UnnecessaryValueParamCheck.cpp

Go to the documentation of this file.

//===----------------------------------------------------------------------===//
//
// 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 "UnnecessaryValueParamCheck.h"
#include "../utils/DeclRefExprUtils.h"
#include "../utils/FixItHintUtils.h"
#include "../utils/Matchers.h"
#include "../utils/OptionsUtils.h"
#include "../utils/TypeTraits.h"
#include "clang/Frontend/CompilerInstance.h"
#include "clang/Lex/Lexer.h"
#include "clang/Lex/Preprocessor.h"
#include <optional>
 
using namespace clang::ast_matchers;
 
namespace clang::tidy::performance {
 
static std::string paramNameOrIndex(StringRef Name, size_t Index) {
  return (Name.empty() ? llvm::Twine('#') + llvm::Twine(Index + 1)
                       : llvm::Twine('\'') + Name + llvm::Twine('\''))
      .str();
}
 
static bool hasLoopStmtAncestor(const DeclRefExpr &DeclRef, const Decl &Decl,
                                ASTContext &Context) {
  auto Matches = match(
      traverse(TK_AsIs,
               decl(forEachDescendant(declRefExpr(
                   equalsNode(&DeclRef),
                   unless(hasAncestor(stmt(anyOf(forStmt(), cxxForRangeStmt(),
                                                 whileStmt(), doStmt())))))))),
      Decl, Context);
  return Matches.empty();
}
 
UnnecessaryValueParamCheck::UnnecessaryValueParamCheck(
    StringRef Name, ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      Inserter(Options.getLocalOrGlobal("IncludeStyle",
                                        utils::IncludeSorter::IS_LLVM),
               areDiagsSelfContained()),
      AllowedTypes(
          utils::options::parseStringList(Options.get("AllowedTypes", ""))),
      IgnoreCoroutines(Options.get("IgnoreCoroutines", true)) {}
 
void UnnecessaryValueParamCheck::registerMatchers(MatchFinder *Finder) {
  const auto ExpensiveValueParamDecl = parmVarDecl(
      hasType(qualType(
          hasCanonicalType(matchers::isExpensiveToCopy()),
          unless(anyOf(hasCanonicalType(referenceType()),
                       hasDeclaration(namedDecl(
                           matchers::matchesAnyListedName(AllowedTypes))))))),
      decl().bind("param"));
  Finder->addMatcher(
      traverse(TK_AsIs,
               functionDecl(
                   hasBody(IgnoreCoroutines ? stmt(unless(coroutineBodyStmt()))
                                            : stmt()),
                   isDefinition(), unless(isImplicit()),
                   unless(cxxMethodDecl(anyOf(isOverride(), isFinal()))),
                   has(typeLoc(forEach(ExpensiveValueParamDecl))),
                   decl().bind("functionDecl"))),
      this);
}
 
void UnnecessaryValueParamCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *Param = Result.Nodes.getNodeAs<ParmVarDecl>("param");
  const auto *Function = Result.Nodes.getNodeAs<FunctionDecl>("functionDecl");
 
  TraversalKindScope RAII(*Result.Context, TK_AsIs);
 
  FunctionParmMutationAnalyzer *Analyzer =
      FunctionParmMutationAnalyzer::getFunctionParmMutationAnalyzer(
          *Function, *Result.Context, MutationAnalyzerCache);
  if (Analyzer->isMutated(Param))
    return;
 
  const bool IsConstQualified =
      Param->getType().getCanonicalType().isConstQualified();
 
  // If the parameter is non-const, check if it has a move constructor and is
  // only referenced once to copy-construct another object or whether it has a
  // move assignment operator and is only referenced once when copy-assigned.
  // In this case wrap DeclRefExpr with std::move() to avoid the unnecessary
  // copy.
  if (!IsConstQualified) {
    auto AllDeclRefExprs = utils::decl_ref_expr::allDeclRefExprs(
        *Param, *Function, *Result.Context);
    if (AllDeclRefExprs.size() == 1) {
      auto CanonicalType = Param->getType().getCanonicalType();
      const auto &DeclRefExpr = **AllDeclRefExprs.begin();
 
