<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64177>64177</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Instcombine] Undef shuffle may lead to unsound transform
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            miscompilation,
            llvm:instcombine
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          alexey-bataev
      </td>
    </tr>
</table>

<pre>
    Shuffle with undef vector (not poison) and non-poison index leads to unsound transformation

define <3 x i32> @select_and_insert_values(<3 x i32> %a, <3 x i32> %b, <3 x i32> %c) {
  %c0 = extractelement <3 x i32> %c, i32 0
  %cmp0 = icmp ne i32 %c0, 0
  %1 = insertelement <2 x i1> poison, i1 %cmp0, i32 0
  %2 = shufflevector <2 x i1> %1, <2 x i1> poison, <2 x i32> zeroinitializer
  %3 = shufflevector <3 x i32> %a, <3 x i32> poison, <2 x i32> zeroinitializer
  %4 = shufflevector <3 x i32> %b, <3 x i32> poison, <2 x i32> <i32 0, i32 1>
  %5 = select <2 x i1> %2, <2 x i32> %3, <2 x i32> %4
  %6 = shufflevector <2 x i32> %5, <2 x i32> undef, <3 x i32> <i32 0, i32 1, i32 2>
  ret <3 x i32> %6
}

https://alive2.llvm.org/ce/z/jUBXvb
</pre>
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