[clang-tidy] Add 'performance-const-parameter-value-or-ref' for checking const-qualified parameters

[cjdb]

Summary

Checks `const`-qualified parameters to determine if they should be
passed by value or by reference-to-`const` for function definitions.
Ignores proper function declarations, parameters without `const` in
their type specifier, and dependent types.

[cjdb]

Work left to do in this patch: add user options. Figured getting a non-config version up for review first would be a good idea.

[cjdb]

Those CI failures are kinda embarrassing. Is there any way to leverage clang-format in a case like this?

[Eugene.Zelenko]

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

57

Please use static, not anonymous namespace for functions.

58

Please don't use auto unless type is spelled in same statement or iterator.

clang-tools-extra/clang-tidy/performance/ConstParameterValueOrRef.h

1

See other headers as example for C++ definition for code editors.

18

Please separate with newline.

43

Seems to be unused.

46

Please separate with newline.

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

16

Please remove newline.

67

Please use single back-tick for option values.

[jmarrec]

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

90

101

104

typo here, not sure what the intent was.

[cjdb]

• adds user options

• removes use of auto
• fixes header comment
• removes unused type
• fixes documentation

[cjdb]

@jmarrec I believe you wanted to import a discussion from D107900?

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

57

Hmm... there are lots of instances of namespace { in clang-tidy.

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

104

I think this is something I put at the end first, and then moved to the start of the sentence, but forgot to delete.

[cjdb]

removes WIP tag from patch name

[whisperity]

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

57

Yeah, because you can't do static class/struct X.

74

QualType is small (AFAIK it's just 2 pointers), no need for a reference on this, it can live on the stack.

93

This reminds me eerily of performance-unnecessary-value-param!

clang-tools-extra/clang-tidy/performance/ConstParameterValueOrRef.h

20–23

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

60

(Minor suggestion: maybe MaxSmallSize would be a better name here than SmallMaxSize.)

103–104

Will GitHub and Godbolt outlive LLVM? I am not comfortable with depending on external sources (such as shortlinks like that git.io one that I was a bit anxious to click...) to explain decisions. I get it that the code is long to be put into the documentation, though... But perhaps the benchmark's tables could go in here, at the very bottom, as an appendix.

[aaron.ballman]

It seems like there may be some considerable overlap between this check and the one proposed in https://reviews.llvm.org/D54943. I know the other check is more about C++ Core Guidelines so it's not perfect overlap. But I do wonder if it'd make sense to combine the efforts given that the goal of both checks is to find const correctness issues. Do you think there's a chance for sharing implementation efforts here?

[jmarrec]

Imported some discussion from https://reviews.llvm.org/D107900#inline-1029358. Sorry it took that long

clang-tools-extra/clang-tidy/performance/ConstParameterValueOrRef.h

41

https://reviews.llvm.org/D107900#inline-1029358

On mine, I had another option RestrictToBuiltInTypes which I find an interesting option. It has a builtinType() matcher in the registerMatchers function

44

Perhaps two additional options to turn off either "side" would be helpful? eg CheckPassByValueIsAppropriate and CheckPassByRefIsAppropriate, both defaulted to true

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

45

https://reviews.llvm.org/D107900#inline-1029358

I am removing the const as well in addition to the &. I guess you may be just relying on using it in conjunction with readability-avoid-const-params-in-decls but I think that makes it harder to use (you can't just do -checks=-*,<yourcheck> you have to remember to add the readability-avoid-const-params-in-decls too...).

Taht being said, perhaps that's how it's meant to be...

[cjdb]

clang-tools-extra/clang-tidy/performance/ConstParameterValueOrRef.h

41

Could you please elaborate on why you find this interesting? I don't see any benefit to ignoring struct S { char c; }.

