1 //===--- ForRangeCopyCheck.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 "ForRangeCopyCheck.h"
10 #include "../utils/DeclRefExprUtils.h"
11 #include "../utils/FixItHintUtils.h"
12 #include "../utils/Matchers.h"
13 #include "../utils/OptionsUtils.h"
14 #include "../utils/TypeTraits.h"
15 #include "clang/Analysis/Analyses/ExprMutationAnalyzer.h"
16 #include "clang/Basic/Diagnostic.h"
17 
18 using namespace clang::ast_matchers;
19 
20 namespace clang {
21 namespace tidy {
22 namespace performance {
23 
24 ForRangeCopyCheck::ForRangeCopyCheck(StringRef Name, ClangTidyContext *Context)
25     : ClangTidyCheck(Name, Context),
26       WarnOnAllAutoCopies(Options.get("WarnOnAllAutoCopies", false)),
27       AllowedTypes(
28           utils::options::parseStringList(Options.get("AllowedTypes", ""))) {}
29 
30 void ForRangeCopyCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
31   Options.store(Opts, "WarnOnAllAutoCopies", WarnOnAllAutoCopies);
32   Options.store(Opts, "AllowedTypes",
33                 utils::options::serializeStringList(AllowedTypes));
34 }
35 
36 void ForRangeCopyCheck::registerMatchers(MatchFinder *Finder) {
37   // Match loop variables that are not references or pointers or are already
38   // initialized through MaterializeTemporaryExpr which indicates a type
39   // conversion.
40   auto HasReferenceOrPointerTypeOrIsAllowed = hasType(qualType(
41       unless(anyOf(hasCanonicalType(anyOf(referenceType(), pointerType())),
42                    hasDeclaration(namedDecl(
43                        matchers::matchesAnyListedName(AllowedTypes)))))));
44   auto IteratorReturnsValueType = cxxOperatorCallExpr(
45       hasOverloadedOperatorName("*"),
46       callee(
47           cxxMethodDecl(returns(unless(hasCanonicalType(referenceType()))))));
48   auto NotConstructedByCopy = cxxConstructExpr(
49       hasDeclaration(cxxConstructorDecl(unless(isCopyConstructor()))));
50   auto ConstructedByConversion = cxxMemberCallExpr(callee(cxxConversionDecl()));
51   auto LoopVar =
52       varDecl(HasReferenceOrPointerTypeOrIsAllowed,
53               unless(hasInitializer(expr(hasDescendant(expr(
54                   anyOf(materializeTemporaryExpr(), IteratorReturnsValueType,
55                         NotConstructedByCopy, ConstructedByConversion)))))));
56   Finder->addMatcher(
57       traverse(TK_AsIs,
58                cxxForRangeStmt(hasLoopVariable(LoopVar.bind("loopVar")))
59                    .bind("forRange")),
60       this);
61 }
62 
63 void ForRangeCopyCheck::check(const MatchFinder::MatchResult &Result) {
64   const auto *Var = Result.Nodes.getNodeAs<VarDecl>("loopVar");
65 
66   // Ignore code in macros since we can't place the fixes correctly.
67   if (Var->getBeginLoc().isMacroID())
68     return;
69   if (handleConstValueCopy(*Var, *Result.Context))
70     return;
71   const auto *ForRange = Result.Nodes.getNodeAs<CXXForRangeStmt>("forRange");
72   handleCopyIsOnlyConstReferenced(*Var, *ForRange, *Result.Context);
73 }
74 
75 bool ForRangeCopyCheck::handleConstValueCopy(const VarDecl &LoopVar,
76                                              ASTContext &Context) {
77   if (WarnOnAllAutoCopies) {
78     // For aggressive check just test that loop variable has auto type.
79     if (!isa<AutoType>(LoopVar.getType()))
80       return false;
81   } else if (!LoopVar.getType().isConstQualified()) {
82     return false;
83   }
84   llvm::Optional<bool> Expensive =
85       utils::type_traits::isExpensiveToCopy(LoopVar.getType(), Context);
86   if (!Expensive || !*Expensive)
87     return false;
88   auto Diagnostic =
89       diag(LoopVar.getLocation(),
90            "the loop variable's type is not a reference type; this creates a "
91            "copy in each iteration; consider making this a reference")
92       << utils::fixit::changeVarDeclToReference(LoopVar, Context);
93   if (!LoopVar.getType().isConstQualified()) {
94     if (llvm::Optional<FixItHint> Fix = utils::fixit::addQualifierToVarDecl(
95             LoopVar, Context, DeclSpec::TQ::TQ_const))
96       Diagnostic << *Fix;
97   }
98   return true;
99 }
100 
101 bool ForRangeCopyCheck::handleCopyIsOnlyConstReferenced(
102     const VarDecl &LoopVar, const CXXForRangeStmt &ForRange,
103     ASTContext &Context) {
104   llvm::Optional<bool> Expensive =
105       utils::type_traits::isExpensiveToCopy(LoopVar.getType(), Context);
106   if (LoopVar.getType().isConstQualified() || !Expensive || !*Expensive)
107     return false;
108   // We omit the case where the loop variable is not used in the loop body. E.g.
109   //
110   // for (auto _ : benchmark_state) {
111   // }
112   //
113   // Because the fix (changing to `const auto &`) will introduce an unused
114   // compiler warning which can't be suppressed.
115   // Since this case is very rare, it is safe to ignore it.
116   if (!ExprMutationAnalyzer(*ForRange.getBody(), Context).isMutated(&LoopVar) &&
117       !utils::decl_ref_expr::allDeclRefExprs(LoopVar, *ForRange.getBody(),
118                                              Context)
119            .empty()) {
120     auto Diag = diag(
121         LoopVar.getLocation(),
122         "loop variable is copied but only used as const reference; consider "
123         "making it a const reference");
124 
125     if (llvm::Optional<FixItHint> Fix = utils::fixit::addQualifierToVarDecl(
126             LoopVar, Context, DeclSpec::TQ::TQ_const))
127       Diag << *Fix << utils::fixit::changeVarDeclToReference(LoopVar, Context);
128 
129     return true;
130   }
131   return false;
132 }
133 
134 } // namespace performance
135 } // namespace tidy
136 } // namespace clang
137