MisplacedWideningCastCheck.cpp

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//===----------------------------------------------------------------------===//
//
// 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 "MisplacedWideningCastCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
 
using namespace clang::ast_matchers;
 
namespace clang::tidy::bugprone {
 
MisplacedWideningCastCheck::MisplacedWideningCastCheck(
    StringRef Name, ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      CheckImplicitCasts(Options.get("CheckImplicitCasts", false)) {}
 
void MisplacedWideningCastCheck::storeOptions(
    ClangTidyOptions::OptionMap &Opts) {
  Options.store(Opts, "CheckImplicitCasts", CheckImplicitCasts);
}
 
void MisplacedWideningCastCheck::registerMatchers(MatchFinder *Finder) {
  const auto Calc =
      expr(anyOf(binaryOperator(hasAnyOperatorName("+", "-", "*", "<<")),
                 unaryOperator(hasOperatorName("~"))),
           hasType(isInteger()))
          .bind("Calc");
 
  const auto ExplicitCast = explicitCastExpr(hasDestinationType(isInteger()),
                                             has(ignoringParenImpCasts(Calc)));
  const auto ImplicitCast =
      implicitCastExpr(hasImplicitDestinationType(isInteger()),
                       has(ignoringParenImpCasts(Calc)));
  const auto Cast =
      traverse(TK_AsIs, expr(anyOf(ExplicitCast, ImplicitCast)).bind("Cast"));
 
  Finder->addMatcher(varDecl(hasInitializer(Cast)), this);
  Finder->addMatcher(returnStmt(hasReturnValue(Cast)), this);
  Finder->addMatcher(callExpr(hasAnyArgument(Cast)), this);
  // When assigning to a bit field, bind the FieldDecl so check() can use the
  // actual bit width instead of the declared type width.
  Finder->addMatcher(
      binaryOperator(hasOperatorName("="),
                     hasLHS(expr(optionally(memberExpr(hasDeclaration(
                         fieldDecl(isBitField()).bind("BitField")))))),
                     hasRHS(Cast)),
      this);
  Finder->addMatcher(
      binaryOperator(isComparisonOperator(), hasEitherOperand(Cast)), this);
}
 
static unsigned getMaxCalculationWidth(const ASTContext &Context,
                                       const Expr *E) {
  E = E->IgnoreParenImpCasts();
 
  if (const auto *Bop = dyn_cast<BinaryOperator>(E)) {
    const unsigned LHSWidth = getMaxCalculationWidth(Context, Bop->getLHS());
    const unsigned RHSWidth = getMaxCalculationWidth(Context, Bop->getRHS());
    if (Bop->getOpcode() == BO_Mul)
      return LHSWidth + RHSWidth;
    if (Bop->getOpcode() == BO_Add)
      return std::max(LHSWidth, RHSWidth) + 1;
    if (Bop->getOpcode() == BO_Rem) {
      Expr::EvalResult Result;
      if (Bop->getRHS()->EvaluateAsInt(Result, Context))
        return Result.Val.getInt().getActiveBits();
    } else if (Bop->getOpcode() == BO_Shl) {
      Expr::EvalResult Result;
      if (Bop->getRHS()->EvaluateAsInt(Result, Context)) {
        // We don't handle negative values and large values well. It is assumed
        // that compiler warnings are written for such values so the user will
        // fix that.
        return LHSWidth + Result.Val.getInt().getExtValue();
      }
 
      // Unknown bitcount, assume there is truncation.
      return 1024U;
    }
  } else if (const auto *Uop = dyn_cast<UnaryOperator>(E)) {
    // There is truncation when ~ is used.
    if (Uop->getOpcode() == UO_Not)
      return 1024U;
 
    const QualType T = Uop->getType();
    return T->isIntegerType() ? Context.getIntWidth(T) : 1024U;
  } else if (const auto *I = dyn_cast<IntegerLiteral>(E)) {
    return I->getValue().getActiveBits();
  }
 
  return Context.getIntWidth(E->getType());
}
 
static int relativeIntSizes(BuiltinType::Kind Kind) {
  switch (Kind) {
  case BuiltinType::UChar:
    return 1;
  case BuiltinType::SChar:
    return 1;
  case BuiltinType::Char_U:
    return 1;
  case BuiltinType::Char_S:
    return 1;
  case BuiltinType::UShort:
    return 2;
  case BuiltinType::Short:
    return 2;
  case BuiltinType::UInt:
    return 3;
  case BuiltinType::Int:
    return 3;
  case BuiltinType::ULong:
    return 4;
  case BuiltinType::Long:
    return 4;
  case BuiltinType::ULongLong:
    return 5;
  case BuiltinType::LongLong:
    return 5;
  case BuiltinType::UInt128:
    return 6;
  case BuiltinType::Int128:
    return 6;
  default:
    return 0;
  }
}
 