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

45

I don't know if our goals are aligned here. This patch intends to preserve constness in all cases. If a user defines a function as

void f(int const& x) { }

Then there's a good chance that they want the constness. Removing that constness changes the meaning of the program since x would then be modifiable. What's deferred to readability-avoid-const-params-in-decls is const-qualified value parameters in forward declarations, which isn't really in scope here, since forward declarations are used to overrule this check's analysis.

clang-tools-extra/clang-tidy/performance/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   FrontendOpenMP
   Support
   )
 
 add_clang_library(clangTidyPerformanceModule
+  ConstParameterValueOrRef.cpp
   FasterStringFindCheck.cpp
   ForRangeCopyCheck.cpp
   ImplicitConversionInLoopCheck.cpp
   InefficientAlgorithmCheck.cpp
   InefficientStringConcatenationCheck.cpp
   InefficientVectorOperationCheck.cpp
   MoveConstArgCheck.cpp
   MoveConstructorInitCheck.cpp

clang-tools-extra/clang-tidy/performance/ConstParameterValueOrRef.h

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_PERFORMANCE_CONSTPARAMETERVALUEORREF_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_PERFORMANCE_CONSTPARAMETERVALUEORREF_H
+
+#include "../ClangTidyCheck.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+namespace clang {
+namespace tidy {
+namespace performance {
+// Checks ``const``-qualified parameters to determine if they should be passed
+// by value or by reference-to-``const`` for function definitions. Ignores
+// proper function declarations, parameters without ``const`` in their type
+// specifier, and dependent types.
+class ConstParameterValueOrRef : public ClangTidyCheck {
+public:
+  using ClangTidyCheck::ClangTidyCheck;
+
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  Optional<int> MaxSize;
+
+  void checkPassByValueIsAppropriate(const ParmVarDecl &Param,
+                                     const FunctionDecl &Func,
+                                     const ASTContext &Context);
+  void checkPassByRefIsAppropriate(const ParmVarDecl &Param,
+                                   const ASTContext &Context);
+};
+
+struct MaxSize {
+  int Size;
+};
+} // namespace performance
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_PERFORMANCE_CONSTPARAMETERVALUEORREF_H

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

+//===----------------------------------------------------------------------===//
+//
+// 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 "ConstParameterValueOrRef.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Basic/Diagnostic.h"
+#include "llvm/ADT/ArrayRef.h"
+#include <utility>
+
+namespace clang {
+namespace tidy {
+namespace performance {
+
+using ast_matchers::allOf;
+using ast_matchers::forEach;
+using ast_matchers::functionDecl;
+using ast_matchers::has;
+using ast_matchers::hasType;
+using ast_matchers::isConstQualified;
+using ast_matchers::isDefinition;
+using ast_matchers::isTemplateInstantiation;
+using ast_matchers::MatchFinder;
+using ast_matchers::parmVarDecl;
+using ast_matchers::references;
+using ast_matchers::typeLoc;
+using ast_matchers::unless;
+
+void ConstParameterValueOrRef::registerMatchers(MatchFinder *Finder) {
+  const auto ConstValueParamDecl =
+      parmVarDecl(hasType(isConstQualified())).bind("value");
+  Finder->addMatcher(
+      functionDecl(allOf(isDefinition(),
+                         has(typeLoc(forEach(ConstValueParamDecl))),
+                         unless(isTemplateInstantiation())))
+          .bind("func-value"),
+      this);
+
+  const auto RefToConstParamDecl =
+      parmVarDecl(hasType(references(isConstQualified()))).bind("ref");
+  Finder->addMatcher(
+      functionDecl(
+          allOf(isDefinition(), has(typeLoc(forEach(RefToConstParamDecl)))),
+          unless(isTemplateInstantiation()))
+          .bind("func-ref"),
+      this);
+}
+
+namespace {
+bool isSharedPtr(const QualType &T) {
+  if (auto R = T->getAsCXXRecordDecl())
+    return R->getQualifiedNameAsString() == "std::shared_ptr" ||
+           R->getQualifiedNameAsString() == "boost::shared_ptr";
+  return false;
+}
+} // namespace
+
+void ConstParameterValueOrRef::checkPassByValueIsAppropriate(
+    const ParmVarDecl &Param, const FunctionDecl &Func,
+    const ASTContext &Context) {
+  if (!Func.isFirstDecl())
+    return;
+
+  const QualType &T = Param.getType();
+
+  if (isSharedPtr(T))
+    return;
+
+  Optional<CharUnits> Size = Context.getTypeSizeInCharsIfKnown(T);
+  if (!Size.hasValue())
+    return;
+
+  SourceLocation Loc = Param.getLocation();
+  FixItHint Hint = FixItHint::CreateInsertion(Loc, "&");
+  if (Size->getQuantity() > *MaxSize) {
+    diag(Loc, "%0 is a large type: pass by reference-to-const instead")
+        << T << Hint;
+    return;
+  }
+
+  if (!T.isTriviallyCopyableType(Context)) {
+    diag(Loc,
+         "%0 is not trivially-copyable: pass by reference-to-const instead")
+        << T << Hint;
+    return;
+  }
+}
+
+void ConstParameterValueOrRef::checkPassByRefIsAppropriate(
+    const ParmVarDecl &Param, const ASTContext &Context) {
+  const QualType &T = Param.getType().getNonReferenceType();
+
+  Optional<CharUnits> Size = Context.getTypeSizeInCharsIfKnown(T);
+  if (!Size.hasValue())
+    return;
+
+  if (isSharedPtr(T))
+    return;
+
+  if (Size->getQuantity() <= *MaxSize && T.isTriviallyCopyableType(Context)) {
+    diag(Param.getTypeSpecEndLoc(),
+         "%0 is a small, trivially-copyable type: pass by value instead")
+        << T << FixItHint::CreateRemoval(Param.getTypeSpecEndLoc());
+    return;
+  }
+}
+
+void ConstParameterValueOrRef::check(const MatchFinder::MatchResult &Result) {
+  if (!MaxSize.hasValue()) {
+    // If the user hasn't provided a default, set the default to sixteen bytes,
+    // which is the largest power-of-two size before performance is impacted
+    // across multiple chipsets. See user-facing documentation for more info.
+    static constexpr int DefaultMaxSize = 16;
+    MaxSize = DefaultMaxSize;
+  }
+
+  if (const auto *Param = Result.Nodes.getNodeAs<ParmVarDecl>("value")) {
+    const auto *Func = Result.Nodes.getNodeAs<FunctionDecl>("func-value");
+    assert(Func != nullptr);
+    return checkPassByValueIsAppropriate(*Param, *Func, *Result.Context);
+  }
+
+  if (const auto *Param = Result.Nodes.getNodeAs<ParmVarDecl>("ref"))
+    return checkPassByRefIsAppropriate(*Param, *Result.Context);
+}
+} // namespace performance
+} // namespace tidy
+} // namespace clang

