<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/71692>71692</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [CodeGen] Reserved registers frozen multiple times
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:AMDGPU
      </td>
    </tr>

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

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

<pre>
    Based on our documentation, it sounds like the set of ReservedRegs is supposed to stay constant after it has been frozen:
https://github.com/llvm/llvm-project/blob/98d8b688bd351850ca99d3e5e9dfdf582c83c4f4/llvm/include/llvm/CodeGen/MachineRegisterInfo.h#L901-L911

However, in practice we call freezeReservedRegs() at least twice, once during FinalizeISel and then again in RegAlloc. Adding a check to not touch the ReservedRegs if they have already been computed results in a bunch of verifier failures in AMDGPU.

I think *something* is wrong here (possibly just our documentation), but I'm not familiar enough with the subject matter to figure out what...
</pre>
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