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      1 //== SimpleConstraintManager.cpp --------------------------------*- C++ -*--==//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 //  This file defines SimpleConstraintManager, a class that holds code shared
     11 //  between BasicConstraintManager and RangeConstraintManager.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "SimpleConstraintManager.h"
     16 #include "clang/StaticAnalyzer/Core/PathSensitive/APSIntType.h"
     17 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
     18 #include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
     19 
     20 namespace clang {
     21 
     22 namespace ento {
     23 
     24 SimpleConstraintManager::~SimpleConstraintManager() {}
     25 
     26 bool SimpleConstraintManager::canReasonAbout(SVal X) const {
     27   Optional<nonloc::SymbolVal> SymVal = X.getAs<nonloc::SymbolVal>();
     28   if (SymVal && SymVal->isExpression()) {
     29     const SymExpr *SE = SymVal->getSymbol();
     30 
     31     if (const SymIntExpr *SIE = dyn_cast<SymIntExpr>(SE)) {
     32       switch (SIE->getOpcode()) {
     33           // We don't reason yet about bitwise-constraints on symbolic values.
     34         case BO_And:
     35         case BO_Or:
     36         case BO_Xor:
     37           return false;
     38         // We don't reason yet about these arithmetic constraints on
     39         // symbolic values.
     40         case BO_Mul:
     41         case BO_Div:
     42         case BO_Rem:
     43         case BO_Shl:
     44         case BO_Shr:
     45           return false;
     46         // All other cases.
     47         default:
     48           return true;
     49       }
     50     }
     51 
     52     if (const SymSymExpr *SSE = dyn_cast<SymSymExpr>(SE)) {
     53       if (BinaryOperator::isComparisonOp(SSE->getOpcode())) {
     54         // We handle Loc <> Loc comparisons, but not (yet) NonLoc <> NonLoc.
     55         if (Loc::isLocType(SSE->getLHS()->getType())) {
     56           assert(Loc::isLocType(SSE->getRHS()->getType()));
     57           return true;
     58         }
     59       }
     60     }
     61 
     62     return false;
     63   }
     64 
     65   return true;
     66 }
     67 
     68 ProgramStateRef SimpleConstraintManager::assume(ProgramStateRef state,
     69                                                DefinedSVal Cond,
     70                                                bool Assumption) {
     71   // If we have a Loc value, cast it to a bool NonLoc first.
     72   if (Optional<Loc> LV = Cond.getAs<Loc>()) {
     73     SValBuilder &SVB = state->getStateManager().getSValBuilder();
     74     QualType T;
     75     const MemRegion *MR = LV->getAsRegion();
     76     if (const TypedRegion *TR = dyn_cast_or_null<TypedRegion>(MR))
     77       T = TR->getLocationType();
     78     else
     79       T = SVB.getContext().VoidPtrTy;
     80 
     81     Cond = SVB.evalCast(*LV, SVB.getContext().BoolTy, T).castAs<DefinedSVal>();
     82   }
     83 
     84   return assume(state, Cond.castAs<NonLoc>(), Assumption);
     85 }
     86 
     87 ProgramStateRef SimpleConstraintManager::assume(ProgramStateRef state,
     88                                                NonLoc cond,
     89                                                bool assumption) {
     90   state = assumeAux(state, cond, assumption);
     91   if (NotifyAssumeClients && SU)
     92     return SU->processAssume(state, cond, assumption);
     93   return state;
     94 }
     95 
     96 
     97 ProgramStateRef
     98 SimpleConstraintManager::assumeAuxForSymbol(ProgramStateRef State,
     99                                             SymbolRef Sym, bool Assumption) {
    100   BasicValueFactory &BVF = getBasicVals();
    101   QualType T = Sym->getType();
    102 
    103   // None of the constraint solvers currently support non-integer types.
