path: root/lib/Checker/SimpleSValuator.cpp

// SimpleSValuator.cpp - A basic SValuator ------------------------*- 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 SimpleSValuator, a basic implementation of SValuator.
//
//===----------------------------------------------------------------------===//

#include "clang/Checker/PathSensitive/SValuator.h"
#include "clang/Checker/PathSensitive/GRState.h"

using namespace clang;

namespace {
class SimpleSValuator : public SValuator {
protected:
  virtual SVal EvalCastNL(NonLoc val, QualType castTy);
  virtual SVal EvalCastL(Loc val, QualType castTy);

public:
  SimpleSValuator(ValueManager &valMgr) : SValuator(valMgr) {}
  virtual ~SimpleSValuator() {}

  virtual SVal EvalMinus(NonLoc val);
  virtual SVal EvalComplement(NonLoc val);
  virtual SVal EvalBinOpNN(const GRState *state, BinaryOperator::Opcode op,
                           NonLoc lhs, NonLoc rhs, QualType resultTy);
  virtual SVal EvalBinOpLL(const GRState *state, BinaryOperator::Opcode op,
                           Loc lhs, Loc rhs, QualType resultTy);
  virtual SVal EvalBinOpLN(const GRState *state, BinaryOperator::Opcode op,
                           Loc lhs, NonLoc rhs, QualType resultTy);

  /// getKnownValue - Evaluates a given SVal. If the SVal has only one possible
  ///  (integer) value, that value is returned. Otherwise, returns NULL.
  virtual const llvm::APSInt *getKnownValue(const GRState *state, SVal V);
  
  SVal MakeSymIntVal(const SymExpr *LHS, BinaryOperator::Opcode op,
                     const llvm::APSInt &RHS, QualType resultTy);
};
} // end anonymous namespace

SValuator *clang::CreateSimpleSValuator(ValueManager &valMgr) {
  return new SimpleSValuator(valMgr);
}

//===----------------------------------------------------------------------===//
// Transfer function for Casts.
//===----------------------------------------------------------------------===//

SVal SimpleSValuator::EvalCastNL(NonLoc val, QualType castTy) {

  bool isLocType = Loc::IsLocType(castTy);

  if (nonloc::LocAsInteger *LI = dyn_cast<nonloc::LocAsInteger>(&val)) {
    if (isLocType)
      return LI->getLoc();

    // FIXME: Correctly support promotions/truncations.
    ASTContext &Ctx = ValMgr.getContext();
    unsigned castSize = Ctx.getTypeSize(castTy);
    if (castSize == LI->getNumBits())
      return val;

    return ValMgr.makeLocAsInteger(LI->getLoc(), castSize);
  }

  if (const SymExpr *se = val.getAsSymbolicExpression()) {
    ASTContext &Ctx = ValMgr.getContext();
    QualType T = Ctx.getCanonicalType(se->getType(Ctx));
    if (T == Ctx.getCanonicalType(castTy))
      return val;
    
    // FIXME: Remove this hack when we support symbolic truncation/extension.
    // HACK: If both castTy and T are integers, ignore the cast.  This is
    // not a permanent solution.  Eventually we want to precisely handle
    // extension/truncation of symbolic integers.  This prevents us from losing
    // precision when we assign 'x = y' and 'y' is symbolic and x and y are
    // different integer types.
    if (T->isIntegerType() && castTy->isIntegerType())
      return val;

    return UnknownVal();
  }

  if (!isa<nonloc::ConcreteInt>(val))
    return UnknownVal();

  // Only handle casts from integers to integers.
  if (!isLocType && !castTy->isIntegerType())
    return UnknownVal();

  llvm::APSInt i = cast<nonloc::ConcreteInt>(val).getValue();
  i.setIsUnsigned(castTy->isUnsignedIntegerType() || Loc::IsLocType(castTy));
  i.extOrTrunc(ValMgr.getContext().getTypeSize(castTy));

  if (isLocType)
    return ValMgr.makeIntLocVal(i);
  else
    return ValMgr.makeIntVal(i);
}

