clang-tools-extra/clang-tidy/bugprone/TooSmallLoopVariableCheck.cpp

   1 //===--- TooSmallLoopVariableCheck.cpp - clang-tidy -----------------------===//
   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 #include "TooSmallLoopVariableCheck.h"
  10 #include "clang/AST/ASTContext.h"
  11 #include "clang/ASTMatchers/ASTMatchFinder.h"
  12
  13 using namespace clang::ast_matchers;
  14
  15 namespace clang::tidy::bugprone {
  16
  17 static constexpr llvm::StringLiteral LoopName =
  18     llvm::StringLiteral("forLoopName");
  19 static constexpr llvm::StringLiteral LoopVarName =
  20     llvm::StringLiteral("loopVar");
  21 static constexpr llvm::StringLiteral LoopVarCastName =
  22     llvm::StringLiteral("loopVarCast");
  23 static constexpr llvm::StringLiteral LoopUpperBoundName =
  24     llvm::StringLiteral("loopUpperBound");
  25 static constexpr llvm::StringLiteral LoopIncrementName =
  26     llvm::StringLiteral("loopIncrement");
  27
  28 namespace {
  29
  30 struct MagnitudeBits {
  31   unsigned WidthWithoutSignBit = 0U;
  32   unsigned BitFieldWidth = 0U;
  33
  34   bool operator<(const MagnitudeBits &Other) const noexcept {
  35     return WidthWithoutSignBit < Other.WidthWithoutSignBit;
  36   }
  37
  38   bool operator!=(const MagnitudeBits &Other) const noexcept {
  39     return WidthWithoutSignBit != Other.WidthWithoutSignBit ||
  40            BitFieldWidth != Other.BitFieldWidth;
  41   }
  42 };
  43
  44 } // namespace
  45
  46 TooSmallLoopVariableCheck::TooSmallLoopVariableCheck(StringRef Name,
  47                                                      ClangTidyContext *Context)
  48     : ClangTidyCheck(Name, Context),
  49       MagnitudeBitsUpperLimit(Options.get("MagnitudeBitsUpperLimit", 16U)) {}
  50
  51 void TooSmallLoopVariableCheck::storeOptions(
  52     ClangTidyOptions::OptionMap &Opts) {
  53   Options.store(Opts, "MagnitudeBitsUpperLimit", MagnitudeBitsUpperLimit);
  54 }
  55
  56 /// The matcher for loops with suspicious integer loop variable.
  57 ///
  58 /// In this general example, assuming 'j' and 'k' are of integral type:
  59 /// \code
  60 ///   for (...; j < 3 + 2; ++k) { ... }
  61 /// \endcode
  62 /// The following string identifiers are bound to these parts of the AST:
  63 ///   LoopVarName: 'j' (as a VarDecl)
  64 ///   LoopVarCastName: 'j' (after implicit conversion)
  65 ///   LoopUpperBoundName: '3 + 2' (as an Expr)
  66 ///   LoopIncrementName: 'k' (as an Expr)
  67 ///   LoopName: The entire for loop (as a ForStmt)
  68 ///
  69 void TooSmallLoopVariableCheck::registerMatchers(MatchFinder *Finder) {
  70   StatementMatcher LoopVarMatcher =
  71       expr(ignoringParenImpCasts(
  72                anyOf(declRefExpr(to(varDecl(hasType(isInteger())))),
  73                      memberExpr(member(fieldDecl(hasType(isInteger())))))))
  74           .bind(LoopVarName);
  75
  76   // We need to catch only those comparisons which contain any integer cast.
  77   StatementMatcher LoopVarConversionMatcher = traverse(
  78       TK_AsIs, implicitCastExpr(hasImplicitDestinationType(isInteger()),
  79                                 has(ignoringParenImpCasts(LoopVarMatcher)))
  80                    .bind(LoopVarCastName));
  81
  82   // We are interested in only those cases when the loop bound is a variable
  83   // value (not const, enum, etc.).
  84   StatementMatcher LoopBoundMatcher =
  85       expr(ignoringParenImpCasts(allOf(hasType(isInteger()),
  86                                        unless(integerLiteral()),
  87                                        unless(hasType(isConstQualified())),
  88                                        unless(hasType(enumType())))))
  89           .bind(LoopUpperBoundName);
  90
  91   // We use the loop increment expression only to make sure we found the right
  92   // loop variable.
  93   StatementMatcher IncrementMatcher =
  94       expr(ignoringParenImpCasts(hasType(isInteger()))).bind(LoopIncrementName);
  95
  96   Finder->addMatcher(
  97       forStmt(
  98           hasCondition(anyOf(
  99               binaryOperator(hasOperatorName("<"),
 100                              hasLHS(LoopVarConversionMatcher),
 101                              hasRHS(LoopBoundMatcher)),
 102               binaryOperator(hasOperatorName("<="),
 103                              hasLHS(LoopVarConversionMatcher),
 104                              hasRHS(LoopBoundMatcher)),
 105               binaryOperator(hasOperatorName(">"), hasLHS(LoopBoundMatcher),
 106                              hasRHS(LoopVarConversionMatcher)),
 107               binaryOperator(hasOperatorName(">="), hasLHS(LoopBoundMatcher),
 108                              hasRHS(LoopVarConversionMatcher)))),
 109           hasIncrement(IncrementMatcher))
 110           .bind(LoopName),
 111       this);
 112 }
 113
 114 /// Returns the magnitude bits of an integer type.
