<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63495>63495</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Specification of __builtin_addc
        </td>
    </tr>

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

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

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

<pre>
    Hello,
the documentation for the clang extension `__builtin_addc` (https://clang.llvm.org/docs/LanguageExtensions.html#multiprecision-arithmetic-builtins) is very limited, it essentially gives the prototype and nothing else
```c
unsigned           __builtin_addc  (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
```
Gcc is currently implementing this builtin (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173) for compatibility, and they interpret the spec in a rather wide manner as a sum of 3 arbitrary integers, with carryout as 1 if there is an overflow (even when the carry should be 2).
Some of us think that the use of the term "carry" means that if `carryin` is not 0 or 1, this function has undefined behavior.

Could you please clarify the documentation? And in case you go with the second interpretation, could you add that check in UBSAN?
</pre>
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