<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/75411>75411</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            ABI issue in i686 mingw + libstdc++ when using std::optional
        </td>
    </tr>

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

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

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

<pre>
    In summary, `std::optional` when compiled from clang is returned from a function using `edx` and `eax`, while gcc and msvc expect the selector bit to be on the stack.

The setup is a bit annoying because one needs a clang + libstdc++ 32 bit mingw toolchain. But once one has that it's simple to see the difference.

```cpp
#include <optional>

std::optional<const char *> foo(int a){
    if (a)
        return std::nullopt;
 return std::optional<const char *>("a");
} 
```

clang assembly:
```assembly
Disassembly of section .text:

64f01400 <__Z3fooi>:
64f01400: 31 d2                         xorl    %edx, %edx
64f01402: 83 7c 24 04 00                cmpl    $0x0, 0x4(%esp)
64f01407: 0f 94 c2                      sete    %dl
64f0140a: b8 00 40 f0 64                movl    $0x64f04000, %eax       # imm = 0x64F04000
64f0140f: c3                            retl
```

gcc assembly:
```assembly

Disassembly of section .text:

651414e0 <__Z3fooi>:
651414e0: 8b 54 24 08                   movl    0x8(%esp), %edx
651414e4: 8b 44 24 04                   movl    0x4(%esp), %eax
651414e8: 85 d2                         testl   %edx, %edx
651414ea: 74 0c                         je      0x651414f8 <__Z3fooi+0x18>
651414ec: c6 40 04 00                   movb    $0x0, 0x4(%eax)
651414f0: c3                            retl
651414f1: 8d b4 26 00 00 00 00          leal    (%esi,%eiz), %esi
651414f8: c7 00 44 40 14 65             movl    $0x65144044, (%eax)     # imm = 0x65144044
651414fe: c6 40 04 01                   movb    $0x1, 0x4(%eax)
65141502: c3                            retl
```

note the `sete %dl`  vs the `movb $0x0, 0x4(%eax) `


</pre>
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