SVal SimpleSValuator::EvalCastL(Loc val, QualType castTy) {

  // Casts from pointers -> pointers, just return the lval.
  //
  // Casts from pointers -> references, just return the lval.  These
  //   can be introduced by the frontend for corner cases, e.g
  //   casting from va_list* to __builtin_va_list&.
  //
  if (Loc::IsLocType(castTy) || castTy->isReferenceType())
    return val;

  // FIXME: Handle transparent unions where a value can be "transparently"
  //  lifted into a union type.
  if (castTy->isUnionType())
    return UnknownVal();

  if (castTy->isIntegerType()) {
    unsigned BitWidth = ValMgr.getContext().getTypeSize(castTy);

    if (!isa<loc::ConcreteInt>(val))
      return ValMgr.makeLocAsInteger(val, BitWidth);

    llvm::APSInt i = cast<loc::ConcreteInt>(val).getValue();
    i.setIsUnsigned(castTy->isUnsignedIntegerType() || Loc::IsLocType(castTy));
    i.extOrTrunc(BitWidth);
    return ValMgr.makeIntVal(i);
  }

  // All other cases: return 'UnknownVal'.  This includes casting pointers
  // to floats, which is probably badness it itself, but this is a good
  // intermediate solution until we do something better.
  return UnknownVal();
}

//===----------------------------------------------------------------------===//
// Transfer function for unary operators.
//===----------------------------------------------------------------------===//

SVal SimpleSValuator::EvalMinus(NonLoc val) {
  switch (val.getSubKind()) {
  case nonloc::ConcreteIntKind:
    return cast<nonloc::ConcreteInt>(val).evalMinus(ValMgr);
  default:
    return UnknownVal();
  }
}

SVal SimpleSValuator::EvalComplement(NonLoc X) {
  switch (X.getSubKind()) {
  case nonloc::ConcreteIntKind:
    return cast<nonloc::ConcreteInt>(X).evalComplement(ValMgr);
  default:
    return UnknownVal();
  }
}

//===----------------------------------------------------------------------===//
// Transfer function for binary operators.
//===----------------------------------------------------------------------===//

static BinaryOperator::Opcode NegateComparison(BinaryOperator::Opcode op) {
  switch (op) {
  default:
    assert(false && "Invalid opcode.");
  case BO_LT: return BO_GE;
  case BO_GT: return BO_LE;
  case BO_LE: return BO_GT;
  case BO_GE: return BO_LT;
  case BO_EQ: return BO_NE;
  case BO_NE: return BO_EQ;
  }
}

static BinaryOperator::Opcode ReverseComparison(BinaryOperator::Opcode op) {
  switch (op) {
  default:
    assert(false && "Invalid opcode.");
  case BO_LT: return BO_GT;
  case BO_GT: return BO_LT;
  case BO_LE: return BO_GE;
  case BO_GE: return BO_LE;
  case BO_EQ:
  case BO_NE:
    return op;
  }
}

SVal SimpleSValuator::MakeSymIntVal(const SymExpr *LHS,
                                    BinaryOperator::Opcode op,
                                    const llvm::APSInt &RHS,
                                    QualType resultTy) {
  bool isIdempotent = false;

  // Check for a few special cases with known reductions first.
  switch (op) {
  default:
    // We can't reduce this case; just treat it normally.
    break;
  case BO_Mul:
    // a*0 and a*1
    if (RHS == 0)
      return ValMgr.makeIntVal(0, resultTy);
    else if (RHS == 1)
      isIdempotent = true;
    break;
  case BO_Div:
    // a/0 and a/1
    if (RHS == 0)
      // This is also handled elsewhere.
      return UndefinedVal();
    else if (RHS == 1)
      isIdempotent = true;
    break;
  case BO_Rem:
    // a%0 and a%1
    if (RHS == 0)
      // This is also handled elsewhere.
      return UndefinedVal();
    else if (RHS == 1)
      return ValMgr.makeIntVal(0, resultTy);
    break;
  case BO_Add:
  case BO_Sub:
  case BO_Shl:
  case BO_Shr:
  case BO_Xor:
    // a+0, a-0, a<<0, a>>0, a^0
    if (RHS == 0)
      isIdempotent = true;
    break;
  case BO_And:
    // a&0 and a&(~0)
    if (RHS == 0)
      return ValMgr.makeIntVal(0, resultTy);
    else if (RHS.isAllOnesValue())
      isIdempotent = true;
    break;
  case BO_Or:
    // a|0 and a|(~0)
    if (RHS == 0)
      isIdempotent = true;
    else if (RHS.isAllOnesValue()) {
      BasicValueFactory &BVF = ValMgr.getBasicValueFactory();
      const llvm::APSInt &Result = BVF.Convert(resultTy, RHS);
      return nonloc::ConcreteInt(Result);
    }
    break;
  }

