================
@@ -3958,6 +3958,50 @@ Note that the `size` argument must be a compile time
constant.
Note that this intrinsic cannot yet be called in a ``constexpr`` context.
+``__is_bitwise_cloneable``
+-------------------------
+
+A type trait is used to check whether a type can be safely copied by memcpy.
+
+**Syntax**:
+
+.. code-block:: c++
+
+ bool __is_bitwise_cloneable(Type)
+
+** Example of Use**:
+
+.. code-block:: c++
+
+ // Return a cloned object of the given default instance.
+ Foo* Clone(const Foo* default_instance, char* buffer, unsigned size) {
+ if constexpr __is_bitwise_cloneable(decltype(*default_instance)) {
+ // Fast path via memcopy, without calling class constructor.
+ memcpy(buffer, default_instance, size);
+ // Explicitly start the lifetime of the cloned object.
+ return __builtin_start_object_lifetime(reinterpret_cast<Foo*>(buffer));
+ }
+ // Fallback the operator new, which invoke the class constructor.
+ return new(buffer) Foo(*default_instance);
+ }
+
+**Description**:
+
+It is common for library owners to perform memcpy/memmove on types that aren't
+trivally copyable for performance reason. However, according to the C++
standard,
----------------
ilya-biryukov wrote:
NIT: I think it would be useful to explain the differences between this and
is_trivially_copyable.
Something like:
```cpp
This trait is similar to `std::is_trivially_copyable`, but additionally allows
to have user-defined constructors,
virtual functions and virtual bases. It is up to the user code to guarantee
that a bitwise copy results in
non-broken object and that the lifetime of an object is properly started.
```
https://github.com/llvm/llvm-project/pull/86512
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