clang  8.0.0
ConversionChecker.cpp
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1 //=== ConversionChecker.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 // Check that there is no loss of sign/precision in assignments, comparisons
11 // and multiplications.
12 //
13 // ConversionChecker uses path sensitive analysis to determine possible values
14 // of expressions. A warning is reported when:
15 // * a negative value is implicitly converted to an unsigned value in an
16 // assignment, comparison or multiplication.
17 // * assignment / initialization when the source value is greater than the max
18 // value of the target integer type
19 // * assignment / initialization when the source integer is above the range
20 // where the target floating point type can represent all integers
21 //
22 // Many compilers and tools have similar checks that are based on semantic
23 // analysis. Those checks are sound but have poor precision. ConversionChecker
24 // is an alternative to those checks.
25 //
26 //===----------------------------------------------------------------------===//
28 #include "clang/AST/ParentMap.h"
33 #include "llvm/ADT/APFloat.h"
34 
35 #include <climits>
36 
37 using namespace clang;
38 using namespace ento;
39 
40 namespace {
41 class ConversionChecker : public Checker<check::PreStmt<ImplicitCastExpr>> {
42 public:
43  void checkPreStmt(const ImplicitCastExpr *Cast, CheckerContext &C) const;
44 
45 private:
46  mutable std::unique_ptr<BuiltinBug> BT;
47 
48  bool isLossOfPrecision(const ImplicitCastExpr *Cast, QualType DestType,
49  CheckerContext &C) const;
50 
51  bool isLossOfSign(const ImplicitCastExpr *Cast, CheckerContext &C) const;
52 
53  void reportBug(ExplodedNode *N, CheckerContext &C, const char Msg[]) const;
54 };
55 }
56 
57 void ConversionChecker::checkPreStmt(const ImplicitCastExpr *Cast,
58  CheckerContext &C) const {
59  // TODO: For now we only warn about DeclRefExpr, to avoid noise. Warn for
60  // calculations also.
61  if (!isa<DeclRefExpr>(Cast->IgnoreParenImpCasts()))
62  return;
63 
64  // Don't warn for loss of sign/precision in macros.
65  if (Cast->getExprLoc().isMacroID())
66  return;
67 
68  // Get Parent.
69  const ParentMap &PM = C.getLocationContext()->getParentMap();
70  const Stmt *Parent = PM.getParent(Cast);
71  if (!Parent)
72  return;
73 
74  bool LossOfSign = false;
75  bool LossOfPrecision = false;
76 
77  // Loss of sign/precision in binary operation.
78  if (const auto *B = dyn_cast<BinaryOperator>(Parent)) {
79  BinaryOperator::Opcode Opc = B->getOpcode();
80  if (Opc == BO_Assign) {
81  LossOfSign = isLossOfSign(Cast, C);
82  LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
83  } else if (Opc == BO_AddAssign || Opc == BO_SubAssign) {
84  // No loss of sign.
85  LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
86  } else if (Opc == BO_MulAssign) {
87  LossOfSign = isLossOfSign(Cast, C);
88  LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
89  } else if (Opc == BO_DivAssign || Opc == BO_RemAssign) {
90  LossOfSign = isLossOfSign(Cast, C);
91  // No loss of precision.
92  } else if (Opc == BO_AndAssign) {
93  LossOfSign = isLossOfSign(Cast, C);
94  // No loss of precision.
95  } else if (Opc == BO_OrAssign || Opc == BO_XorAssign) {
96  LossOfSign = isLossOfSign(Cast, C);
97  LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
98  } else if (B->isRelationalOp() || B->isMultiplicativeOp()) {
99  LossOfSign = isLossOfSign(Cast, C);
100  }
101  } else if (isa<DeclStmt>(Parent)) {
102  LossOfSign = isLossOfSign(Cast, C);
103  LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
104  }
105 
106  if (LossOfSign || LossOfPrecision) {
107  // Generate an error node.
108  ExplodedNode *N = C.generateNonFatalErrorNode(C.getState());
109  if (!N)
110  return;
111  if (LossOfSign)
112  reportBug(N, C, "Loss of sign in implicit conversion");
113  if (LossOfPrecision)
114  reportBug(N, C, "Loss of precision in implicit conversion");
115  }
116 }
117 
118 void ConversionChecker::reportBug(ExplodedNode *N, CheckerContext &C,
119  const char Msg[]) const {
120  if (!BT)
121  BT.reset(
122  new BuiltinBug(this, "Conversion", "Possible loss of sign/precision."));
123 
124  // Generate a report for this bug.
125  auto R = llvm::make_unique<BugReport>(*BT, Msg, N);
126  C.emitReport(std::move(R));
127 }
128 
129 bool ConversionChecker::isLossOfPrecision(const ImplicitCastExpr *Cast,
130  QualType DestType,
131  CheckerContext &C) const {
132  // Don't warn about explicit loss of precision.