    104   if (!T->isIntegralOrEnumerationType())
    105     return State;
    106 
    107   const llvm::APSInt &zero = BVF.getValue(0, T);
    108   if (Assumption)
    109     return assumeSymNE(State, Sym, zero, zero);
    110   else
    111     return assumeSymEQ(State, Sym, zero, zero);
    112 }
    113 
    114 ProgramStateRef SimpleConstraintManager::assumeAux(ProgramStateRef state,
    115                                                   NonLoc Cond,
    116                                                   bool Assumption) {
    117 
    118   // We cannot reason about SymSymExprs, and can only reason about some
    119   // SymIntExprs.
    120   if (!canReasonAbout(Cond)) {
    121     // Just add the constraint to the expression without trying to simplify.
    122     SymbolRef sym = Cond.getAsSymExpr();
    123     return assumeAuxForSymbol(state, sym, Assumption);
    124   }
    125 
    126   switch (Cond.getSubKind()) {
    127   default:
    128     llvm_unreachable("'Assume' not implemented for this NonLoc");
    129 
    130   case nonloc::SymbolValKind: {
    131     nonloc::SymbolVal SV = Cond.castAs<nonloc::SymbolVal>();
    132     SymbolRef sym = SV.getSymbol();
    133     assert(sym);
    134 
    135     // Handle SymbolData.
    136     if (!SV.isExpression()) {
    137       return assumeAuxForSymbol(state, sym, Assumption);
    138 
    139     // Handle symbolic expression.
    140     } else if (const SymIntExpr *SE = dyn_cast<SymIntExpr>(sym)) {
    141       // We can only simplify expressions whose RHS is an integer.
    142 
    143       BinaryOperator::Opcode op = SE->getOpcode();
    144       if (BinaryOperator::isComparisonOp(op)) {
    145         if (!Assumption)
    146           op = BinaryOperator::negateComparisonOp(op);
    147 
    148         return assumeSymRel(state, SE->getLHS(), op, SE->getRHS());
    149       }
    150 
    151     } else if (const SymSymExpr *SSE = dyn_cast<SymSymExpr>(sym)) {
    152       // Translate "a != b" to "(b - a) != 0".
    153       // We invert the order of the operands as a heuristic for how loop
    154       // conditions are usually written ("begin != end") as compared to length
    155       // calculations ("end - begin"). The more correct thing to do would be to
    156       // canonicalize "a - b" and "b - a", which would allow us to treat
    157       // "a != b" and "b != a" the same.
    158       SymbolManager &SymMgr = getSymbolManager();
    159       BinaryOperator::Opcode Op = SSE->getOpcode();
    160       assert(BinaryOperator::isComparisonOp(Op));
    161 
    162       // For now, we only support comparing pointers.
    163       assert(Loc::isLocType(SSE->getLHS()->getType()));
    164       assert(Loc::isLocType(SSE->getRHS()->getType()));
    165       QualType DiffTy = SymMgr.getContext().getPointerDiffType();
    166       SymbolRef Subtraction = SymMgr.getSymSymExpr(SSE->getRHS(), BO_Sub,
    167                                                    SSE->getLHS(), DiffTy);
    168 
    169       const llvm::APSInt &Zero = getBasicVals().getValue(0, DiffTy);
    170       Op = BinaryOperator::reverseComparisonOp(Op);
    171       if (!Assumption)
    172         Op = BinaryOperator::negateComparisonOp(Op);
    173       return assumeSymRel(state, Subtraction, Op, Zero);
    174     }
    175 
    176     // If we get here, there's nothing else we can do but treat the symbol as
    177     // opaque.
    178     return assumeAuxForSymbol(state, sym, Assumption);
    179   }
    180 
    181   case nonloc::ConcreteIntKind: {
    182     bool b = Cond.castAs<nonloc::ConcreteInt>().getValue() != 0;
    183     bool isFeasible = b ? Assumption : !Assumption;
    184     return isFeasible ? state : nullptr;
    185   }
    186 
    187   case nonloc::LocAsIntegerKind:
    188     return assume(state, Cond.castAs<nonloc::LocAsInteger>().getLoc(),
    189                   Assumption);
    190   } // end switch
    191 }
    192 
    193 static void computeAdjustment(SymbolRef &Sym, llvm::APSInt &Adjustment) {
    194   // Is it a "($sym+constant1)" expression?