  // Idempotent ops (like a*1) can still change the type of an expression.
  // Wrap the LHS up in a NonLoc again and let EvalCastNL do the dirty work.
  if (isIdempotent) {
    if (SymbolRef LHSSym = dyn_cast<SymbolData>(LHS))
      return EvalCastNL(nonloc::SymbolVal(LHSSym), resultTy);
    return EvalCastNL(nonloc::SymExprVal(LHS), resultTy);
  }

  // If we reach this point, the expression cannot be simplified.
  // Make a SymExprVal for the entire thing.
  return ValMgr.makeNonLoc(LHS, op, RHS, resultTy);
}

SVal SimpleSValuator::EvalBinOpNN(const GRState *state,
                                  BinaryOperator::Opcode op,
                                  NonLoc lhs, NonLoc rhs,
                                  QualType resultTy)  {
  // Handle trivial case where left-side and right-side are the same.
  if (lhs == rhs)
    switch (op) {
      default:
        break;
      case BO_EQ:
      case BO_LE:
      case BO_GE:
        return ValMgr.makeTruthVal(true, resultTy);
      case BO_LT:
      case BO_GT:
      case BO_NE:
        return ValMgr.makeTruthVal(false, resultTy);
      case BO_Xor:
      case BO_Sub:
        return ValMgr.makeIntVal(0, resultTy);
      case BO_Or:
      case BO_And:
        return EvalCastNL(lhs, resultTy);
    }

  while (1) {
    switch (lhs.getSubKind()) {
    default:
      return UnknownVal();
    case nonloc::LocAsIntegerKind: {
      Loc lhsL = cast<nonloc::LocAsInteger>(lhs).getLoc();
      switch (rhs.getSubKind()) {
        case nonloc::LocAsIntegerKind:
          return EvalBinOpLL(state, op, lhsL,
                             cast<nonloc::LocAsInteger>(rhs).getLoc(),
                             resultTy);
        case nonloc::ConcreteIntKind: {
          // Transform the integer into a location and compare.
          ASTContext& Ctx = ValMgr.getContext();
          llvm::APSInt i = cast<nonloc::ConcreteInt>(rhs).getValue();
          i.setIsUnsigned(true);
          i.extOrTrunc(Ctx.getTypeSize(Ctx.VoidPtrTy));
          return EvalBinOpLL(state, op, lhsL, ValMgr.makeLoc(i), resultTy);
        }
        default:
          switch (op) {
            case BO_EQ:
              return ValMgr.makeTruthVal(false, resultTy);
            case BO_NE:
              return ValMgr.makeTruthVal(true, resultTy);
            default:
              // This case also handles pointer arithmetic.
              return UnknownVal();
          }
      }
    }
    case nonloc::SymExprValKind: {
      nonloc::SymExprVal *selhs = cast<nonloc::SymExprVal>(&lhs);

      // Only handle LHS of the form "$sym op constant", at least for now.
      const SymIntExpr *symIntExpr =
        dyn_cast<SymIntExpr>(selhs->getSymbolicExpression());

      if (!symIntExpr)
        return UnknownVal();

      // Is this a logical not? (!x is represented as x == 0.)
      if (op == BO_EQ && rhs.isZeroConstant()) {
        // We know how to negate certain expressions. Simplify them here.