133  if (Cast->isEvaluatable(C.getASTContext()))
134  return false;
135 
136  QualType SubType = Cast->IgnoreParenImpCasts()->getType();
137 
138  if (!DestType->isRealType() || !SubType->isIntegerType())
139  return false;
140 
141  const bool isFloat = DestType->isFloatingType();
142 
143  const auto &AC = C.getASTContext();
144 
145  // We will find the largest RepresentsUntilExp value such that the DestType
146  // can exactly represent all nonnegative integers below 2^RepresentsUntilExp.
147  unsigned RepresentsUntilExp;
148 
149  if (isFloat) {
150  const llvm::fltSemantics &Sema = AC.getFloatTypeSemantics(DestType);
151  RepresentsUntilExp = llvm::APFloat::semanticsPrecision(Sema);
152  } else {
153  RepresentsUntilExp = AC.getIntWidth(DestType);
154  if (RepresentsUntilExp == 1) {
155  // This is just casting a number to bool, probably not a bug.
156  return false;
157  }
158  if (DestType->isSignedIntegerType())
159  RepresentsUntilExp--;
160  }
161 
162  if (RepresentsUntilExp >= sizeof(unsigned long long) * CHAR_BIT) {
163  // Avoid overflow in our later calculations.
164  return false;
165  }
166 
167  unsigned CorrectedSrcWidth = AC.getIntWidth(SubType);
168  if (SubType->isSignedIntegerType())
169  CorrectedSrcWidth--;
170 
171  if (RepresentsUntilExp >= CorrectedSrcWidth) {
172  // Simple case: the destination can store all values of the source type.
173  return false;
174  }
175 
176  unsigned long long MaxVal = 1ULL << RepresentsUntilExp;
177  if (isFloat) {
178  // If this is a floating point type, it can also represent MaxVal exactly.
179  MaxVal++;
180  }
181  return C.isGreaterOrEqual(Cast->getSubExpr(), MaxVal);
182  // TODO: maybe also check negative values with too large magnitude.
183 }
184 
185 bool ConversionChecker::isLossOfSign(const ImplicitCastExpr *Cast,
186  CheckerContext &C) const {
187  QualType CastType = Cast->getType();
188  QualType SubType = Cast->IgnoreParenImpCasts()->getType();
189 
190  if (!CastType->isUnsignedIntegerType() || !SubType->isSignedIntegerType())
191  return false;
192 
193  return C.isNegative(Cast->getSubExpr());
194 }
195 
196 void ento::registerConversionChecker(CheckerManager &mgr) {
197  mgr.registerChecker<ConversionChecker>();
198 }
A (possibly-)qualified type.
Definition: Type.h:638
Stmt - This represents one statement.
Definition: Stmt.h:66
Stmt * getParent(Stmt *) const
Definition: ParentMap.cpp:123
bool isUnsignedIntegerType() const
Return true if this is an integer type that is unsigned, according to C99 6.2.5p6 [which returns true...
Definition: Type.cpp:1884
Expr * getSubExpr()
Definition: Expr.h:3050
BinaryOperatorKind
#define CHAR_BIT
Definition: limits.h:79
NodeId Parent
Definition: ASTDiff.cpp:192
Sema - This implements semantic analysis and AST building for C.
Definition: Sema.h:278
bool isSignedIntegerType() const
Return true if this is an integer type that is signed, according to C99 6.2.5p4 [char, signed char, short, int, long..], or an enum decl which has a signed representation.
Definition: Type.cpp:1844
QualType getType() const
Definition: Expr.h:128
ImplicitCastExpr - Allows us to explicitly represent implicit type conversions, which have no direct ...
Definition: Expr.h:3115
SourceLocation getExprLoc() const LLVM_READONLY
getExprLoc - Return the preferred location for the arrow when diagnosing a problem with a generic exp...
Definition: Expr.cpp:215
bool isRealType() const
Definition: Type.cpp:1943
Dataflow Directional Tag Classes.
Expr * IgnoreParenImpCasts() LLVM_READONLY
IgnoreParenImpCasts - Ignore parentheses and implicit casts.
Definition: Expr.cpp:2693
bool isIntegerType() const
isIntegerType() does not include complex integers (a GCC extension).
Definition: Type.h:6578
bool isEvaluatable(const ASTContext &Ctx, SideEffectsKind AllowSideEffects=SE_NoSideEffects) const
isEvaluatable - Call EvaluateAsRValue to see if this expression can be constant folded without side-e...
bool isFloatingType() const
Definition: Type.cpp:1921
CastType
Definition: SemaCast.cpp:46