<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/146141>146141</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] Look into merging the atomics in __libcpp_contention_table_entry
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
            huixie90
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ldionne
      </td>
    </tr>
</table>

<pre>
    While discussing with @huixie90 and Apple folks about https://github.com/llvm/llvm-project/issues/109290, it was noticed that `__libcpp_contention_table_entry` defined [here](https://github.com/llvm/llvm-project/blob/52040b44f6060540a0b9d56fdd2e0eb5a540e84c/libcxx/src/atomic.cpp#L121C54-L121C85) contained two atomics when in reality we could merge both into a single atomic:

```
struct alignas(64) /*  aim to avoid false sharing */ __libcpp_contention_table_entry {
  __cxx_atomic_contention_t __contention_state;
  __cxx_atomic_contention_t __platform_state;
  inline constexpr __libcpp_contention_table_entry() : __contention_state(0), __platform_state(0) {}
};
```

On Linux, `__cxx_atomic_contention_t` is 32 bits, so we could merge both into a single 64 bits value and perform atomic operations on that directly.

On Apple arm64, `__cxx_atomic_contention_t` is 64 bits, so we could merge both into a single 128 bits value and perform 128-bit atomic operations on that. On Apple x86_64, I don't think we can use this trick (?).

This can potentially simplify the implementation of the contention table algorithm and might be faster.
</pre>
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