        BinaryOperator::Opcode opc = symIntExpr->getOpcode();
        switch (opc) {
        default:
          // We don't know how to negate this operation.
          // Just handle it as if it were a normal comparison to 0.
          break;
        case BO_LAnd:
        case BO_LOr:
          assert(false && "Logical operators handled by branching logic.");
          return UnknownVal();
        case BO_Assign:
        case BO_MulAssign:
        case BO_DivAssign:
        case BO_RemAssign:
        case BO_AddAssign:
        case BO_SubAssign:
        case BO_ShlAssign:
        case BO_ShrAssign:
        case BO_AndAssign:
        case BO_XorAssign:
        case BO_OrAssign:
        case BO_Comma:
          assert(false && "'=' and ',' operators handled by GRExprEngine.");
          return UnknownVal();
        case BO_PtrMemD:
        case BO_PtrMemI:
          assert(false && "Pointer arithmetic not handled here.");
          return UnknownVal();
        case BO_LT:
        case BO_GT:
        case BO_LE:
        case BO_GE:
        case BO_EQ:
        case BO_NE:
          // Negate the comparison and make a value.
          opc = NegateComparison(opc);
          assert(symIntExpr->getType(ValMgr.getContext()) == resultTy);
          return ValMgr.makeNonLoc(symIntExpr->getLHS(), opc,
                                   symIntExpr->getRHS(), resultTy);
        }
      }

      // For now, only handle expressions whose RHS is a constant.
      const nonloc::ConcreteInt *rhsInt = dyn_cast<nonloc::ConcreteInt>(&rhs);
      if (!rhsInt)
        return UnknownVal();

      // If both the LHS and the current expression are additive,
      // fold their constants.
      if (BinaryOperator::isAdditiveOp(op)) {
        BinaryOperator::Opcode lop = symIntExpr->getOpcode();
        if (BinaryOperator::isAdditiveOp(lop)) {
          BasicValueFactory &BVF = ValMgr.getBasicValueFactory();

          // resultTy may not be the best type to convert to, but it's
          // probably the best choice in expressions with mixed type
          // (such as x+1U+2LL). The rules for implicit conversions should
          // choose a reasonable type to preserve the expression, and will
          // at least match how the value is going to be used.
          const llvm::APSInt &first =
            BVF.Convert(resultTy, symIntExpr->getRHS());
          const llvm::APSInt &second =
            BVF.Convert(resultTy, rhsInt->getValue());

          const llvm::APSInt *newRHS;
          if (lop == op)
            newRHS = BVF.EvaluateAPSInt(BO_Add, first, second);
          else
            newRHS = BVF.EvaluateAPSInt(BO_Sub, first, second);
          return MakeSymIntVal(symIntExpr->getLHS(), lop, *newRHS, resultTy);
        }
      }

      // Otherwise, make a SymExprVal out of the expression.
      return MakeSymIntVal(symIntExpr, op, rhsInt->getValue(), resultTy);
    }
    case nonloc::ConcreteIntKind: {
      const nonloc::ConcreteInt& lhsInt = cast<nonloc::ConcreteInt>(lhs);

      if (isa<nonloc::ConcreteInt>(rhs)) {
        return lhsInt.evalBinOp(ValMgr, op, cast<nonloc::ConcreteInt>(rhs));
      } else {
        const llvm::APSInt& lhsValue = lhsInt.getValue();
        
        // Swap the left and right sides and flip the operator if doing so
        // allows us to better reason about the expression (this is a form
        // of expression canonicalization).
        // While we're at it, catch some special cases for non-commutative ops.
        NonLoc tmp = rhs;
        rhs = lhs;
        lhs = tmp;

        switch (op) {
          case BO_LT:
          case BO_GT:
          case BO_LE:
          case BO_GE:
            op = ReverseComparison(op);
            continue;
          case BO_EQ:
          case BO_NE:
          case BO_Add:
          case BO_Mul:
          case BO_And:
          case BO_Xor:
          case BO_Or:
            continue;
          case BO_Shr:
            if (lhsValue.isAllOnesValue() && lhsValue.isSigned())
              // At this point lhs and rhs have been swapped.
              return rhs;
            // FALL-THROUGH
          case BO_Shl:
            if (lhsValue == 0)
              // At this point lhs and rhs have been swapped.
              return rhs;
            return UnknownVal();
          default:
            return UnknownVal();
        }
      }
    }
    case nonloc::SymbolValKind: {
      nonloc::SymbolVal *slhs = cast<nonloc::SymbolVal>(&lhs);
      SymbolRef Sym = slhs->getSymbol();

