<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62022>62022</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] ordering comparisons should be marked __attribute__((pure))
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    https://godbolt.org/z/e4rTvMeoW

The following code generates Wassume warnings suggesting there are side effects when converting from a strong_ordering to a bool. The same is true of partial_ordering and weak_ordering.

```cpp
#include <compare>

struct S {
    __attribute__((pure)) bool g() { return true; }

    __attribute__((pure)) auto operator<=>(const S& rhs) const {
        return std::strong_ordering::equal;
 }
};


constexpr __attribute__((pure)) __attribute__((noinline)) auto f(
    const S&, const S&) noexcept {
    return std::strong_ordering::equal;
}

int main() {
    S s1, s2;
 __builtin_assume(f(s1, s2) == 0);
    __builtin_assume(f(s1, s2) == std::strong_ordering::equal);
    __builtin_assume(s1 <= s2);
    int a, b;
    __builtin_assume(a <= b);  // Only this one is accepted without warning.

    __builtin_assume(std::strong_ordering::equivalent == 0);
    __builtin_assume(std::weak_ordering::equivalent == 0);
 __builtin_assume(std::partial_ordering::equivalent == 0);
}
```

I tried to find the right place to insert `__attribute__((pure))`. I think all of these:
- https://github.com/llvm/llvm-project/blob/e3d2b584aae36a3d7d2a28b23ff1c5ec4f70974a/libcxx/include/__compare/ordering.h#L157-L213
- https://github.com/llvm/llvm-project/blob/e3d2b584aae36a3d7d2a28b23ff1c5ec4f70974a/libcxx/include/__compare/ordering.h#L72-L129
- https://github.com/llvm/llvm-project/blob/e3d2b584aae36a3d7d2a28b23ff1c5ec4f70974a/libcxx/include/__compare/ordering.h#L249-L305


</pre>
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