clang 19.0.0git
ConversionChecker.cpp
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1//=== ConversionChecker.cpp -------------------------------------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// Check that there is no loss of sign/precision in assignments, comparisons
10// and multiplications.
11//
12// ConversionChecker uses path sensitive analysis to determine possible values
13// of expressions. A warning is reported when:
14// * a negative value is implicitly converted to an unsigned value in an
15// assignment, comparison or multiplication.
16// * assignment / initialization when the source value is greater than the max
17// value of the target integer type
18// * assignment / initialization when the source integer is above the range
19// where the target floating point type can represent all integers
20//
21// Many compilers and tools have similar checks that are based on semantic
22// analysis. Those checks are sound but have poor precision. ConversionChecker
23// is an alternative to those checks.
24//
25//===----------------------------------------------------------------------===//
27#include "clang/AST/ParentMap.h"
32#include "llvm/ADT/APFloat.h"
33
34#include <climits>
35
36using namespace clang;
37using namespace ento;
38
39namespace {
40class ConversionChecker : public Checker<check::PreStmt<ImplicitCastExpr>> {
41public:
42 void checkPreStmt(const ImplicitCastExpr *Cast, CheckerContext &C) const;
43
44private:
45 const BugType BT{this, "Conversion"};
46
47 bool isLossOfPrecision(const ImplicitCastExpr *Cast, QualType DestType,
48 CheckerContext &C) const;
49
50 bool isLossOfSign(const ImplicitCastExpr *Cast, CheckerContext &C) const;
51
52 void reportBug(ExplodedNode *N, const Expr *E, CheckerContext &C,
53 const char Msg[]) const;
54};
55}
56
57void ConversionChecker::checkPreStmt(const ImplicitCastExpr *Cast,
58 CheckerContext &C) const {
59 // Don't warn for implicit conversions to bool
60 if (Cast->getType()->isBooleanType())
61 return;
62
63 // Don't warn for loss of sign/precision in macros.
64 if (Cast->getExprLoc().isMacroID())
65 return;
66
67 // Get Parent.
68 const ParentMap &PM = C.getLocationContext()->getParentMap();
69 const Stmt *Parent = PM.getParent(Cast);
70 if (!Parent)
71 return;
72 // Dont warn if this is part of an explicit cast
73 if (isa<ExplicitCastExpr>(Parent))
74 return;
75
76 bool LossOfSign = false;
77 bool LossOfPrecision = false;
78
79 // Loss of sign/precision in binary operation.
80 if (const auto *B = dyn_cast<BinaryOperator>(Parent)) {
81 BinaryOperator::Opcode Opc = B->getOpcode();
82 if (Opc == BO_Assign) {
83 if (!Cast->IgnoreParenImpCasts()->isEvaluatable(C.getASTContext())) {
84 LossOfSign = isLossOfSign(Cast, C);
85 LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
86 }
87 } else if (Opc == BO_AddAssign || Opc == BO_SubAssign) {
88 // No loss of sign.
89 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
90 } else if (Opc == BO_MulAssign) {
91 LossOfSign = isLossOfSign(Cast, C);
92 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
93 } else if (Opc == BO_DivAssign || Opc == BO_RemAssign) {
94 LossOfSign = isLossOfSign(Cast, C);
95 // No loss of precision.
96 } else if (Opc == BO_AndAssign) {
97 LossOfSign = isLossOfSign(Cast, C);
98 // No loss of precision.
99 } else if (Opc == BO_OrAssign || Opc == BO_XorAssign) {
100 LossOfSign = isLossOfSign(Cast, C);
101 LossOfPrecision = isLossOfPrecision(Cast, B->getLHS()->getType(), C);
102 } else if (B->isRelationalOp() || B->isMultiplicativeOp()) {
103 LossOfSign = isLossOfSign(Cast, C);
104 }
105 } else if (isa<DeclStmt, ReturnStmt>(Parent)) {
106 if (!Cast->IgnoreParenImpCasts()->isEvaluatable(C.getASTContext())) {
107 LossOfSign = isLossOfSign(Cast, C);
108 LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
109 }
110 } else {
111 LossOfSign = isLossOfSign(Cast, C);
112 LossOfPrecision = isLossOfPrecision(Cast, Cast->getType(), C);
113 }
114
115 if (LossOfSign || LossOfPrecision) {
116 // Generate an error node.
117 ExplodedNode *N = C.generateNonFatalErrorNode(C.getState());
118 if (!N)
119 return;
120 if (LossOfSign)
121 reportBug(N, Cast, C, "Loss of sign in implicit conversion");
122 if (LossOfPrecision)
123 reportBug(N, Cast, C, "Loss of precision in implicit conversion");
124 }
125}
126
127void ConversionChecker::reportBug(ExplodedNode *N, const Expr *E,
128 CheckerContext &C, const char Msg[]) const {
129 // Generate a report for this bug.
