[PATCH] D59096: [SelectionDAG][RISCV] Add a SELECT_PARTS SelectionDAG node

[Alex Bradbury via Phabricator via llvm-commits]

asb added a comment.

In D59096#1421940 <https://reviews.llvm.org/D59096#1421940>, @efriedma wrote:

> I'm not sure generating SELECT_PARTS in type legalization is the best approach.  Instead of specifically considering the case of a SELECT whose result is split by legalization exactly once, you could more generally consider DAGs which, after type legalization, contain multiple SELECTs with the same condition.
>
> The x86 backend actually has some code to try improve the lowering for this sort of construct after isel, in cases where there is no native cmov instruction; see X86TargetLowering::EmitLoweredSelect.  As written, it's not completely reliable; it depends on the DAG scheduler to schedule the relevant nodes next to each other. Not suggesting that's the best approach, but maybe a useful reference point.


Thanks Eli. One reason we were drawn to this approach is it seemed a reasonable analogue of the situation with SHL_PARTS. That code has of course been in LLVM for a very very long time, and perhaps would be done another way now, and of course there are other differences.

I hadn't realised X86 made a similar optimisation. Handling it in EmitInstrWithCustomInserter seems a promising approach (or as you suggest, at least something along those lines).


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