[libcxx-commits] [clang] [libcxx] [clang] [libc++] fix _Atomic c11 compare exchange does not update expected results (PR #78707)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// https://github.com/llvm/llvm-project/issues/30023
+// compare exchange does not work with types of which the size is not a power of 2
+
+#include <atomic>
+#include <cstring>
+#include <cassert>
+
+template <int Size>
+struct S {
+  char data[Size];
+
+  explicit S(char v = 0) noexcept { memset(&data[0], v, sizeof(data)); }
+
+  // only used in the test to check the results. Not used in atomic operations.
+  friend bool operator==(const S& lhs, const S& rhs) noexcept {
+    return memcmp(&lhs.data[0], &rhs.data[0], sizeof(lhs.data)) == 0;
+  }
+  friend bool operator!=(const S& lhs, const S& rhs) noexcept { return !(lhs == rhs); }
+};
+
+template <int Size>
+struct Expected {
+  S<Size> s;
+  bool b = true; // used to validate that s's operation won't overwrite the memory next to it
+};
+
+template <int Size>
+void test() {
+  using T = S<Size>;
+  std::atomic<T> a(T(0));
+  Expected<Size> expected{T(17)};
+
+  assert(a.load() != expected.s);
+  assert(expected.b);
+
+  auto r1 = a.compare_exchange_strong(expected.s, T(18), std::memory_order_relaxed);
+
+  assert(!r1);
+  assert(expected.s == T(0)); // expected.s is modified by compare_exchange_strong
+  assert(expected.b);
+  assert(a.load() == T(0));
+
+  auto r2 = a.compare_exchange_strong(expected.s, T(18), std::memory_order_relaxed);
+  assert(r2);
+  assert(a.load() == T(18));
+  assert(expected.s == T(0));
+  assert(expected.b);
+}
+
+int main() {
+  test<1>();
+  test<2>();
+  test<3>();
+  test<4>();
+  test<5>();
+  test<6>();
+}
----------------
ldionne wrote:

```suggestion
  test<6>();
  return 0;
}
```

https://github.com/llvm/llvm-project/pull/78707