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//===--- TooSmallLoopVariableCheck.cpp - clang-tidy -----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "TooSmallLoopVariableCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
using namespace clang::ast_matchers;
namespace clang::tidy::bugprone {
static constexpr llvm::StringLiteral LoopName =
llvm::StringLiteral("forLoopName");
static constexpr llvm::StringLiteral LoopVarName =
llvm::StringLiteral("loopVar");
static constexpr llvm::StringLiteral LoopVarCastName =
llvm::StringLiteral("loopVarCast");
static constexpr llvm::StringLiteral LoopUpperBoundName =
llvm::StringLiteral("loopUpperBound");
static constexpr llvm::StringLiteral LoopIncrementName =
llvm::StringLiteral("loopIncrement");
namespace {
struct MagnitudeBits {
unsigned WidthWithoutSignBit = 0U;
unsigned BitFieldWidth = 0U;
bool operator<(const MagnitudeBits &Other) const noexcept {
return WidthWithoutSignBit < Other.WidthWithoutSignBit;
}
bool operator!=(const MagnitudeBits &Other) const noexcept {
return WidthWithoutSignBit != Other.WidthWithoutSignBit ||
BitFieldWidth != Other.BitFieldWidth;
}
};
} // namespace
TooSmallLoopVariableCheck::TooSmallLoopVariableCheck(StringRef Name,
ClangTidyContext *Context)
: ClangTidyCheck(Name, Context),
MagnitudeBitsUpperLimit(Options.get("MagnitudeBitsUpperLimit", 16U)) {}
void TooSmallLoopVariableCheck::storeOptions(
ClangTidyOptions::OptionMap &Opts) {
Options.store(Opts, "MagnitudeBitsUpperLimit", MagnitudeBitsUpperLimit);
}
/// The matcher for loops with suspicious integer loop variable.
///
/// In this general example, assuming 'j' and 'k' are of integral type:
/// \code
/// for (...; j < 3 + 2; ++k) { ... }
/// \endcode
/// The following string identifiers are bound to these parts of the AST:
/// LoopVarName: 'j' (as a VarDecl)
/// LoopVarCastName: 'j' (after implicit conversion)
/// LoopUpperBoundName: '3 + 2' (as an Expr)
/// LoopIncrementName: 'k' (as an Expr)
/// LoopName: The entire for loop (as a ForStmt)
///
void TooSmallLoopVariableCheck::registerMatchers(MatchFinder *Finder) {
StatementMatcher LoopVarMatcher =
expr(ignoringParenImpCasts(
anyOf(declRefExpr(to(varDecl(hasType(isInteger())))),
memberExpr(member(fieldDecl(hasType(isInteger())))))))
.bind(LoopVarName);
// We need to catch only those comparisons which contain any integer cast.
StatementMatcher LoopVarConversionMatcher = traverse(
TK_AsIs, implicitCastExpr(hasImplicitDestinationType(isInteger()),
has(ignoringParenImpCasts(LoopVarMatcher)))
.bind(LoopVarCastName));
// We are interested in only those cases when the loop bound is a variable
// value (not const, enum, etc.).
StatementMatcher LoopBoundMatcher =
expr(ignoringParenImpCasts(allOf(hasType(isInteger()),
unless(integerLiteral()),
unless(hasType(isConstQualified())),
unless(hasType(enumType())))))
.bind(LoopUpperBoundName);
// We use the loop increment expression only to make sure we found the right
// loop variable.
StatementMatcher IncrementMatcher =
expr(ignoringParenImpCasts(hasType(isInteger()))).bind(LoopIncrementName);
Finder->addMatcher(
forStmt(
hasCondition(anyOf(
binaryOperator(hasOperatorName("<"),
hasLHS(LoopVarConversionMatcher),
hasRHS(LoopBoundMatcher)),
binaryOperator(hasOperatorName("<="),
hasLHS(LoopVarConversionMatcher),
hasRHS(LoopBoundMatcher)),
binaryOperator(hasOperatorName(">"), hasLHS(LoopBoundMatcher),
hasRHS(LoopVarConversionMatcher)),
binaryOperator(hasOperatorName(">="), hasLHS(LoopBoundMatcher),
hasRHS(LoopVarConversionMatcher)))),
hasIncrement(IncrementMatcher))
.bind(LoopName),
this);
}
/// Returns the magnitude bits of an integer type.