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

 //===-- PerformanceTidyModule.cpp - clang-tidy ----------------------------===//
 //
 // 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 "../ClangTidy.h"
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
+#include "ConstParameterValueOrRef.h"
 #include "FasterStringFindCheck.h"
 #include "ForRangeCopyCheck.h"
 #include "ImplicitConversionInLoopCheck.h"
 #include "InefficientAlgorithmCheck.h"
 #include "InefficientStringConcatenationCheck.h"
 #include "InefficientVectorOperationCheck.h"
 #include "MoveConstArgCheck.h"
 #include "MoveConstructorInitCheck.h"
 #include "NoAutomaticMoveCheck.h"
 #include "NoIntToPtrCheck.h"
 #include "NoexceptMoveConstructorCheck.h"
 #include "TriviallyDestructibleCheck.h"
 #include "TypePromotionInMathFnCheck.h"
 #include "UnnecessaryCopyInitialization.h"
 #include "UnnecessaryValueParamCheck.h"
 
 namespace clang {
 namespace tidy {
 namespace performance {
 
 class PerformanceModule : public ClangTidyModule {
 public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
+    CheckFactories.registerCheck<ConstParameterValueOrRef>(
+        "performance-const-parameter-value-or-ref");
     CheckFactories.registerCheck<FasterStringFindCheck>(
         "performance-faster-string-find");
     CheckFactories.registerCheck<ForRangeCopyCheck>(
         "performance-for-range-copy");
     CheckFactories.registerCheck<ImplicitConversionInLoopCheck>(
         "performance-implicit-conversion-in-loop");
     CheckFactories.registerCheck<InefficientAlgorithmCheck>(
         "performance-inefficient-algorithm");

clang-tools-extra/docs/clang-tidy/checks/performance-const-parameter-value-or-ref.rst

