While this function can easily be written with vectors, it just fails to
get any performance improvement.
For reference, this is a simpler loop-free implementation that does get
better performance than the current one depending on hardware, but still
more or less the same metrics as the C code:
func ff_sbr_neg_odd_64_rvv, zve64x
li a1, 32
addi a0, a0, 7
li t0, 8
vsetvli zero, a1, e8, m2, ta, ma
li t1, 0x80
vlse8.v v8, (a0), t0
vxor.vx v8, v8, t1
vsse8.v v8, (a0), t0
ret
endfunc
This reverts commit d06fd18f8f4c6a81ef94cbb600620d83ad51269d.
7591eb4055