static MagnitudeBits calcMagnitudeBits(const ASTContext &Context,
const QualType &IntExprType,
const Expr *IntExpr) {
assert(IntExprType->isIntegerType());
unsigned SignedBits = IntExprType->isUnsignedIntegerType() ? 0U : 1U;
if (const auto *BitField = IntExpr->getSourceBitField()) {
unsigned BitFieldWidth = BitField->getBitWidthValue(Context);
return {BitFieldWidth - SignedBits, BitFieldWidth};
}
unsigned IntWidth = Context.getIntWidth(IntExprType);
return {IntWidth - SignedBits, 0U};
}
/// Calculate the upper bound expression's magnitude bits, but ignore
/// constant like values to reduce false positives.
static MagnitudeBits
calcUpperBoundMagnitudeBits(const ASTContext &Context, const Expr *UpperBound,
const QualType &UpperBoundType) {
// Ignore casting caused by constant values inside a binary operator.
// We are interested in variable values' magnitude bits.
if (const auto *BinOperator = dyn_cast<BinaryOperator>(UpperBound)) {
const Expr *RHSE = BinOperator->getRHS()->IgnoreParenImpCasts();
const Expr *LHSE = BinOperator->getLHS()->IgnoreParenImpCasts();
QualType RHSEType = RHSE->getType();
QualType LHSEType = LHSE->getType();
if (!RHSEType->isIntegerType() || !LHSEType->isIntegerType())
return {};
bool RHSEIsConstantValue = RHSEType->isEnumeralType() ||
RHSEType.isConstQualified() ||
isa<IntegerLiteral>(RHSE);
bool LHSEIsConstantValue = LHSEType->isEnumeralType() ||
LHSEType.isConstQualified() ||
isa<IntegerLiteral>(LHSE);
// Avoid false positives produced by two constant values.
if (RHSEIsConstantValue && LHSEIsConstantValue)
return {};
if (RHSEIsConstantValue)
return calcMagnitudeBits(Context, LHSEType, LHSE);
if (LHSEIsConstantValue)
return calcMagnitudeBits(Context, RHSEType, RHSE);
return std::max(calcMagnitudeBits(Context, LHSEType, LHSE),
calcMagnitudeBits(Context, RHSEType, RHSE));
}
return calcMagnitudeBits(Context, UpperBoundType, UpperBound);
}
static std::string formatIntegralType(const QualType &Type,
const MagnitudeBits &Info) {
std::string Name = Type.getAsString();
if (!Info.BitFieldWidth)
return Name;
Name += ':';
Name += std::to_string(Info.BitFieldWidth);
return Name;
}
void TooSmallLoopVariableCheck::check(const MatchFinder::MatchResult &Result) {
const auto *LoopVar = Result.Nodes.getNodeAs<Expr>(LoopVarName);
const auto *UpperBound =
Result.Nodes.getNodeAs<Expr>(LoopUpperBoundName)->IgnoreParenImpCasts();
const auto *LoopIncrement =
Result.Nodes.getNodeAs<Expr>(LoopIncrementName)->IgnoreParenImpCasts();
// We matched the loop variable incorrectly.
if (LoopVar->getType() != LoopIncrement->getType())
return;
ASTContext &Context = *Result.Context;
const QualType LoopVarType = LoopVar->getType();
const MagnitudeBits LoopVarMagnitudeBits =
calcMagnitudeBits(Context, LoopVarType, LoopVar);
const MagnitudeBits LoopIncrementMagnitudeBits =
calcMagnitudeBits(Context, LoopIncrement->getType(), LoopIncrement);
// We matched the loop variable incorrectly.
if (LoopIncrementMagnitudeBits != LoopVarMagnitudeBits)
return;
const QualType UpperBoundType = UpperBound->getType();
const MagnitudeBits UpperBoundMagnitudeBits =
calcUpperBoundMagnitudeBits(Context, UpperBound, UpperBoundType);
if ((0U == UpperBoundMagnitudeBits.WidthWithoutSignBit) ||
(LoopVarMagnitudeBits.WidthWithoutSignBit > MagnitudeBitsUpperLimit) ||
(LoopVarMagnitudeBits.WidthWithoutSignBit >=
UpperBoundMagnitudeBits.WidthWithoutSignBit))
return;
diag(LoopVar->getBeginLoc(),
"loop variable has narrower type '%0' than iteration's upper bound '%1'")
<< formatIntegralType(LoopVarType, LoopVarMagnitudeBits)
<< formatIntegralType(UpperBoundType, UpperBoundMagnitudeBits);
}
} // namespace clang::tidy::bugprone
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