<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/65863>65863</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Simplify add-with-overflow pattern (uaddo)
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue,
            llvm:instcombine,
            missed-optimization
      </td>
    </tr>

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

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

<pre>
    For unsigned integers to check if an addition overflowed (get carry bit) it is enough to compare the sum with one of its arguments `carry = sum < a`. However, it is not obvious that it is enough to only check one of the arguments and we may encounter a pattern where both arguments are checked.

```llvm
define i1 @src(i64 %a, i64 %b){
  %s = add i64 %a, %b
  %cond_a = icmp ult i64 %s, %a
  %cond_b = icmp ult i64 %s, %b
  %cond = or i1 %cond_a, %cond_b
  ret i1 %cond
}

define i1 @tgt(i64 %a, i64 %b){
  %s = add i64 %a, %b
  %cond_a = icmp ult i64 %s, %a
  ret i1 %cond_a
}
```

Playground: https://godbolt.org/z/TW8enc6zo
Proof: https://alive2.llvm.org/ce/z/iPAE7t
</pre>
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