[RISCV] Vector sub (zext, zext) -> sext (sub (zext, zext)) (#82455)

This is legal as long as the inner zext retains at least one bit of
increase so that the sub overflow case (0 - UINT_MAX) can be
represented. Alive2 proof: https://alive2.llvm.org/ce/z/BKeV3W

For RVV, restrict this to power of two sizes with the operation type
being at least e8 to stick to legal extends. We could arguably handle i1
source types with some care if we wanted to.

This is likely profitable because it may allow us to perform the sub
instruction in a narrow LMUL (equivalently, in fewer DLEN-sized pieces)
before widening for the user. We could arguably avoid narrowing below
DLEN, but the transform should at worst introduce one extra extend and
one extra vsetvli toggle if the source could previously be handled via
loads explicit w/EEW.

0 files changed, 0 insertions, 0 deletions