clang-tools-extra/clang-tidy/performance/ForRangeCopyCheck.cpp

//===--- ForRangeCopyCheck.cpp - clang-tidy--------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "ForRangeCopyCheck.h"
#include "../utils/DeclRefExprUtils.h"
#include "../utils/ExprMutationAnalyzer.h"
#include "../utils/FixItHintUtils.h"
#include "../utils/TypeTraits.h"

using namespace clang::ast_matchers;

namespace clang {
namespace tidy {
namespace performance {

ForRangeCopyCheck::ForRangeCopyCheck(StringRef Name, ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      WarnOnAllAutoCopies(Options.get("WarnOnAllAutoCopies", 0)) {}

void ForRangeCopyCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
  Options.store(Opts, "WarnOnAllAutoCopies", WarnOnAllAutoCopies);
}

void ForRangeCopyCheck::registerMatchers(MatchFinder *Finder) {
  // Match loop variables that are not references or pointers or are already
  // initialized through MaterializeTemporaryExpr which indicates a type
  // conversion.
  auto LoopVar = varDecl(
      hasType(hasCanonicalType(unless(anyOf(referenceType(), pointerType())))),
      unless(hasInitializer(expr(hasDescendant(materializeTemporaryExpr())))));
  Finder->addMatcher(cxxForRangeStmt(hasLoopVariable(LoopVar.bind("loopVar")))
                         .bind("forRange"),
                     this);
}

void ForRangeCopyCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *Var = Result.Nodes.getNodeAs<VarDecl>("loopVar");
  // Ignore code in macros since we can't place the fixes correctly.
  if (Var->getBeginLoc().isMacroID())
    return;
  if (handleConstValueCopy(*Var, *Result.Context))
    return;
  const auto *ForRange = Result.Nodes.getNodeAs<CXXForRangeStmt>("forRange");
  handleCopyIsOnlyConstReferenced(*Var, *ForRange, *Result.Context);
}

bool ForRangeCopyCheck::handleConstValueCopy(const VarDecl &LoopVar,
                                             ASTContext &Context) {
  if (WarnOnAllAutoCopies) {
    // For aggressive check just test that loop variable has auto type.
    if (!isa<AutoType>(LoopVar.getType()))
      return false;
  } else if (!LoopVar.getType().isConstQualified()) {
    return false;
  }
  llvm::Optional<bool> Expensive =
      utils::type_traits::isExpensiveToCopy(LoopVar.getType(), Context);
  if (!Expensive || !*Expensive)
    return false;
  auto Diagnostic =
      diag(LoopVar.getLocation(),
           "the loop variable's type is not a reference type; this creates a "
           "copy in each iteration; consider making this a reference")
      << utils::fixit::changeVarDeclToReference(LoopVar, Context);
  if (!LoopVar.getType().isConstQualified())
    Diagnostic << utils::fixit::changeVarDeclToConst(LoopVar);
  return true;
}

bool ForRangeCopyCheck::handleCopyIsOnlyConstReferenced(
    const VarDecl &LoopVar, const CXXForRangeStmt &ForRange,
    ASTContext &Context) {
  llvm::Optional<bool> Expensive =
      utils::type_traits::isExpensiveToCopy(LoopVar.getType(), Context);
  if (LoopVar.getType().isConstQualified() || !Expensive || !*Expensive)
    return false;
  // We omit the case where the loop variable is not used in the loop body. E.g.
  //
  // for (auto _ : benchmark_state) {
  // }
  //
  // Because the fix (changing to `const auto &`) will introduce an unused
  // compiler warning which can't be suppressed.
  // Since this case is very rare, it is safe to ignore it.
  if (!utils::ExprMutationAnalyzer(ForRange.getBody(), &Context)
          .isMutated(&LoopVar) &&
      !utils::decl_ref_expr::allDeclRefExprs(LoopVar, *ForRange.getBody(),
                                             Context)
           .empty()) {
    diag(LoopVar.getLocation(),
         "loop variable is copied but only used as const reference; consider "
         "making it a const reference")
        << utils::fixit::changeVarDeclToConst(LoopVar)
        << utils::fixit::changeVarDeclToReference(LoopVar, Context);
    return true;
  }
  return false;
}

} // namespace performance
} // namespace tidy
} // namespace clang