      ASTContext& Ctx = ValMgr.getContext();

      // Does the symbol simplify to a constant?  If so, "fold" the constant
      // by setting 'lhs' to a ConcreteInt and try again.
      if (Sym->getType(Ctx)->isIntegerType())
        if (const llvm::APSInt *Constant = state->getSymVal(Sym)) {
          // The symbol evaluates to a constant. If necessary, promote the
          // folded constant (LHS) to the result type.
          BasicValueFactory &BVF = ValMgr.getBasicValueFactory();
          const llvm::APSInt &lhs_I = BVF.Convert(resultTy, *Constant);
          lhs = nonloc::ConcreteInt(lhs_I);
          
          // Also promote the RHS (if necessary).

          // For shifts, it is not necessary to promote the RHS.
          if (BinaryOperator::isShiftOp(op))
            continue;
          
          // Other operators: do an implicit conversion.  This shouldn't be
          // necessary once we support truncation/extension of symbolic values.
          if (nonloc::ConcreteInt *rhs_I = dyn_cast<nonloc::ConcreteInt>(&rhs)){
            rhs = nonloc::ConcreteInt(BVF.Convert(resultTy, rhs_I->getValue()));
          }
          
          continue;
        }

      // Is the RHS a symbol we can simplify?
      if (const nonloc::SymbolVal *srhs = dyn_cast<nonloc::SymbolVal>(&rhs)) {
        SymbolRef RSym = srhs->getSymbol();
        if (RSym->getType(Ctx)->isIntegerType()) {
          if (const llvm::APSInt *Constant = state->getSymVal(RSym)) {
            // The symbol evaluates to a constant.
            BasicValueFactory &BVF = ValMgr.getBasicValueFactory();
            const llvm::APSInt &rhs_I = BVF.Convert(resultTy, *Constant);
            rhs = nonloc::ConcreteInt(rhs_I);
          }
        }
      }

      if (isa<nonloc::ConcreteInt>(rhs)) {
        return MakeSymIntVal(slhs->getSymbol(), op,
                             cast<nonloc::ConcreteInt>(rhs).getValue(),
                             resultTy);
      }

      return UnknownVal();
    }
    }
  }
}

// FIXME: all this logic will change if/when we have MemRegion::getLocation().
SVal SimpleSValuator::EvalBinOpLL(const GRState *state,
                                  BinaryOperator::Opcode op,
                                  Loc lhs, Loc rhs,
                                  QualType resultTy) {
  // Only comparisons and subtractions are valid operations on two pointers.
  // See [C99 6.5.5 through 6.5.14] or [C++0x 5.6 through 5.15].
  // However, if a pointer is casted to an integer, EvalBinOpNN may end up
  // calling this function with another operation (PR7527). We don't attempt to
  // model this for now, but it could be useful, particularly when the
  // "location" is actually an integer value that's been passed through a void*.
  if (!(BinaryOperator::isComparisonOp(op) || op == BO_Sub))
    return UnknownVal();

  // Special cases for when both sides are identical.
  if (lhs == rhs) {
    switch (op) {
    default:
      assert(false && "Unimplemented operation for two identical values");
      return UnknownVal();
    case BO_Sub:
      return ValMgr.makeZeroVal(resultTy);
    case BO_EQ:
    case BO_LE:
    case BO_GE:
      return ValMgr.makeTruthVal(true, resultTy);
    case BO_NE:
    case BO_LT:
    case BO_GT:
      return ValMgr.makeTruthVal(false, resultTy);
    }
  }

  switch (lhs.getSubKind()) {
  default:
    assert(false && "Ordering not implemented for this Loc.");
    return UnknownVal();