    195   if (const SymIntExpr *SE = dyn_cast<SymIntExpr>(Sym)) {
    196     BinaryOperator::Opcode Op = SE->getOpcode();
    197     if (Op == BO_Add || Op == BO_Sub) {
    198       Sym = SE->getLHS();
    199       Adjustment = APSIntType(Adjustment).convert(SE->getRHS());
    200 
    201       // Don't forget to negate the adjustment if it's being subtracted.
    202       // This should happen /after/ promotion, in case the value being
    203       // subtracted is, say, CHAR_MIN, and the promoted type is 'int'.
    204       if (Op == BO_Sub)
    205         Adjustment = -Adjustment;
    206     }
    207   }
    208 }
    209 
    210 ProgramStateRef SimpleConstraintManager::assumeSymRel(ProgramStateRef state,
    211                                                      const SymExpr *LHS,
    212                                                      BinaryOperator::Opcode op,
    213                                                      const llvm::APSInt& Int) {
    214   assert(BinaryOperator::isComparisonOp(op) &&
    215          "Non-comparison ops should be rewritten as comparisons to zero.");
    216 
    217   // Get the type used for calculating wraparound.
    218   BasicValueFactory &BVF = getBasicVals();
    219   APSIntType WraparoundType = BVF.getAPSIntType(LHS->getType());
    220 
    221   // We only handle simple comparisons of the form "$sym == constant"
    222   // or "($sym+constant1) == constant2".
    223   // The adjustment is "constant1" in the above expression. It's used to
    224   // "slide" the solution range around for modular arithmetic. For example,
    225   // x < 4 has the solution [0, 3]. x+2 < 4 has the solution [0-2, 3-2], which
    226   // in modular arithmetic is [0, 1] U [UINT_MAX-1, UINT_MAX]. It's up to
    227   // the subclasses of SimpleConstraintManager to handle the adjustment.
    228   SymbolRef Sym = LHS;
    229   llvm::APSInt Adjustment = WraparoundType.getZeroValue();
    230   computeAdjustment(Sym, Adjustment);
    231 
    232   // Convert the right-hand side integer as necessary.
    233   APSIntType ComparisonType = std::max(WraparoundType, APSIntType(Int));
    234   llvm::APSInt ConvertedInt = ComparisonType.convert(Int);
    235 
    236   // Prefer unsigned comparisons.
    237   if (ComparisonType.getBitWidth() == WraparoundType.getBitWidth() &&
    238       ComparisonType.isUnsigned() && !WraparoundType.isUnsigned())
    239     Adjustment.setIsSigned(false);
    240 
    241   switch (op) {
    242   default:
    243     llvm_unreachable("invalid operation not caught by assertion above");
    244 
    245   case BO_EQ:
    246     return assumeSymEQ(state, Sym, ConvertedInt, Adjustment);
    247 
    248   case BO_NE:
    249     return assumeSymNE(state, Sym, ConvertedInt, Adjustment);
    250 
    251   case BO_GT:
    252     return assumeSymGT(state, Sym, ConvertedInt, Adjustment);
    253 
    254   case BO_GE:
    255     return assumeSymGE(state, Sym, ConvertedInt, Adjustment);
    256 
    257   case BO_LT:
    258     return assumeSymLT(state, Sym, ConvertedInt, Adjustment);
    259 
    260   case BO_LE:
    261     return assumeSymLE(state, Sym, ConvertedInt, Adjustment);
    262   } // end switch
    263 }
    264 
    265 } // end of namespace ento
    266 
    267 } // end of namespace clang
    268