T-Head C908 (cycles):
vc1dsp.vc1_inv_trans_4x4_c: 310.7
vc1dsp.vc1_inv_trans_4x4_rvv_i32: 120.0
We could use 1 `vlseg4e64.v` instead of 4 `vle16.v`, but that seems to
be about 7% slower.
The 8x8 pixel arrays are not necessarily aligned to 64 bits, so the
current code leads to Bus error on real hardware. This reproducible
with FATE's vc1_ilaced_twomv test case.
The new "pessimist" code can trivially be shared for 16x16 pixel
arrays so we also do that. FWIW, this also nominally reduces the
hardware requirement from Zve64x to Zve32x.
T-Head C908:
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_c: 14.7
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_rvv_i32: 3.5
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_c: 3.7
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_rvv_i32: 1.5
SpacemiT X60:
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_c: 13.0
vc1dsp.avg_vc1_mspel_pixels_tab[0][0]_rvv_i32: 3.0
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_c: 3.2
vc1dsp.avg_vc1_mspel_pixels_tab[1][0]_rvv_i32: 1.2
Notes:
- The loop is biased toward no unescaped bytes as that should be most common.
- The input byte array is slid rather than the (8 times smaller) bit-mask,
as RISC-V V does not provide a bit-mask (or bit-wise) slide instruction.
- There are two comparisons with 0 per iteration, for the same reason.
- In case of match, bytes are copied until the first match, and the loop is
restarted after the escape byte. Vector compression (vcompress.vm) could
discard all escape bytes but that is slower if escape bytes are rare.
Further optimisations should be possible, e.g.:
- processing 2 bytes fewer per iteration to get rid of a 2 slides,
- taking a short cut if the input vector contains less than 2 zeroes.
But this is a good starting point:
T-Head C908:
vc1dsp.vc1_unescape_buffer_c: 12749.5
vc1dsp.vc1_unescape_buffer_rvv_i32: 6009.0
SpacemiT X60:
vc1dsp.vc1_unescape_buffer_c: 11038.0
vc1dsp.vc1_unescape_buffer_rvv_i32: 2061.0
The main loop processes 8 bytes in 5 instructions.
For comparison, the optimal plain strnlen() requires 4 instructions per
byte (6.4x worse): LBU; ADDI; BEQZ; BNE. The current libavcodec C code
involves 5 instructions per byte (8x worse). Actual benchmarks may be
slightly less favourable due to latency from ORC.B to BNE.
We can't call ff_get_rv_vlenb() if we don't have RVV available
at all.
Acked-by: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: Martin Storsjö <martin@martin.st>