  case loc::GotoLabelKind:
    // The only thing we know about labels is that they're non-null.
    if (rhs.isZeroConstant()) {
      switch (op) {
      default:
        break;
      case BO_Sub:
        return EvalCastL(lhs, resultTy);
      case BO_EQ:
      case BO_LE:
      case BO_LT:
        return ValMgr.makeTruthVal(false, resultTy);
      case BO_NE:
      case BO_GT:
      case BO_GE:
        return ValMgr.makeTruthVal(true, resultTy);
      }
    }
    // There may be two labels for the same location, and a function region may
    // have the same address as a label at the start of the function (depending
    // on the ABI).
    // FIXME: we can probably do a comparison against other MemRegions, though.
    // FIXME: is there a way to tell if two labels refer to the same location?
    return UnknownVal(); 

  case loc::ConcreteIntKind: {
    // If one of the operands is a symbol and the other is a constant,
    // build an expression for use by the constraint manager.
    if (SymbolRef rSym = rhs.getAsLocSymbol()) {
      // We can only build expressions with symbols on the left,
      // so we need a reversible operator.
      if (!BinaryOperator::isComparisonOp(op))
        return UnknownVal();

      const llvm::APSInt &lVal = cast<loc::ConcreteInt>(lhs).getValue();
      return ValMgr.makeNonLoc(rSym, ReverseComparison(op), lVal, resultTy);
    }

    // If both operands are constants, just perform the operation.
    if (loc::ConcreteInt *rInt = dyn_cast<loc::ConcreteInt>(&rhs)) {
      BasicValueFactory &BVF = ValMgr.getBasicValueFactory();
      SVal ResultVal = cast<loc::ConcreteInt>(lhs).EvalBinOp(BVF, op, *rInt);
      if (Loc *Result = dyn_cast<Loc>(&ResultVal))
        return EvalCastL(*Result, resultTy);
      else
        return UnknownVal();
    }

    // Special case comparisons against NULL.
    // This must come after the test if the RHS is a symbol, which is used to
    // build constraints. The address of any non-symbolic region is guaranteed
    // to be non-NULL, as is any label.
    assert(isa<loc::MemRegionVal>(rhs) || isa<loc::GotoLabel>(rhs));
    if (lhs.isZeroConstant()) {
      switch (op) {
      default:
        break;
      case BO_EQ:
      case BO_GT:
      case BO_GE:
        return ValMgr.makeTruthVal(false, resultTy);
      case BO_NE:
      case BO_LT:
      case BO_LE:
        return ValMgr.makeTruthVal(true, resultTy);
      }
    }

    // Comparing an arbitrary integer to a region or label address is
    // completely unknowable.
    return UnknownVal();
  }
  case loc::MemRegionKind: {
    if (loc::ConcreteInt *rInt = dyn_cast<loc::ConcreteInt>(&rhs)) {
      // If one of the operands is a symbol and the other is a constant,
      // build an expression for use by the constraint manager.
      if (SymbolRef lSym = lhs.getAsLocSymbol())
        return MakeSymIntVal(lSym, op, rInt->getValue(), resultTy);

      // Special case comparisons to NULL.
      // This must come after the test if the LHS is a symbol, which is used to
      // build constraints. The address of any non-symbolic region is guaranteed
      // to be non-NULL.
      if (rInt->isZeroConstant()) {
        switch (op) {
        default:
          break;
        case BO_Sub:
          return EvalCastL(lhs, resultTy);
        case BO_EQ:
        case BO_LT:
        case BO_LE:
          return ValMgr.makeTruthVal(false, resultTy);
        case BO_NE:
        case BO_GT:
        case BO_GE:
          return ValMgr.makeTruthVal(true, resultTy);
        }
      }

      // Comparing a region to an arbitrary integer is completely unknowable.
      return UnknownVal();
    }

    // Get both values as regions, if possible.
    const MemRegion *LeftMR = lhs.getAsRegion();
    assert(LeftMR && "MemRegionKind SVal doesn't have a region!");

    const MemRegion *RightMR = rhs.getAsRegion();
    if (!RightMR)
      // The RHS is probably a label, which in theory could address a region.
      // FIXME: we can probably make a more useful statement about non-code
      // regions, though.
      return UnknownVal();