 115 static MagnitudeBits calcMagnitudeBits(const ASTContext &Context,
 116                                        const QualType &IntExprType,
 117                                        const Expr *IntExpr) {
 118   assert(IntExprType->isIntegerType());
 119
 120   unsigned SignedBits = IntExprType->isUnsignedIntegerType() ? 0U : 1U;
 121
 122   if (const auto *BitField = IntExpr->getSourceBitField()) {
 123     unsigned BitFieldWidth = BitField->getBitWidthValue(Context);
 124     return {BitFieldWidth - SignedBits, BitFieldWidth};
 125   }
 126
 127   unsigned IntWidth = Context.getIntWidth(IntExprType);
 128   return {IntWidth - SignedBits, 0U};
 129 }
 130
 131 /// Calculate the upper bound expression's magnitude bits, but ignore
 132 /// constant like values to reduce false positives.
 133 static MagnitudeBits
 134 calcUpperBoundMagnitudeBits(const ASTContext &Context, const Expr *UpperBound,
 135                             const QualType &UpperBoundType) {
 136   // Ignore casting caused by constant values inside a binary operator.
 137   // We are interested in variable values' magnitude bits.
 138   if (const auto *BinOperator = dyn_cast<BinaryOperator>(UpperBound)) {
 139     const Expr *RHSE = BinOperator->getRHS()->IgnoreParenImpCasts();
 140     const Expr *LHSE = BinOperator->getLHS()->IgnoreParenImpCasts();
 141
 142     QualType RHSEType = RHSE->getType();
 143     QualType LHSEType = LHSE->getType();
 144
 145     if (!RHSEType->isIntegerType() || !LHSEType->isIntegerType())
 146       return {};
 147
 148     bool RHSEIsConstantValue = RHSEType->isEnumeralType() ||
 149                                RHSEType.isConstQualified() ||
 150                                isa<IntegerLiteral>(RHSE);
 151     bool LHSEIsConstantValue = LHSEType->isEnumeralType() ||
 152                                LHSEType.isConstQualified() ||
 153                                isa<IntegerLiteral>(LHSE);
 154
 155     // Avoid false positives produced by two constant values.
 156     if (RHSEIsConstantValue && LHSEIsConstantValue)
 157       return {};
 158     if (RHSEIsConstantValue)
 159       return calcMagnitudeBits(Context, LHSEType, LHSE);
 160     if (LHSEIsConstantValue)
 161       return calcMagnitudeBits(Context, RHSEType, RHSE);
 162
 163     return std::max(calcMagnitudeBits(Context, LHSEType, LHSE),
 164                     calcMagnitudeBits(Context, RHSEType, RHSE));
 165   }
 166
 167   return calcMagnitudeBits(Context, UpperBoundType, UpperBound);
 168 }
 169
 170 static std::string formatIntegralType(const QualType &Type,
 171                                       const MagnitudeBits &Info) {
 172   std::string Name = Type.getAsString();
 173   if (!Info.BitFieldWidth)
 174     return Name;
 175
 176   Name += ':';
 177   Name += std::to_string(Info.BitFieldWidth);
 178   return Name;
 179 }
 180
 181 void TooSmallLoopVariableCheck::check(const MatchFinder::MatchResult &Result) {
 182   const auto *LoopVar = Result.Nodes.getNodeAs<Expr>(LoopVarName);
 183   const auto *UpperBound =
 184       Result.Nodes.getNodeAs<Expr>(LoopUpperBoundName)->IgnoreParenImpCasts();
 185   const auto *LoopIncrement =
 186       Result.Nodes.getNodeAs<Expr>(LoopIncrementName)->IgnoreParenImpCasts();
 187
 188   // We matched the loop variable incorrectly.
 189   if (LoopVar->getType() != LoopIncrement->getType())
 190     return;
 191
 192   ASTContext &Context = *Result.Context;
 193
 194   const QualType LoopVarType = LoopVar->getType();
 195   const MagnitudeBits LoopVarMagnitudeBits =
 196       calcMagnitudeBits(Context, LoopVarType, LoopVar);
 197
 198   const MagnitudeBits LoopIncrementMagnitudeBits =
 199       calcMagnitudeBits(Context, LoopIncrement->getType(), LoopIncrement);
 200   // We matched the loop variable incorrectly.
 201   if (LoopIncrementMagnitudeBits != LoopVarMagnitudeBits)
 202     return;
 203
 204   const QualType UpperBoundType = UpperBound->getType();
 205   const MagnitudeBits UpperBoundMagnitudeBits =
 206       calcUpperBoundMagnitudeBits(Context, UpperBound, UpperBoundType);
 207
 208   if ((0U == UpperBoundMagnitudeBits.WidthWithoutSignBit) ||
 209       (LoopVarMagnitudeBits.WidthWithoutSignBit > MagnitudeBitsUpperLimit) ||
 210       (LoopVarMagnitudeBits.WidthWithoutSignBit >=
 211        UpperBoundMagnitudeBits.WidthWithoutSignBit))
 212     return;
 213
 214   diag(LoopVar->getBeginLoc(),
 215        "loop variable has narrower type '%0' than iteration's upper bound '%1'")
 216       << formatIntegralType(LoopVarType, LoopVarMagnitudeBits)
 217       << formatIntegralType(UpperBoundType, UpperBoundMagnitudeBits);
 218 }
 219
 220 } // namespace clang::tidy::bugprone