = cast<LVal>(SubV); 
      RVal V  = GetRVal(St, SubLV, U->getType());
      
      if (V.isUnknown()) {
        Dst.Add(N1);
        continue;
      }

      // Propagate undefined values.      
      if (V.isUndef()) {
        Nodify(Dst, U, N1, SetRVal(St, U, V));
        continue;
      }
      
      // Handle all other values.
      
      BinaryOperator::Opcode Op = U->isIncrementOp() ? BinaryOperator::Add
                                                     : BinaryOperator::Sub;
      
      RVal Result = EvalBinOp(Op, V, MakeConstantVal(1U, U));
      
      if (U->isPostfix())
        St = SetRVal(SetRVal(St, U, V), SubLV, Result);
      else
        St = SetRVal(SetRVal(St, U, Result), SubLV, Result);
        
      Nodify(Dst, U, N1, St);
      continue;
    }    
    
    // Handle all other unary operators.
    
    switch (U->getOpcode()) {
        
      case UnaryOperator::Extension:
        St = SetRVal(St, U, SubV);
        break;

      case UnaryOperator::Minus:
        St = SetRVal(St, U, EvalMinus(U, cast<NonLVal>(SubV)));
        break;
        
      case UnaryOperator::Not:
        St = SetRVal(St, U, EvalComplement(cast<NonLVal>(SubV)));
        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".

        if (isa<LVal>(SubV)) {
          lval::ConcreteInt V(BasicVals.getZeroWithPtrWidth());
          RVal Result = EvalBinOp(BinaryOperator::EQ, cast<LVal>(SubV), V);
          St = SetRVal(St, U, Result);
        }
        else {
          Expr* Ex = U->getSubExpr();
          nonlval::ConcreteInt V(BasicVals.getValue(0, Ex->getType()));
          RVal Result = EvalBinOp(BinaryOperator::EQ, cast<NonLVal>(SubV), V);
          St = SetRVal(St, U, Result);
        }
        
        break;
        
      case UnaryOperator::AddrOf: {
        assert (isa<LVal>(SubV));
        St = SetRVal(St, U, SubV);
        break;
      }
                
      default: ;
        assert (false && "Not implemented.");
    } 
    
    Nodify(Dst, U, N1, St);
  }
}

void GRExprEngine::VisitSizeOfExpr(UnaryOperator* U, NodeTy* Pred,
                                   NodeSet& Dst) {
  
  QualType T = U->getSubExpr()->getType();
  
  // FIXME: Add support for VLAs.
  if (!T.getTypePtr()->isConstantSizeType())
    return;
  
  uint64_t size = getContext().getTypeSize(T) / 8;                
  ValueState* St = GetState(Pred);
  St = SetRVal(St, U, NonLVal::MakeVal(BasicVals, size, U->getType()));

  Nodify(Dst, U, Pred, St);
}

void GRExprEngine::VisitLVal(Expr* Ex, NodeTy* Pred, NodeSet& Dst) {

  if (Ex != CurrentStmt && getCFG().isBlkExpr(Ex)) {
    Dst.Add(Pred);
    return;
  }
  
  Ex = Ex->IgnoreParens();