    // If both values wrap regions, see if they're from different base regions.
    const MemRegion *LeftBase = LeftMR->getBaseRegion();
    const MemRegion *RightBase = RightMR->getBaseRegion();
    if (LeftBase != RightBase &&
        !isa<SymbolicRegion>(LeftBase) && !isa<SymbolicRegion>(RightBase)) {
      switch (op) {
      default:
        return UnknownVal();
      case BO_EQ:
        return ValMgr.makeTruthVal(false, resultTy);
      case BO_NE:
        return ValMgr.makeTruthVal(true, resultTy);
      }
    }

    // The two regions are from the same base region. See if they're both a
    // type of region we know how to compare.

    // FIXME: If/when there is a getAsRawOffset() for FieldRegions, this
    // ElementRegion path and the FieldRegion path below should be unified.
    if (const ElementRegion *LeftER = dyn_cast<ElementRegion>(LeftMR)) {
      // First see if the right region is also an ElementRegion.
      const ElementRegion *RightER = dyn_cast<ElementRegion>(RightMR);
      if (!RightER)
        return UnknownVal();

      // Next, see if the two ERs have the same super-region and matching types.
      // FIXME: This should do something useful even if the types don't match,
      // though if both indexes are constant the RegionRawOffset path will
      // give the correct answer.
      if (LeftER->getSuperRegion() == RightER->getSuperRegion() &&
          LeftER->getElementType() == RightER->getElementType()) {
        // Get the left index and cast it to the correct type.
        // If the index is unknown or undefined, bail out here.
        SVal LeftIndexVal = LeftER->getIndex();
        NonLoc *LeftIndex = dyn_cast<NonLoc>(&LeftIndexVal);
        if (!LeftIndex)
          return UnknownVal();
        LeftIndexVal = EvalCastNL(*LeftIndex, resultTy);
        LeftIndex = dyn_cast<NonLoc>(&LeftIndexVal);
        if (!LeftIndex)
          return UnknownVal();

        // Do the same for the right index.
        SVal RightIndexVal = RightER->getIndex();
        NonLoc *RightIndex = dyn_cast<NonLoc>(&RightIndexVal);
        if (!RightIndex)
          return UnknownVal();
        RightIndexVal = EvalCastNL(*RightIndex, resultTy);
        RightIndex = dyn_cast<NonLoc>(&RightIndexVal);
        if (!RightIndex)
          return UnknownVal();

        // Actually perform the operation.
        // EvalBinOpNN expects the two indexes to already be the right type.
        return EvalBinOpNN(state, op, *LeftIndex, *RightIndex, resultTy);
      }

      // If the element indexes aren't comparable, see if the raw offsets are.
      RegionRawOffset LeftOffset = LeftER->getAsArrayOffset();
      RegionRawOffset RightOffset = RightER->getAsArrayOffset();

      if (LeftOffset.getRegion() != NULL &&
          LeftOffset.getRegion() == RightOffset.getRegion()) {
        int64_t left = LeftOffset.getByteOffset();
        int64_t right = RightOffset.getByteOffset();

        switch (op) {
        default:
          return UnknownVal();
        case BO_LT:
          return ValMgr.makeTruthVal(left < right, resultTy);
        case BO_GT:
          return ValMgr.makeTruthVal(left > right, resultTy);
        case BO_LE:
          return ValMgr.makeTruthVal(left <= right, resultTy);
        case BO_GE:
          return ValMgr.makeTruthVal(left >= right, resultTy);
        case BO_EQ:
          return ValMgr.makeTruthVal(left == right, resultTy);
        case BO_NE:
          return ValMgr.makeTruthVal(left != right, resultTy);
        }
      }

      // If we get here, we have no way of comparing the ElementRegions.
      return UnknownVal();
    }