static int relativeCharSizes(BuiltinType::Kind Kind) {
  switch (Kind) {
  case BuiltinType::UChar:
    return 1;
  case BuiltinType::SChar:
    return 1;
  case BuiltinType::Char_U:
    return 1;
  case BuiltinType::Char_S:
    return 1;
  case BuiltinType::Char16:
    return 2;
  case BuiltinType::Char32:
    return 3;
  default:
    return 0;
  }
}
 
static int relativeCharSizesW(BuiltinType::Kind Kind) {
  switch (Kind) {
  case BuiltinType::UChar:
    return 1;
  case BuiltinType::SChar:
    return 1;
  case BuiltinType::Char_U:
    return 1;
  case BuiltinType::Char_S:
    return 1;
  case BuiltinType::WChar_U:
    return 2;
  case BuiltinType::WChar_S:
    return 2;
  default:
    return 0;
  }
}
 
static bool isFirstWider(BuiltinType::Kind First, BuiltinType::Kind Second) {
  int FirstSize = 0, SecondSize = 0;
  if ((FirstSize = relativeIntSizes(First)) != 0 &&
      (SecondSize = relativeIntSizes(Second)) != 0)
    return FirstSize > SecondSize;
  if ((FirstSize = relativeCharSizes(First)) != 0 &&
      (SecondSize = relativeCharSizes(Second)) != 0)
    return FirstSize > SecondSize;
  if ((FirstSize = relativeCharSizesW(First)) != 0 &&
      (SecondSize = relativeCharSizesW(Second)) != 0)
    return FirstSize > SecondSize;
  return false;
}
 
void MisplacedWideningCastCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *Cast = Result.Nodes.getNodeAs<CastExpr>("Cast");
  if (!CheckImplicitCasts && isa<ImplicitCastExpr>(Cast))
    return;
  if (Cast->getBeginLoc().isMacroID())
    return;
 
  const auto *Calc = Result.Nodes.getNodeAs<Expr>("Calc");
  if (Calc->getBeginLoc().isMacroID())
    return;
 
  if (Cast->isTypeDependent() || Cast->isValueDependent() ||
      Calc->isTypeDependent() || Calc->isValueDependent())
    return;
 
  const ASTContext &Context = *Result.Context;
 
  const QualType CastType = Cast->getType();
  const QualType CalcType = Calc->getType();
 
  // If assigning to a bit field, use the bit field width as the effective
  // target width. The declared type may be wider than the actual bit field
  // storage.
  unsigned TargetWidth = Context.getIntWidth(CastType);
  bool IsBitfieldAssign = false;
  if (const auto *FD = Result.Nodes.getNodeAs<FieldDecl>("BitField")) {
    TargetWidth = FD->getBitWidthValue();
    IsBitfieldAssign = true;
  }
 
  // Explicit truncation using cast.
  if (TargetWidth < Context.getIntWidth(CalcType))
    return;
 
  // If CalcType and CastType have same size then there is no real danger, but
  // there can be a portability problem.
 
  if (TargetWidth == Context.getIntWidth(CalcType)) {
    // Bit field width is fixed across platforms — no portability concern.
    if (IsBitfieldAssign)
      return;
    const auto *CastBuiltinType =
        dyn_cast<BuiltinType>(CastType->getUnqualifiedDesugaredType());
    const auto *CalcBuiltinType =
        dyn_cast<BuiltinType>(CalcType->getUnqualifiedDesugaredType());
    if (!CastBuiltinType || !CalcBuiltinType)
      return;
    if (!isFirstWider(CastBuiltinType->getKind(), CalcBuiltinType->getKind()))
      return;
  }
 
  // Don't write a warning if we can easily see that the result is not
  // truncated.
  if (Context.getIntWidth(CalcType) >= getMaxCalculationWidth(Context, Calc))
    return;
 
  diag(Cast->getBeginLoc(), "either cast from %0 to %1 is ineffective, or "
                            "there is loss of precision before the conversion")
      << CalcType << CastType;
}
 
} // namespace clang::tidy::bugprone