<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/120405>120405</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [RISCV] vp.merge of nxv16i1 tries to be unrolled
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:RISC-V
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          lukel97
      </td>
    </tr>
</table>

<pre>
    After #101641, this vp merge began to be emitted from the loop vectorizer with EVL tail folding from SPEC CPU 2017:

```llvm
define <vscale x 16 x i1> @f(<vscale x 16 x i1> %m, <vscale x 16 x i1> %x, <vscale x 16 x i1> %y, i32 %evl) {
  %1 = tail call <vscale x 16 x i1> @llvm.vp.merge.nxv16i1(<vscale x 16 x i1> %m, <vscale x 16 x i1> %x, <vscale x 16 x i1> %y, i32 %evl)
  ret <vscale x 16 x i1> %1
}
```

It crashes with `llc -mtriple=riscv64 -mattr=+v` with `LLVM ERROR: Invalid size request on a scalable vector.` because it tries to unroll it when legalizing vector ops.  The DAG after type legalization looks like:

```
Type-legalized selection DAG: %bb.0 'f:'
SelectionDAG has 15 nodes:
  t0: ch,glue = EntryToken
      t2: nxv16i1,ch = CopyFromReg t0, Register:nxv16i1 %0
      t4: nxv16i1,ch = CopyFromReg t0, Register:nxv16i1 %1
      t6: nxv16i1,ch = CopyFromReg t0, Register:nxv16i1 %2
        t8: i64,ch = CopyFromReg t0, Register:i64 %3
 t18: i64 = and t8, Constant:i64<4294967295>
    t12: nxv16i1 = vp_merge t2, t4, t6, t18
  t15: ch,glue = CopyToReg t0, Register:nxv16i1 $v0, t12
 t16: ch = RISCVISD::RET_GLUE t15, Register:nxv16i1 $v0, t15:1
```

</pre>
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