    // See if both regions are fields of the same structure.
    // FIXME: This doesn't handle nesting, inheritance, or Objective-C ivars.
    if (const FieldRegion *LeftFR = dyn_cast<FieldRegion>(LeftMR)) {
      // Only comparisons are meaningful here!
      if (!BinaryOperator::isComparisonOp(op))
        return UnknownVal();

      // First see if the right region is also a FieldRegion.
      const FieldRegion *RightFR = dyn_cast<FieldRegion>(RightMR);
      if (!RightFR)
        return UnknownVal();

      // Next, see if the two FRs have the same super-region.
      // FIXME: This doesn't handle casts yet, and simply stripping the casts
      // doesn't help.
      if (LeftFR->getSuperRegion() != RightFR->getSuperRegion())
        return UnknownVal();

      const FieldDecl *LeftFD = LeftFR->getDecl();
      const FieldDecl *RightFD = RightFR->getDecl();
      const RecordDecl *RD = LeftFD->getParent();

      // Make sure the two FRs are from the same kind of record. Just in case!
      // FIXME: This is probably where inheritance would be a problem.
      if (RD != RightFD->getParent())
        return UnknownVal();

      // We know for sure that the two fields are not the same, since that
      // would have given us the same SVal.
      if (op == BO_EQ)
        return ValMgr.makeTruthVal(false, resultTy);
      if (op == BO_NE)
        return ValMgr.makeTruthVal(true, resultTy);

      // Iterate through the fields and see which one comes first.
      // [C99 6.7.2.1.13] "Within a structure object, the non-bit-field
      // members and the units in which bit-fields reside have addresses that
      // increase in the order in which they are declared."
      bool leftFirst = (op == BO_LT || op == BO_LE);
      for (RecordDecl::field_iterator I = RD->field_begin(),
           E = RD->field_end(); I!=E; ++I) {
        if (*I == LeftFD)
          return ValMgr.makeTruthVal(leftFirst, resultTy);
        if (*I == RightFD)
          return ValMgr.makeTruthVal(!leftFirst, resultTy);
      }

      assert(false && "Fields not found in parent record's definition");
    }

    // If we get here, we have no way of comparing the regions.
    return UnknownVal();
  }
  }
}

SVal SimpleSValuator::EvalBinOpLN(const GRState *state,
                                  BinaryOperator::Opcode op,
                                  Loc lhs, NonLoc rhs, QualType resultTy) {
  // Special case: 'rhs' is an integer that has the same width as a pointer and
  // we are using the integer location in a comparison.  Normally this cannot be
  // triggered, but transfer functions like those for OSCommpareAndSwapBarrier32
  // can generate comparisons that trigger this code.
  // FIXME: Are all locations guaranteed to have pointer width?
  if (BinaryOperator::isComparisonOp(op)) {
    if (nonloc::ConcreteInt *rhsInt = dyn_cast<nonloc::ConcreteInt>(&rhs)) {
      const llvm::APSInt *x = &rhsInt->getValue();
      ASTContext &ctx = ValMgr.getContext();
      if (ctx.getTypeSize(ctx.VoidPtrTy) == x->getBitWidth()) {
        // Convert the signedness of the integer (if necessary).
        if (x->isSigned())
          x = &ValMgr.getBasicValueFactory().getValue(*x, true);

        return EvalBinOpLL(state, op, lhs, loc::ConcreteInt(*x), resultTy);
      }
    }
  }

  // Delegate pointer arithmetic to the StoreManager.
  return state->getStateManager().getStoreManager().EvalBinOp(op, lhs,
                                                              rhs, resultTy);
}

const llvm::APSInt *SimpleSValuator::getKnownValue(const GRState *state,
                                                   SVal V) {
  if (V.isUnknownOrUndef())
    return NULL;

  if (loc::ConcreteInt* X = dyn_cast<loc::ConcreteInt>(&V))
    return &X->getValue();

  if (nonloc::ConcreteInt* X = dyn_cast<nonloc::ConcreteInt>(&V))
    return &X->getValue();

  if (SymbolRef Sym = V.getAsSymbol())
    return state->getSymVal(Sym);

  // FIXME: Add support for SymExprs.
  return NULL;
}