[clang-tidy] ObjC ARC objects should not trigger performance-unnecessary-value-param

[benhamilton]

Summary

The following Objective-C code currently incorrectly triggers
clang-tidy's performance-unnecessary-value-param check:

% cat /tmp/performance-unnecessary-value-param-arc.m
void foo(id object) { }

clang-tidy /tmp/performance-unnecessary-value-param-arc.m
-checks=-\*,performance-unnecessary-value-param -- -xobjective-c
-fobjc-abi-version=2 -fobjc-arc
1 warning generated.
/src/llvm/tools/clang/tools/extra/test/clang-tidy/performance-unnecessary-value-param-arc.m:10:13:
warning: the parameter 'object' is copied for each invocation but only
used as a const reference; consider making it a const reference
[performance-unnecessary-value-param]
void foo(id object) { }
         ~~ ^
         const &

This is wrong for a few reasons:

1. Objective-C doesn't have references, so const & is not going to help
2. ARC heavily optimizes the "expensive" copy which triggers the warning

This fixes the issue by disabling the warning for non-C++, as well as
disabling it for objects under ARC memory management for
Objective-C++.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32075

Test Plan: New tests added. Ran tests with make -j12 check-clang-tools.

[hokein]

LGTM

clang-tools-extra/trunk/clang-tidy/performance/UnnecessaryValueParamCheck.cpp

 
 UnnecessaryValueParamCheck::UnnecessaryValueParamCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IncludeStyle(utils::IncludeSorter::parseIncludeStyle(
           Options.getLocalOrGlobal("IncludeStyle", "llvm"))) {}
 
 void UnnecessaryValueParamCheck::registerMatchers(MatchFinder *Finder) {
+  // This check is specific to C++ and doesn't apply to languages like
+  // Objective-C.
+  if (!getLangOpts().CPlusPlus)
+    return;
   const auto ExpensiveValueParamDecl =
       parmVarDecl(hasType(hasCanonicalType(allOf(
                       unless(referenceType()), matchers::isExpensiveToCopy()))),
                   decl().bind("param"));
   Finder->addMatcher(
       functionDecl(hasBody(stmt()), isDefinition(), unless(isImplicit()),
                    unless(cxxMethodDecl(anyOf(isOverride(), isFinal()))),
                    has(typeLoc(forEach(ExpensiveValueParamDecl))),

clang-tools-extra/trunk/clang-tidy/utils/TypeTraits.cpp

 } // namespace
 
 llvm::Optional<bool> isExpensiveToCopy(QualType Type,
                                        const ASTContext &Context) {
   if (Type->isDependentType() || Type->isIncompleteType())
     return llvm::None;
   return !Type.isTriviallyCopyableType(Context) &&
          !classHasTrivialCopyAndDestroy(Type) &&
-         !hasDeletedCopyConstructor(Type);
+         !hasDeletedCopyConstructor(Type) &&
+         !Type->isObjCLifetimeType();
 }
 
 bool recordIsTriviallyDefaultConstructible(const RecordDecl &RecordDecl,
                                            const ASTContext &Context) {
   const auto *ClassDecl = dyn_cast<CXXRecordDecl>(&RecordDecl);
   // Non-C++ records are always trivially constructible.
   if (!ClassDecl)
     return true;

clang-tools-extra/trunk/test/clang-tidy/performance-unnecessary-value-param-arc.m

+// RUN: clang-tidy %s -checks=-*,performance-unnecessary-value-param -- \
+// RUN:   -xobjective-c -fobjc-abi-version=2 -fobjc-arc | count 0
+
+#if !__has_feature(objc_arc)
+#error Objective-C ARC not enabled as expected
+#endif
+
+// Passing an Objective-C ARC-managed object to a C function should
+// not raise performance-unnecessary-value-param.
+void foo(id object) { }
+
+// Same for explcitly non-ARC-managed Objective-C objects.
+void bar(__unsafe_unretained id object) { }
+
+// Same for Objective-c classes.
+void baz(Class c) { }

clang-tools-extra/trunk/test/clang-tidy/performance-unnecessary-value-param-arc.mm

+// RUN: clang-tidy %s -checks=-*,performance-unnecessary-value-param -- \
+// RUN:   -xobjective-c++ -fobjc-abi-version=2 -fobjc-arc | count 0
+
+#if !__has_feature(objc_arc)
+#error Objective-C ARC not enabled as expected
+#endif
+
+// Passing an Objective-C ARC-managed object to a C function should
+// not raise performance-unnecessary-value-param.
+void foo(id object) { }
+
+// Same for explcitly non-ARC-managed Objective-C objects.
+void bar(__unsafe_unretained id object) { }
+
+// Same for Objective-c classes.
+void baz(Class c) { }