+.. title:: clang-tidy - performance-const-parameter-value-or-ref
+
+performance-const-parameter-value-or-ref
+========================================
+
+Checks ``const``-qualified parameters to determine if they should be passed by
+value or by reference-to-``const`` for function definitions. Ignores proper
+function declarations, parameters without ``const`` in their type specifier, and
+dependent types.
+
+A type is recommended to be passed by value if it is both "small" and trivially
+copyable; otherwise the type recommendation flips to being pass by
+reference-to-``const``.
+
+This check works well with ``readability-avoid-const-params-in-decls``.
+
+
+Examples
+--------
+
+.. code-block:: c++
+
+  // No warnings for proper function declarations
+  void f1(const int &);
+  void f1(const std::string); // use readability-avoid-const-params-in-decls
+
+  // No warnings when value parameter is non-const
+  void f2(int i) {}
+  void f2(std::string s) {}
+
+  // No warnings when passing by value and parameter is "small" + trivially
+  // copyable
+  void f3(const char c) {}
+  void f3(const std::array<int, 8> a) {}
+
+  // No warning when passing by reference-to-const and parameter is not "small"
+  void f4(const std::array<int, 9> &a) {}
+
+  // No warning when passing by reference-to-const and type isn't trivially
+  // copyable
+  void f5(const std::string &) {}
+
+  void f6(int const& i) {}
+  // warning: 'const int' is a small, trivially copyable type: pass by value instead
+  // fix-it: void f6(const int i) {}
+
+  void f7(const std::array<int, 9> a) {}
+  // warning: 'const std::array<int, 9>' is a large type: pass by reference-to-const instead
+  // fix-it: void f7(const std::array<int, 9> &a) {}
+
+  void f8(const std::string s) {}
+  // warning: 'const std::string' is not trivially copyable: pass by reference-to-const instead
+
+  void f9(const std::shared_ptr<int>) {} // no warning by default: see below
+  void f10(const std::shared_ptr<int> &) {} // no warning by default: see below
+
+Options
+-------
+
+.. option:: SmallMaxSize
+
+Positive integer that indicates the largest size an object can be while being
+passed by ``const``-qualified value. The default value is 32 bytes.
+
+.. option:: EnableSharedPtrWarningFor
+
+**Accepted values**: ``'ConstValue'``, ``'RefToConst'``, ``Neither``
+
+Directs clang-tidy to warn about ``std::shared_ptr`` and ``boost::shared_ptr``
+on a particular condition.
+
+
+Design
+------
+
+Liberal use of ``const`` is good: it improves the integrity of a program and
+gives readers confidence that certain objects won't be modified throughout their
+lifetime. To mitigate performance bugs with respect to unnecessarily passing by
+value without compromising the aforementioned integrity, this check performs two
+tests: a ``const``-qualified value parameter is allowed if the programmer
+considers the type to be small, and if the type is trivially copyable.
+
+A type that's not trivially copyable is difficult to reason about at a glance.
+Well-designed types usually use copy constructors and copy-assignment operators
+to allocate new resources and copy the data of the other object in some
+non-constant time fashion. There are a few exceptions to this rule, such as
+``std::shared_ptr``, which often needs to be passed by value to increase the
+reference count (but can be marked as ``const`` to prevent releasing, etc.).
+Passing a ``shared_ptr`` by reference-to-``const`` is also acceptable in certain
+situaitons. As such, ``std::shared_ptr<T>`` and ``boost::shared_ptr<T>`` will
+not warn in either case by default.
+
+If a function has a visible forward declaration, then the check will ignore a
+const-qualified value parameter that's ``const``-qualified.
+
+.. code-block:: c++
+  // No warning because there's a forward declaration
+  void f(std::vector<int>);
+  void f(const std::vector<int>) {}
+
+The default value for ``SmallMaxSize`` was determined by through benchmarking
+when passing by value was no longer competetive with passing by reference for
+various `boards <https://git.io/JRKGv>`_. The benchmark can be found on
+`Compiler Explorer <https://godbolt.org/z/rfW1Wqajh>`_, with the used to inform
+the threshold.
+
+
+Limitations and future work
+---------------------------
+
+The forward declaration approach to asserting to the compiler that a parameter
+is intended to be passed by value is only applicable to free functions and
+member functions defined out-of-line. Inline member functions, hidden friends,
+and lambdas miss out on this in-source warning suppression.
+
+This check only supports ``std::shared_ptr`` and ``boost::shared_ptr`` as
+flexible types. Future work will establish a way for users to declare
+non-trivially copyable types that can be flexible.
+
+Template type parameters types aren't yet supported. It isn't clear how to
+support type parameters types in the general case (especially without proper
+static analysis), but there may be a way to set up mnemonics for clang-tidy to
+follow when using concepts. For example, a type parameter requiring
+``std::integral`` is always an integer, so it might be possible to use this
+knowledge to our advantage. Future work will look into this.

