<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/158472>158472</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] [x86-64] enum layout without fixed type differs from GCC
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    clang computes a different layout than GCC for this enum:
```
enum big {
    A = 9223372036854775808u,
    B = -1,
};
```
([Godbolt](https://godbolt.org/z/GqK8dMbTe))

GCC gives this size 16 without any warning. clang gives it size 8 and says
```
<source>:3:6: warning: enumeration values exceed range of largest integer [-Wenum-too-large]
    3 | enum big {
      |      ^
```
(On x86-32, clang and GCC both give it size 8 and both emit a warning.)

Whether this is legal C or not depends on whether you consider `__int128` to be an "extended integer type" or not... GCC apparently [does not](https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html), though they do treat it like an extended integer type in many respects, so 🤷 . But legal C or not, it seems surprising that the two compiler would disagree  here, and it could lead to ABI issues.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJxsVFGPpDYM_jWeF2sQEwgwDzwws8fqWlWVqkr3eArEQNqQ0CTs7tyvrwLcVtquFIkQO46_z58tvFejIaqB34A_ncQaJuvqP4QeflnNeOqsfNS9FmbE3s7LGsijQKmGgRyZgFo87BowTMLg8_2Og3UYJuWRzDpD1kDaQJEeK23iKXZqRChvkDaIiA1C9oRXxrKsZGlWVDwvS16l1QrsfvjcNp_zZT-B8gmy24fIwCrgt2crO6sD8Cdg1RTC4mMOrAXWjrspsW4E1v4A1j7_82slf-v-JGDXuNKYbQQxqhfyOwyvfhBeCnxVYYpAhXngq3BGmTHBnZfdW4Xdt0JhJHrx8B8TzO7erq4nyL5A1mSQNQVkzc9ocRvZISeCsgZfhF7JI731RBKdMCOhHVALN5IPqEygkRwCv52_xXvnYO15s0bwO20ZQnnHTzjHzbBv-Jf_M_m7wbeqOGcM2P1AGVFFbjobpg3yB8TbOc0qoHgn6J3UbxOFiQ5lKI-aRqHxjtahsQElLWSkR2vw9fB82BV7a7ySEWSRfv-uTLiwCooUg8WOUBgExugtkJEk3wkJj4WAsSN0kiRb1mJZRNSrfkTGpCUfrZ_KpO-T0ayHTKzRypC0vd9NwNqv-0P-rOZF00wmbAVLpjDrTUh3jFIZJwwTPVBaDI5EiHRp9feW96dJozI4R3k58gv1wcdI3iK0KVxbaHK4lZjgbQ0f6It-sRZEs0e_usUpr8wYezI2JmF4tVvzKk0OX-2qJUrlxeiIECdyFCPEGqqA_WbWJGSkubl9ReX9Sj45yTqT1-wqTlRfSl5Wec4yfprqrKqkKHrJClbJ9HrtRVekokivF973nF9OqmYp4-n1kl_SNOM8yQs-DIIPJSu5vFQV5CnNQulE65c5En_a3qwvvMpLdtKiI-23AcXYJkZgLM4qV8cL524dPeSpVj74_0IEFfQ21fYb_CkWPoq6yOPP1hTH8PrZ24N6I7nXYh9wHgdn5yig0-p0_UEoKkxrl_R2BtbGZ4_PeXH2L-oDsHanDlh7IHmp2b8BAAD__w0NtQQ">