<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/127486>127486</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            GlobalISel should have G_POISON
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            good first issue,
            llvm:globalisel
      </td>
    </tr>

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

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

<pre>
    Currently GlobalISel has G_IMPLICIT_DEF, which acts like `undef`. This is used to lower both `poison` and `undef` from the IR. To mirror the semantics of the IR, we need a separate representation of poison. We should introduce G_POISON to enable better optimizations, particularly when legalization introduces padding vector elements.

This change would mostly be finding all the places G_IMPLICIT_DEF is handled, and equivalently handling G_POISON. We should then migrate vector widening and other transforms to introduce poison instead of undef. 
</pre>
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