[PATCH] D58419: [GISel]: Allow G_EXTRACT_VEC_ELT's result to be larger the source element type

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added a comment.

In D58419#1404416 <https://reviews.llvm.org/D58419#1404416>, @aditya_nandakumar wrote:

> In D58419#1404103 <https://reviews.llvm.org/D58419#1404103>, @arsenm wrote:
>
> > I always thought this behavior in SelectionDAG was error prone.
>
>
> Personally in an out of tree backend, I find BUILD_VEC_TRUNC to be very elegant when trying to legalize/optimize vector types whose scalar types are illegal in the architecture otherwise needing several unmerges + shifts + ands.
>  I'm fine with adding a separate opcode (say G_EXTRACT_VECTOR_ELT_ANYEXT - suggestions accepted).
>  We need to do something similar for INSERT_VEC_ELT which can implicitly truncate.


One question I've been thinking about is whether implicit extension is needed for G_INSERT/G_EXTRACT. I figured extract_vector_elt/insert_vector_elt with a constant could always be implemented as a G_EXTRACT/G_INSERT.


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