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

[Damien L-G via libcxx-commits]

================
@@ -0,0 +1,335 @@
+// -*- 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>;
----------------
dalg24 wrote:

Do you prefer this instead?
```diff
diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
index 126fa60e1816..f376051a902e 100644
--- a/libcxx/include/__atomic/atomic_ref.h
+++ b/libcxx/include/__atomic/atomic_ref.h
@@ -45,20 +45,10 @@ 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>;
-#  else
-      false;
-#  endif
-
   _LIBCPP_HIDE_FROM_ABI static _Tp* __clear_padding(_Tp& __val) noexcept {
     _Tp* __ptr = std::addressof(__val);
 #  if __has_builtin(__builtin_clear_padding)
-    if constexpr (__might_have_padding) {
-      __builtin_clear_padding(__ptr);
-    }
+    __builtin_clear_padding(__ptr);
 #  endif
     return __ptr;
   }
@@ -109,7 +99,13 @@ struct __atomic_ref_base {

   _LIBCPP_HIDE_FROM_ABI static bool __compare_exchange(
       _Tp* __ptr, _Tp* __expected, _Tp* __desired, bool __is_weak, int __success, int __failure) noexcept {
-    if constexpr (!__might_have_padding) {
+    if constexpr (
+#  if __has_builtin(__builtin_clear_padding)
+        has_unique_object_representations_v<_Tp> || same_as<_Tp, float> || same_as<_Tp, double>;
+#  else
+        true
+#  endif
+    ) {
       return __atomic_compare_exchange(__ptr, __expected, __desired, __is_weak, __success, __failure);
     } else { // _Tp has padding bits and __builtin_clear_padding is available
       __clear_padding(*__desired);

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