<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/85551>85551</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Indexing bounds checks are not hoisted out of a loop
        </td>
    </tr>

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

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

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

<pre>
    I played around with some variants on using std::span bounds checks, and it seems Clang is not yet very good at hoisting checks out of loops.

Consider a (slightly silly) function like this:
```
int sum_prefix(std::span<const int> s, size_t n) {
    int ret = 0;
    for (size_t i = 0; i < n; i++) {
        ret += s[i];
 }
    return ret;
}
```

With bounds checks on, `s[i]` will check that `i < s.size()` at every loop iteration. But ideally we'd do a single check that `n <= s.size()` once, before the loop. That then allows Clang to vectorize the loop and do other things.

Here's a godbolt with some more illustrative variations:
https://godbolt.org/z/zYbfzco4e
</pre>
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