[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) {
----------------
ldionne wrote:

```suggestion
_LIBCPP_HIDE_FROM_ABI bool __atomic_cas_with_clear_padding(_Tp* __expected, _Up __value, _CasFunc&& __cas_func) {
```

I would suggest taking by value here. That makes it easier to understand since you're clearing the padding below (otherwise we have to know that the caller of this function already made a copy that they are about to discard, which requires non local reasoning). I suspect it doesn't make a difference in the codegen since they are small functions and are probably inlined anyway.

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