[libcxx-commits] [libcxx] [libc++] Add the __is_replaceable type trait (PR #132408)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_trivially_copyable.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is replaceable if, with `x` and `y` being different objects, `x = std::move(y)` is equivalent to:
+//
+//  std::destroy_at(&x)
+//  std::construct_at(&x, std::move(y))
+//
+// This allows turning a move-assignment into a sequence of destroy + move-construct, which
+// is often more efficient. This is especially relevant when the move-construct is in fact
+// part of a trivial relocation from somewhere else, in which case there is a huge win.
+//
+// Note that this requires language support in order to be really effective, but we
+// currently emulate the base template with something very conservative.
+template <class _Tp, class = void>
+struct __is_replaceable : is_trivially_copyable<_Tp> {};
+
+template <class _Tp>
+struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {};
+
+template <class _Tp>
+inline const bool __is_replaceable_v = __is_replaceable<_Tp>::value;
+
+// Determines whether an allocator member of a container is replaceable.
+//
+// We take into account whether the allocator is propagated on assignments. If the allocator
+// always compares equal, then it doesn't matter whether we propagate it or not on assignments,
+// the result will be the same and we can just as much move-construct it instead.
+//
+// If the allocator does not always compare equal, we check whether it propagates on assignment
+// and it is replaceable.
+template <class _AllocatorTraits>
+struct __container_allocator_is_replaceable
+    : integral_constant<bool,
+                        _AllocatorTraits::is_always_equal::value ||
+                            (_AllocatorTraits::propagate_on_container_move_assignment::value &&
+                             _AllocatorTraits::propagate_on_container_copy_assignment::value &&
+                             __is_replaceable_v<typename _AllocatorTraits::allocator_type>)> {};
----------------
ldionne wrote:

Ack, implemented the change in my last update.

https://github.com/llvm/llvm-project/pull/132408