clang-tools-extra/test/clang-tidy/checkers/performance-const-parameter-value-or-ref.cpp

+// RUN: %check_clang_tidy %s performance-const-parameter-value-or-ref %t
+
+template <class T, int N>
+struct array {
+  T base[N];
+};
+
+using MaxSmall = array<char, 16>;
+using MinLarge = array<char, 17>;
+
+class NotTriviallyCopyable {
+public:
+  NotTriviallyCopyable() = default;
+
+  NotTriviallyCopyable(const NotTriviallyCopyable &) {}
+};
+
+// No warnings for forward declarations: this is the domain of
+// readability-avoid-const-params-in-decls
+void f1(const __int128);
+void f1(const MaxSmall);
+void f1(const MinLarge);
+void f1(const NotTriviallyCopyable);
+
+struct S1 {
+  void f(const __int128);
+  void f(const MaxSmall);
+  void f(const MinLarge);
+  void f(const NotTriviallyCopyable);
+};
+
+void f2(const __int128 &);
+void f2(const MaxSmall &);
+void f2(const MinLarge &);
+void f2(const NotTriviallyCopyable &);
+void f2(const array<NotTriviallyCopyable, 1> &);
+
+struct S2 {
+  void f(const __int128 &);
+  void f(const MaxSmall &);
+  void f(const MinLarge &);
+  void f(const NotTriviallyCopyable &);
+  void f(const array<NotTriviallyCopyable, 1> &);
+};
+
+// No warnings for function definitions that have been forward declared (pass
+// by value only).
+void f1(const MinLarge) {}
+void f1(const NotTriviallyCopyable) {}
+
+// No warnings when passing by value + mutable parameter
+void f3(__int128) {}
+void f3(MaxSmall) {}
+void f3(MinLarge) {}
+void f3(NotTriviallyCopyable) {}
+void f3(array<NotTriviallyCopyable, 1>) {}
+
+struct S3 {
+  void f(__int128) {}
+  void f(MaxSmall) {}
+  void f(MinLarge) {}
+  void f(NotTriviallyCopyable) {}
+  void f(array<NotTriviallyCopyable, 1>) {}
+};
+
+// No warnings when passing by value + "small" parameter
+void f4(const __int128) {}
+void f4(const MaxSmall) {}
+
+struct S4 {
+  void f(const __int128) {}
+  void f(const MaxSmall) {}
+};
+
+// No warnings when passing by reference-to-const + "large"/non-trivial parameter
+void f5(const MinLarge &) {}
+void f5(const NotTriviallyCopyable &) {}
+void f5(const array<NotTriviallyCopyable, 1> &) {}
+
+struct S5 {
+  void f(const MinLarge &) {}
+  void f(const NotTriviallyCopyable &) {}
+  void f(const array<NotTriviallyCopyable, 1> &) {}
+};
+
+// No warnings for dependent types
+template <class T>
+T f6(const T t) { return t; }
+
+const auto R1 = f6(false);
+const auto R2 = f6(MaxSmall());
+const auto R3 = f6(MinLarge());
+const auto R4 = f6(NotTriviallyCopyable());
+
+struct S6 {
+  template <class T>
+  T f(const T t) { return t; }
+
+  S6() {
+    const auto R1 = f(false);
+    const auto R2 = f(MaxSmall());
+    const auto R3 = f(MinLarge());
+    const auto R4 = f(NotTriviallyCopyable());
+  }
+};
+
+template <class T>
+T f7(const T &t) { return t; }
+
+const auto R5 = f7(false);
+const auto R6 = f7(MaxSmall());
+const auto R7 = f7(MinLarge());
+const auto R8 = f7(NotTriviallyCopyable());
+
+struct S7 {
+  template <class T>
+  T f(const T &t) { return t; }
+
+  S7() {
+    const auto R5 = f(false);
+    const auto R6 = f(MaxSmall());
+    const auto R7 = f(MinLarge());
+    const auto R8 = f(NotTriviallyCopyable());
+  }
+};
+
+////////////////////////////////////////////////////////////////////////////////
+// Warn when passing a "small" parameter by reference-to-const.
+////////////////////////////////////////////////////////////////////////////////
+void f2(const __int128 &) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: 'const __int128' is a small, trivially-copyable type: pass by value instead
+// CHECK-FIXES: void f2(const __int128) {}
+
+void f8(const __int128 &) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: 'const __int128' is a small, trivially-copyable type: pass by value instead
