[libcxx-commits] [libcxx] [libc++] Implement P0528R3 `std::atomic` CAS for types with paddings (PR #76180)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,81 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_COMMON_H
+#define _LIBCPP___ATOMIC_SUPPORT_COMMON_H
+
+#include <__config>
+#include <__memory/addressof.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/has_unique_object_representation.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/negation.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
+#include <cstring>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20 && __has_builtin(__builtin_clear_padding)
+
+template <class _Tp>
+struct __needs_clear_padding
+    : _And<_Not<has_unique_object_representations<_Tp>>, _Not<is_same<_Tp, float>>, _Not<is_same<_Tp, double>>> {};
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr void __clear_padding_if_needed(_Tp&& __obj) noexcept {
+  if constexpr (__needs_clear_padding<remove_cvref_t<_Tp>>::value) {
+    if (!__builtin_is_constant_evaluated()) {
+      __builtin_clear_padding(std::addressof(__obj));
+    }
+  }
+}
+
+template <class _Tp, class _Up, class _CasFunc>
+_LIBCPP_HIDE_FROM_ABI bool __atomic_cas_with_clear_padding(_Tp* __expected, _Up&& __value, _CasFunc&& __cas_func) {
+  if constexpr (!__needs_clear_padding<remove_cv_t<_Tp>>::value) {
+    return __cas_func(__expected, std::forward<_Up>(__value));
+  } else {
+    std::__clear_padding_if_needed(__value);
+    remove_cv_t<_Tp> __expected_copy = *__expected;
+    std::__clear_padding_if_needed(__expected_copy);
+    if (__cas_func(std::addressof(__expected_copy), std::forward<_Up>(__value))) {
+      return true;
+    } else {
+      std::memcpy(__expected, std::addressof(__expected_copy), sizeof(remove_cv_t<_Tp>));
+      return false;
+    }
+  }
+}
+
+#else // _LIBCPP_STD_VER >= 20 && __has_builtin(__builtin_clear_padding)
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI
+#  if _LIBCPP_STD_VER >= 14
+// c++11 does not allow constexpr functions to return void
+constexpr
----------------
ldionne wrote:

You can use `_LIBCPP_CONSTEXPR_SINCE_CXX14` for this.

https://github.com/llvm/llvm-project/pull/76180