[libcxx-commits] [libcxx] [libc++] Extract a clean base support API for std::atomic (PR #118129)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,265 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___ATOMIC_SUPPORT_GCC_H
+#define _LIBCPP___ATOMIC_SUPPORT_GCC_H
+
+#include <__atomic/memory_order.h>
+#include <__atomic/to_gcc_order.h>
+#include <__config>
+#include <__memory/addressof.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/remove_const.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+//
+// This file implements support for GCC-style atomics
+//
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// [atomics.types.generic]p1 guarantees _Tp is trivially copyable. Because
+// the default operator= in an object is not volatile, a byte-by-byte copy
+// is required.
+template <typename _Tp, typename _Tv, __enable_if_t<is_assignable<_Tp&, _Tv>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_assign_volatile(_Tp& __a_value, _Tv const& __val) {
+  __a_value = __val;
+}
+template <typename _Tp, typename _Tv, __enable_if_t<is_assignable<_Tp&, _Tv>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_assign_volatile(_Tp volatile& __a_value, _Tv volatile const& __val) {
+  volatile char* __to         = reinterpret_cast<volatile char*>(std::addressof(__a_value));
+  volatile char* __end        = __to + sizeof(_Tp);
+  volatile const char* __from = reinterpret_cast<volatile const char*>(std::addressof(__val));
+  while (__to != __end)
+    *__to++ = *__from++;
+}
+
+template <typename _Tp>
+struct __cxx_atomic_base_impl {
+  _LIBCPP_HIDE_FROM_ABI
+#ifndef _LIBCPP_CXX03_LANG
----------------
ldionne wrote:

For now, yes. We still maintain C++03 support for 1 or 2 releases (I forgot what we agreed on in the RFC).

https://github.com/llvm/llvm-project/pull/118129