+// CHECK-FIXES: void f8(const __int128) {}
+
+void f8(const MaxSmall &) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: 'const MaxSmall' (aka 'const array<char, 16>') is a small, trivially-copyable type: pass by value instead
+// CHECK-FIXES: void f8(const MaxSmall) {}
+
+template <class T>
+void f8(T, const __int128 &) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:26: warning: 'const __int128' is a small, trivially-copyable type: pass by value instead
+// CHECK-FIXES: void f8(T, const __int128) {}
+
+template <class T>
+void f8(const MaxSmall &, T) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: 'const MaxSmall' (aka 'const array<char, 16>') is a small, trivially-copyable type: pass by value instead
+// CHECK-FIXES: void f8(const MaxSmall, T) {}
+
+struct S8 {
+  void f128(const __int128 &) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: 'const __int128' is a small, trivially-copyable type: pass by value instead
+  // CHECK-FIXES: void f128(const __int128) {}
+
+  void fMaxSmall(const MaxSmall &) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:32: warning: 'const MaxSmall' (aka 'const array<char, 16>') is a small, trivially-copyable type: pass by value instead
+  // CHECK-FIXES: void fMaxSmall(const MaxSmall) {}
+
+  template <class T>
+  void template128(T, const __int128 &) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:37: warning: 'const __int128' is a small, trivially-copyable type: pass by value instead
+  // CHECK-FIXES: void template128(T, const __int128) {}
+
+  template <class T>
+  void templateMaxSmall(const MaxSmall &, T) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:39: warning: 'const MaxSmall' (aka 'const array<char, 16>') is a small, trivially-copyable type: pass by value instead
+  // CHECK-FIXES: void templateMaxSmall(const MaxSmall, T) {}
+};
+
+////////////////////////////////////////////////////////////////////////////////
+// Warn when passing a "large" or non-trivially-copyable parameter by value (and
+// it's const-qualified).
+////////////////////////////////////////////////////////////////////////////////
+void f9(const MinLarge x) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: 'const MinLarge' (aka 'const array<char, 17>') is a large type: pass by reference-to-const instead
+// CHECK-FIXES: void f9(const MinLarge &x) {}
+
+void f9(const NotTriviallyCopyable x) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:36: warning: 'const NotTriviallyCopyable' is not trivially-copyable: pass by reference-to-const instead
+// CHECK-FIXES: void f9(const NotTriviallyCopyable &x) {}
+
+void f9(const array<NotTriviallyCopyable, 1> x) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:46: warning: 'const array<NotTriviallyCopyable, 1>' is not trivially-copyable: pass by reference-to-const instead
+// CHECK-FIXES: void f9(const array<NotTriviallyCopyable, 1> &x) {}
+
+template <class T>
+void f9(T, const MinLarge x) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:27: warning: 'const MinLarge' (aka 'const array<char, 17>') is a large type: pass by reference-to-const instead
+// CHECK-FIXES: void f9(T, const MinLarge &x) {}
+
+template <class T>
+void f9(const NotTriviallyCopyable x, T) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:36: warning: 'const NotTriviallyCopyable' is not trivially-copyable: pass by reference-to-const instead
+// CHECK-FIXES: void f9(const NotTriviallyCopyable &x, T) {}
+
+template <class T>
+void f9(const array<NotTriviallyCopyable, 1> x, T) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:46: warning: 'const array<NotTriviallyCopyable, 1>' is not trivially-copyable: pass by reference-to-const instead
