<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/58338>58338</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Redundant movzx instructions when operating on i8s
        </td>
    </tr>

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

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

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

<pre>
    When casting argument from i8 to i32, codegen doesn't seem to leverage the fact that the rest of `edi` is zeroed, emitting `movzx eax, dil` ([playground]). Gcc in similar case [emits simply `mov eax, edi`][gcc].

In the [playground] there's one more case (`ternary_and`), roughly `x < 3 ? 0 : x & 2`, where two redundant cmovx are emitted. (They're redundant, as high bits of eax are already zeroed). The real world case I initially noticed this behaviour was using `sub nuw` instead of `and`. Here also high bits of eax should be zeroed (although `opt` turns this `sub nuw` into `add`, losing `nuw` annotation).

Perhaps related: https://github.com/llvm/llvm-project/issues/56498

[playground]: https://alive2.llvm.org/ce/z/Hi9yux
[gcc]: https://gcc.godbolt.org/z/ndGK8c3MG
</pre>
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