<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/153059>153059</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [X86] Add opcode "udb" (permanent #UD in 64-bit mode)
        </td>
    </tr>

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

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

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

<pre>
    The opcode D6 has been officially reserved as a single-byte permanent
undefined (#UD) opcode in 64-bit mode with the mnemonic UDB.  This is
already the behavior of all known 64-bit implementations; this is thus
merely an official statement of forward compatibility and the
assignment of a mnemonic.

This will be documented in the next version of the Intel Software
Developer's Manual; in the meantime I DO speak officially for Intel on
this issue.

The x86 Advisory Council has ratified this decision, and so it is
expected to be honored across vendors, but I obviously cannot make any
official statement on any other vendor's behalf.

</pre>
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