In this case, the inner loop computing the scalar product can be reduced
to just one multiplication and one sum even with 128-bit vectors. The
result is a lot simpler, but also brings more modest performance gains:
flac_lpc_16_13_c: 15241.0
flac_lpc_16_13_rvv_i32: 11230.0
flac_lpc_16_16_c: 17884.0
flac_lpc_16_16_rvv_i32: 12125.7
flac_lpc_16_29_c: 27847.7
flac_lpc_16_29_rvv_i32: 10494.0
flac_lpc_16_32_c: 30051.5
flac_lpc_16_32_rvv_i32: 10355.0
The entire set of 32 coefficients and corresponding past 32 samples can
fit in a single vector (with LMUL=8) exactly, but... since widening
double the needed vector sizes, we still end up too short with 128-bit
vectors. This adds a very simple version for future 256+-bit hardware,
and for pred_orders values up to 16, and a bit more involved loop for
for 128-bit hardware with pred_orders between 17 and 32.
With 128-bit hardware, the benchmarks look like this:
flac_lpc_32_13_c: 30152.0
flac_lpc_32_13_rvv_i32: 10244.7
flac_lpc_32_16_c: 37314.2
flac_lpc_32_16_rvv_i32: 10126.2
flac_lpc_32_29_c: 61910.0
flac_lpc_32_29_rvv_i32: 14495.2
flac_lpc_32_32_c: 68204.0
flac_lpc_32_32_rvv_i32: 13273.7
