<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/95417>95417</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LLVM does not support pseudo REX2 prefix
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:X86,
            mc
      </td>
    </tr>

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

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

<pre>
     .byte 0xf3,0xd5,0x80,0xb8,0xc1
  {rex2} popcnt %ecx,%eax

Expect the second has the same encoding as the first one.  GNU binutils supports it while LLVM doesn't support it.

https://www.godbolt.org/z/xd5Tj78cr
</pre>
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