130 auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, N);
132 C.emitReport(std::move(R));
133}
134
135bool ConversionChecker::isLossOfPrecision(const ImplicitCastExpr *Cast,
136 QualType DestType,
137 CheckerContext &C) const {
138 // Don't warn about explicit loss of precision.
139 if (Cast->isEvaluatable(C.getASTContext()))
140 return false;
141
142 QualType SubType = Cast->IgnoreParenImpCasts()->getType();
143
144 if (!DestType->isRealType() || !SubType->isIntegerType())
145 return false;
146
147 const bool isFloat = DestType->isFloatingType();
148
149 const auto &AC = C.getASTContext();
150
151 // We will find the largest RepresentsUntilExp value such that the DestType
152 // can exactly represent all nonnegative integers below 2^RepresentsUntilExp.
153 unsigned RepresentsUntilExp;
154
155 if (isFloat) {
156 const llvm::fltSemantics &Sema = AC.getFloatTypeSemantics(DestType);
157 RepresentsUntilExp = llvm::APFloat::semanticsPrecision(Sema);
158 } else {
159 RepresentsUntilExp = AC.getIntWidth(DestType);
160 if (RepresentsUntilExp == 1) {
161 // This is just casting a number to bool, probably not a bug.
162 return false;
163 }
164 if (DestType->isSignedIntegerType())
165 RepresentsUntilExp--;
166 }
167
168 if (RepresentsUntilExp >= sizeof(unsigned long long) * CHAR_BIT) {
169 // Avoid overflow in our later calculations.
170 return false;
171 }
172
173 unsigned CorrectedSrcWidth = AC.getIntWidth(SubType);
174 if (SubType->isSignedIntegerType())
175 CorrectedSrcWidth--;
176
177 if (RepresentsUntilExp >= CorrectedSrcWidth) {
178 // Simple case: the destination can store all values of the source type.
179 return false;
180 }
181
182 unsigned long long MaxVal = 1ULL << RepresentsUntilExp;
183 if (isFloat) {
184 // If this is a floating point type, it can also represent MaxVal exactly.
185 MaxVal++;
186 }
187 return C.isGreaterOrEqual(Cast->getSubExpr(), MaxVal);
188 // TODO: maybe also check negative values with too large magnitude.
189}
190
191bool ConversionChecker::isLossOfSign(const ImplicitCastExpr *Cast,
192 CheckerContext &C) const {
193 QualType CastType = Cast->getType();
194 QualType SubType = Cast->IgnoreParenImpCasts()->getType();
195
196 if (!CastType->isUnsignedIntegerType() || !SubType->isSignedIntegerType())
197 return false;
198
199 return C.isNegative(Cast->getSubExpr());
200}
201
202void ento::registerConversionChecker(CheckerManager &mgr) {
203 mgr.registerChecker<ConversionChecker>();
204}
205
206bool ento::shouldRegisterConversionChecker(const CheckerManager &mgr) {
207 return true;
208}
NodeId Parent
Definition: ASTDiff.cpp:191
Expr * E
CastType
Definition: SemaCast.cpp:49
This represents one expression.
Definition: Expr.h:110
ImplicitCastExpr - Allows us to explicitly represent implicit type conversions, which have no direct ...
Definition: Expr.h:3660
Stmt * getParent(Stmt *) const
Definition: ParentMap.cpp:136
A (possibly-)qualified type.
Definition: Type.h:942
Sema - This implements semantic analysis and AST building for C.
Definition: Sema.h:535
Stmt - This represents one statement.
Definition: Stmt.h:84
bool isSignedIntegerType() const
Return true if this is an integer type that is signed, according to C99 6.2.5p4 [char,...
Definition: Type.cpp:2145
bool isIntegerType() const
isIntegerType() does not include complex integers (a GCC extension).
Definition: Type.h:8336
bool isFloatingType() const
Definition: Type.cpp:2248
bool isRealType() const
Definition: Type.cpp:2271
CHECKER * registerChecker(AT &&... Args)
Used to register checkers.
#define CHAR_BIT
Definition: limits.h:71
bool trackExpressionValue(const ExplodedNode *N, const Expr *E, PathSensitiveBugReport &R, TrackingOptions Opts={})
Attempts to add visitors to track expression value back to its point of origin.
bool Cast(InterpState &S, CodePtr OpPC)
Definition: Interp.h:1961
The JSON file list parser is used to communicate input to InstallAPI.
BinaryOperatorKind