[libcxx-commits] [libcxx] [libc++] Implement C++20 atomic_ref (PR #76647)

[via libcxx-commits]

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ATOMIC_ATOMIC_REF_H
+#define _LIBCPP___ATOMIC_ATOMIC_REF_H
+
+#include <__assert>
+#include <__atomic/atomic_sync.h>
+#include <__atomic/check_memory_order.h>
+#include <__atomic/to_gcc_order.h>
+#include <__concepts/arithmetic.h>
+#include <__concepts/same_as.h>
+#include <__config>
+#include <__memory/addressof.h>
+#include <__type_traits/has_unique_object_representation.h>
+#include <__type_traits/is_trivially_copyable.h>
+#include <cstddef>
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Tp>
+struct __atomic_ref_base {
+  _Tp* __ptr_;
+
+  // whether the object pointed to has padding AND we will actually ignore them
+  static constexpr bool __might_have_padding =
+#  if __has_builtin(__builtin_clear_padding)
+      !has_unique_object_representations_v<_Tp> && !same_as<_Tp, float> && !same_as<_Tp, double>;
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huixie90 wrote:

for `long double` , only fp80 format has padding.
However,  IIUC this is just an optimization so it does not hurt.

BTW, this logic is likely to be duplicated in the `__atomic_base`. maybe we can consider putting it in a shared place later when we need

https://github.com/llvm/llvm-project/pull/76647