[clang] Implement a bitwise_copyable builtin type trait.

This patch implements a `__is_bitwise_copyable` builtin in clang.

The bitwise copyable types act as the trivially copyable types, but
they support a wider range of types (e.g. classes with virtual methods) --
their underlying types can be safely copied by `memcopy` or `memmove`,
the clang compiler guarantees that both source and destination objects
have the same *object* representations after the copy operation, and the
lifetime of the destination object implicitly starts.

A particular use case of this builtin is to clone an object via memcopy
(without running the constructor):

```
Message* clone(const Message* src, char* buffer, int size) {
  if constexpr __is_bitwise_copyable(Message) {
    // bitwise copy to buffer, and implicitly create objects at the buffer
    __builtin_memcpy(buffer, src, size);
    return std::launder(reinterpret_cast<Message*>(buffer));
  }
  // Fallback the operator new, which calls the constructor to start the lifetime.
  return new(buffer) Message(src);
}
```

Note that the definition of bitwise copyable is not tied to the Rule Of
Five, so users of this builtin must guarantee that program semantic constraints
are satisfied, e.g. no double resource deallocations.

Context: https://discourse.llvm.org/t/extension-for-creating-objects-via-memcpy

Author: hokein

clang/include/clang/AST/Type.h

@@ -1120,6 +1120,18 @@ class QualType {
   /// Return true if this is a trivially copyable type (C++0x [basic.types]p9)
   bool isTriviallyCopyableType(const ASTContext &Context) const;
 
+  /// Return true if this is a bitwise copyable type.
+  ///
+  /// This is an extension in clang: bitwise copyable types act as trivially
+  /// copyable types, underlying bytes of bitwise copyable type can be safely
+  /// copied by memcpy or memmove. Clang guarantees that both source and
+  /// destination objects have the same **object** representations after the
+  /// copy, and the lifetime of the destination object implicitly starts.
+  ///
+  /// bitwise copyable types cover a wider range of types, e.g. classes with
+  /// virtual methods.
+  bool isBitwiseCopyableType(const ASTContext &Context) const;
+
   /// Return true if this is a trivially copyable type
   bool isTriviallyCopyConstructibleType(const ASTContext &Context) const;
 

clang/include/clang/Basic/TokenKinds.def

@@ -527,6 +527,7 @@ TYPE_TRAIT_2(__is_pointer_interconvertible_base_of, IsPointerInterconvertibleBas
 #include "clang/Basic/TransformTypeTraits.def"
 
 // Clang-only C++ Type Traits
+TYPE_TRAIT_1(__is_bitwise_copyable, IsBitwiseCopyable, KEYCXX)
 TYPE_TRAIT_1(__is_trivially_relocatable, IsTriviallyRelocatable, KEYCXX)
 TYPE_TRAIT_1(__is_trivially_equality_comparable, IsTriviallyEqualityComparable, KEYCXX)
 TYPE_TRAIT_1(__is_bounded_array, IsBoundedArray, KEYCXX)

clang/lib/AST/Type.cpp

@@ -2731,6 +2731,29 @@ bool QualType::isTriviallyCopyableType(const ASTContext &Context) const {
                                      /*IsCopyConstructible=*/false);
 }
 
+bool QualType::isBitwiseCopyableType(const ASTContext & Context) const {
+  QualType CanonicalType = getCanonicalType();
+  if (CanonicalType->isIncompleteType() || CanonicalType->isDependentType())
+    return false;
+  // Trivially copyable types are bitwise copyable, e.g. scalar types.
+  if (CanonicalType.isTriviallyCopyableType(Context))
+    return true;
+
+  if (CanonicalType->isArrayType())
+    return Context.getBaseElementType(CanonicalType)
+        .isBitwiseCopyableType(Context);
+
+  if (const auto *RD = CanonicalType->getAsCXXRecordDecl()) {
+    for (auto *const Field : RD->fields()) {
+      QualType T = Context.getBaseElementType(Field->getType());
+      if (!T.isBitwiseCopyableType(Context))
+        return false;
+    }
+    return true;
+  }
+  return false;
+}
+
 bool QualType::isTriviallyCopyConstructibleType(
     const ASTContext &Context) const {
   return isTriviallyCopyableTypeImpl(*this, Context,

clang/lib/Sema/SemaExprCXX.cpp

@@ -5128,6 +5128,8 @@ static bool CheckUnaryTypeTraitTypeCompleteness(Sema &S, TypeTrait UTT,
   case UTT_IsStandardLayout:
   case UTT_IsPOD:
   case UTT_IsLiteral:
+  // Clang extension:
+  case UTT_IsBitwiseCopyable:
   // By analogy, is_trivially_relocatable and is_trivially_equality_comparable
   // impose the same constraints.
   case UTT_IsTriviallyRelocatable:
@@ -5613,6 +5615,8 @@ static bool EvaluateUnaryTypeTrait(Sema &Self, TypeTrait UTT,
     return C.hasUniqueObjectRepresentations(T);
   case UTT_IsTriviallyRelocatable:
     return T.isTriviallyRelocatableType(C);
+  case UTT_IsBitwiseCopyable:
+    return T.isBitwiseCopyableType(C);
   case UTT_IsReferenceable:
     return T.isReferenceable();
   case UTT_CanPassInRegs:

clang/test/SemaCXX/builtin-is-bitwise-copyable.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+// Scalar types are bitwise copyable.
+static_assert(__is_bitwise_copyable(int));
+static_assert(__is_bitwise_copyable(int*));
+// array
+static_assert(__is_bitwise_copyable(int[10]));
+
+
+struct Forward; // expected-note 2{{forward declaration of 'Forward'}}
+static_assert(!__is_bitwise_copyable(Forward)); // expected-error {{incomplete type 'Forward' used in type trait expression}}
+
+struct Foo { int a; };
+static_assert(__is_bitwise_copyable(Foo));
+
+struct DynamicClass { virtual int Foo(); };
+static_assert(__is_bitwise_copyable(DynamicClass));
+
+template <typename T>
+void TemplateFunction() {
+  static_assert(__is_bitwise_copyable(T)); // expected-error {{incomplete type 'Forward' used in type trait expression}}
+}
+void CallTemplateFunc() {
+  TemplateFunction<Forward>(); // expected-note {{in instantiation of function template specialization}}
+  TemplateFunction<Foo>();
+}