Given the size of the data set, strided memory accesses cannot be avoided.
We can still do better than the current code.
ps_hybrid_synthesis_deint_c: 12065.5
ps_hybrid_synthesis_deint_rvv_i32: 13650.2 (before)
ps_hybrid_synthesis_deint_rvv_i64: 8181.0 (after)
Segmented loads may be slower than not. So this advantageously uses a
unit-strided load and narrowing shifts instead.
Before:
ps_add_squares_c: 60757.7
ps_add_squares_rvv_f32: 22242.5
After:
ps_add_squares_c: 60516.0
ps_add_squares_rvv_i64: 17067.7
The code was blindly assuming that Zbb or V implied Zba. While the
earlier is practically always true, the later broke some QEMU setups,
as V was introduced earlier than Zba.
This starts with one-time initialisation of the 26 constant factors
like 08edacc248. That is done with
the scalar instruction set. While the formula can readily be vectored,
the gains would (probably) be more than lost in transfering the results
back to FP registers (or suitably reshuffling them into vector
registers).
Note that the main loop could likely be scheduled sligthly better by
expanding the filter macro and interleaving loads with arithmetic.
It is not clear yet if that would be relevant for vector processing (as
opposed to traditional SIMD).
We could also use fewer vectors, but there is not much point in sparing
them (they are *all* callee-clobbered).