      if (!hasLoopStmtAncestor(DeclRefExpr, *Function, *Result.Context) &&
          ((utils::type_traits::hasNonTrivialMoveConstructor(CanonicalType) &&
            utils::decl_ref_expr::isCopyConstructorArgument(
                DeclRefExpr, *Function, *Result.Context)) ||
           (utils::type_traits::hasNonTrivialMoveAssignment(CanonicalType) &&
            utils::decl_ref_expr::isCopyAssignmentArgument(
                DeclRefExpr, *Function, *Result.Context)))) {
        handleMoveFix(*Param, DeclRefExpr, *Result.Context);
        return;
      }
    }
  }
 
  handleConstRefFix(*Function, *Param, *Result.Context);
}
 
void UnnecessaryValueParamCheck::registerPPCallbacks(
    const SourceManager &SM, Preprocessor *PP, Preprocessor *ModuleExpanderPP) {
  Inserter.registerPreprocessor(PP);
}
 
void UnnecessaryValueParamCheck::storeOptions(
    ClangTidyOptions::OptionMap &Opts) {
  Options.store(Opts, "IncludeStyle", Inserter.getStyle());
  Options.store(Opts, "AllowedTypes",
                utils::options::serializeStringList(AllowedTypes));
  Options.store(Opts, "IgnoreCoroutines", IgnoreCoroutines);
}
 
void UnnecessaryValueParamCheck::onEndOfTranslationUnit() {
  MutationAnalyzerCache.clear();
}
 
void UnnecessaryValueParamCheck::handleConstRefFix(const FunctionDecl &Function,
                                                   const ParmVarDecl &Param,
                                                   ASTContext &Context) {
  const size_t Index =
      llvm::find(Function.parameters(), &Param) - Function.parameters().begin();
  const bool IsConstQualified =
      Param.getType().getCanonicalType().isConstQualified();
 
  auto Diag =
      diag(Param.getLocation(),
           "the %select{|const qualified }0parameter %1 of type %2 is copied "
           "for each "
           "invocation%select{ but only used as a const reference|}0; consider "
           "making it a %select{const |}0reference")
      << IsConstQualified << paramNameOrIndex(Param.getName(), Index)
      << Param.getType();
  // Do not propose fixes when:
  // 1. the ParmVarDecl is in a macro, since we cannot place them correctly
  // 2. the function is virtual as it might break overrides
  // 3. the function is an explicit template/ specialization.
  const auto *Method = llvm::dyn_cast<CXXMethodDecl>(&Function);
  if (Param.getBeginLoc().isMacroID() || (Method && Method->isVirtual()) ||
      Function.getTemplateSpecializationKind() == TSK_ExplicitSpecialization)
    return;
  for (const auto *FunctionDecl = &Function; FunctionDecl != nullptr;
       FunctionDecl = FunctionDecl->getPreviousDecl()) {
    const auto &CurrentParam = *FunctionDecl->getParamDecl(Index);
    Diag << utils::fixit::changeVarDeclToReference(CurrentParam, Context);
    // The parameter of each declaration needs to be checked individually as to
    // whether it is const or not as constness can differ between definition and
    // declaration.
    if (!CurrentParam.getType().getCanonicalType().isConstQualified()) {
      if (std::optional<FixItHint> Fix = utils::fixit::addQualifierToVarDecl(
              CurrentParam, Context, Qualifiers::Const))
        Diag << *Fix;
    }
  }
}
 
void UnnecessaryValueParamCheck::handleMoveFix(const ParmVarDecl &Param,
                                               const DeclRefExpr &CopyArgument,
                                               ASTContext &Context) {
  auto Diag =
      diag(CopyArgument.getBeginLoc(),
           "parameter %0 of type %1 is passed by value and only copied once; "
           "consider moving it to avoid unnecessary copies")
      << &Param << Param.getType();
  // Do not propose fixes in macros since we cannot place them correctly.
  if (CopyArgument.getBeginLoc().isMacroID())
    return;
  const auto &SM = Context.getSourceManager();
  auto EndLoc = Lexer::getLocForEndOfToken(CopyArgument.getLocation(), 0, SM,
                                           Context.getLangOpts());
  Diag << FixItHint::CreateInsertion(CopyArgument.getBeginLoc(), "std::move(")
       << FixItHint::CreateInsertion(EndLoc, ")")
       << Inserter.createIncludeInsertion(
              SM.getFileID(CopyArgument.getBeginLoc()), "<utility>");
}
 
} // namespace clang::tidy::performance