[RISCV] Enable sub(max, min) lowering for ABDS and ABDU (#86592)

We have the ISD nodes for representing signed and unsigned absolute
difference. For RISCV, we have vector min/max in the base vector
extension, so we can expand to the sub(max,min) lowering.

We could almost use the default expansion, but since fixed length
min/max are custom (not legal), the default expansion doesn't cover the
fixed vector cases. The expansion here is just a copy of the generic
code specialized to allow the custom min/max nodes to be created so they
can in turn be legalized to the _vl variants.

Existing DAG combines handle the recognition of absolute difference
idioms and conversion into the respective ISD::ABDS and ISD::ABDU nodes.

This change does have the net effect of potentially pushing a free
floating zero/sign extend after the expansion, and we don't do a great
job of folding that into later expressions. However, since in general
narrowing can reduce required work (by reducing LMUL) this seems like
the right general tradeoff.

0 files changed, 0 insertions, 0 deletions