[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 +1,243 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// atomic_init is deprecated
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
+#include <atomic>
+#include <cassert>
+#include <chrono>
+#include <cstring>
+#include <type_traits>
+
+struct Foo {
+  int i;
+  char c;
+};
+
+static_assert(!std::has_unique_object_representations_v<Foo>);
+static_assert(sizeof(Foo) > sizeof(int) + sizeof(char));
+
+Foo make_foo(int i, char c, unsigned char pad_byte) {
+  Foo f;
+  std::memset(&f, pad_byte, sizeof(Foo));
+  f.i = i;
+  f.c = c;
+  return f;
+}
+
+void assert_foo_padding(const Foo& f, unsigned char pad_byte) {
+  alignas(Foo) unsigned char buf[sizeof(Foo)];
+  std::memset(buf, pad_byte, sizeof(Foo));
+  Foo& reference = *reinterpret_cast<Foo*>(buf);
+  reference.i    = f.i;
+  reference.c    = f.c;
+  assert(std::memcmp(&f, &reference, sizeof(Foo)) == 0);
+}
+
+#if __has_builtin(__builtin_clear_padding)
+
+void test_default_constructor() {
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ldionne wrote:

I would like to suggest using nested scopes inside a templated `test<Foo|Bar>` function directly, with plain english comments explaining what each test case is doing.

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