<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63113>63113</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            ACLE and UBSan disagree about clz with input zero
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          tmatheson-arm
      </td>
    </tr>
</table>

<pre>
    [D152023](https://reviews.llvm.org/D152023) made calls to `__builtin_clz` and `__builtin_ctz` with input 0 to be undefined behaviour. However we use them in the `arm-acle.h` to implement `__clz`:

https://github.com/llvm/llvm-project/blob/c91246b71eec9fc1d4454ddcdc574733a699e190/clang/lib/Headers/arm_acle.h#L143

The [ACLE](https://github.com/ARM-software/acle/blob/main/main/acle.md#miscellaneous-data-processing-intrinsics) documentation for `__clz` states:

> When x is zero it returns the argument width, i.e. 32 or 64.

The definitions in the header will need to be updated to handle the zero case.
</pre>
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