+// CHECK-FIXES: void f9(const array<NotTriviallyCopyable, 1> &x, T) {}
+
+struct S9 {
+  void fMinLarge(const MinLarge x) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:33: warning: 'const MinLarge' (aka 'const array<char, 17>') is a large type: pass by reference-to-const instead
+  // CHECK-FIXES: void fMinLarge(const MinLarge &x) {}
+
+  void fNTC(const NotTriviallyCopyable x) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:40: warning: 'const NotTriviallyCopyable' is not trivially-copyable: pass by reference-to-const instead
+  // CHECK-FIXES: void fNTC(const NotTriviallyCopyable &x) {}
+
+  void fTNTC(const array<NotTriviallyCopyable, 1> x) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:51: warning: 'const array<NotTriviallyCopyable, 1>' is not trivially-copyable: pass by reference-to-const instead
+  // CHECK-FIXES: void fTNTC(const array<NotTriviallyCopyable, 1> &x) {}
+
+  template <class T>
+  void templateMinLarge(T, const MinLarge x) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:43: warning: 'const MinLarge' (aka 'const array<char, 17>') is a large type: pass by reference-to-const instead
+  // CHECK-FIXES: void templateMinLarge(T, const MinLarge &x) {}
+
+  template <class T>
+  void templateNTC(const NotTriviallyCopyable x, T) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:47: warning: 'const NotTriviallyCopyable' is not trivially-copyable: pass by reference-to-const instead
+  // CHECK-FIXES: void templateNTC(const NotTriviallyCopyable &x, T) {}
+
+  template <class T>
+  void templateTNTC(const array<NotTriviallyCopyable, 1> x, T) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:58: warning: 'const array<NotTriviallyCopyable, 1>' is not trivially-copyable: pass by reference-to-const instead
+  // CHECK-FIXES: void templateTNTC(const array<NotTriviallyCopyable, 1> &x, T) {}
+};
+
+////////////////////////////////////////////////////////////////////////////////
+// Default non-trivially copyable exemptions: no warnings expected
+////////////////////////////////////////////////////////////////////////////////
+
+namespace boost {
+template <class>
+struct shared_ptr {
+  shared_ptr(const shared_ptr &);
+};
+
+void can_pass_by_value(const shared_ptr<int>) {}
+void can_pass_by_value(const shared_ptr<shared_ptr<double>>) {}
+
+void can_pass_by_ref_to_const(const shared_ptr<int> &) {}
+void can_pass_by_ref_to_const(const shared_ptr<shared_ptr<double>> &) {}
+} // namespace boost
+
+namespace std {
+template <class>
+struct shared_ptr {
+  shared_ptr(const shared_ptr &);
+};
+
+void can_pass_by_value(const shared_ptr<int>) {}
+void can_pass_by_value(const shared_ptr<shared_ptr<double>>) {}
+
+void can_pass_by_ref_to_const(const shared_ptr<int> &) {}
+void can_pass_by_ref_to_const(const shared_ptr<shared_ptr<double>> &) {}
+} // namespace std

llvm/utils/gn/secondary/clang-tools-extra/clang-tidy/performance/BUILD.gn

 static_library("performance") {
   output_name = "clangTidyPerformanceModule"
   configs += [ "//llvm/utils/gn/build:clang_code" ]
   deps = [
     "//clang-tools-extra/clang-tidy",
     "//clang-tools-extra/clang-tidy/utils",
     "//clang/lib/AST",
     "//clang/lib/ASTMatchers",
     "//clang/lib/Analysis",
     "//clang/lib/Basic",
     "//clang/lib/Lex",
     "//llvm/lib/Support",
   ]
   sources = [
+    "ConstParameterValueOrRef.cpp",
     "FasterStringFindCheck.cpp",
     "ForRangeCopyCheck.cpp",
     "ImplicitConversionInLoopCheck.cpp",
     "InefficientAlgorithmCheck.cpp",
     "InefficientStringConcatenationCheck.cpp",
     "InefficientVectorOperationCheck.cpp",
     "MoveConstArgCheck.cpp",
     "MoveConstructorInitCheck.cpp",