arm: vp9: Add NEON loop filters

Multimedia / Libav - Martin Storsjö [martin.st] - 11 November 2016 06:16 UTC

This work is sponsored by, and copyright, Google.

The implementation tries to have smart handling of cases where no pixels need the full filtering for the 8/16 width filters, skipping both calculation and writeback of the unmodified pixels in those cases. The actual effect of this is hard to test with checkasm though, since it tests the full filtering, and the benefit depends on how many filtered blocks use the shortcut.

Examples of relative speedup compared to the C version, from checkasm: Cortex A7 A8 A9 A53
vp9_loop_filter_h_4_8_neon: 2.72 2.68 1.78 3.15
vp9_loop_filter_h_8_8_neon: 2.36 2.38 1.70 2.91
vp9_loop_filter_h_16_8_neon: 1.80 1.89 1.45 2.01
vp9_loop_filter_h_16_16_neon: 2.81 2.78 2.18 3.16
vp9_loop_filter_mix2_h_44_16_neon: 2.65 2.67 1.93 3.05
vp9_loop_filter_mix2_h_48_16_neon: 2.46 2.38 1.81 2.85
vp9_loop_filter_mix2_h_84_16_neon: 2.50 2.41 1.73 2.85
vp9_loop_filter_mix2_h_88_16_neon: 2.77 2.66 1.96 3.23
vp9_loop_filter_mix2_v_44_16_neon: 4.28 4.46 3.22 5.70
vp9_loop_filter_mix2_v_48_16_neon: 3.92 4.00 3.03 5.19
vp9_loop_filter_mix2_v_84_16_neon: 3.97 4.31 2.98 5.33
vp9_loop_filter_mix2_v_88_16_neon: 3.91 4.19 3.06 5.18
vp9_loop_filter_v_4_8_neon: 4.53 4.47 3.31 6.05
vp9_loop_filter_v_8_8_neon: 3.58 3.99 2.92 5.17
vp9_loop_filter_v_16_8_neon: 3.40 3.50 2.81 4.68
vp9_loop_filter_v_16_16_neon: 4.66 4.41 3.74 6.02

The speedup vs C code is around 2-6x. The numbers are quite inconclusive though, since the checkasm test runs multiple filterings on top of each other, so later rounds might end up with different codepaths (different decisions on which filter to apply, based on input pixel differences). Disabling the early-exit in the asm doesn't give a fair comparison either though, since the C code only does the necessary calcuations for each row.

Based on START_TIMER/STOP_TIMER wrapping around a few individual functions, the speedup vs C code is around 4-9x.

This is pretty similar in runtime to the corresponding routines in libvpx. (This is comparing vpx_lpf_vertical_16_neon,
vpx_lpf_horizontal_edge_8_neon and vpx_lpf_horizontal_edge_16_neon to vp9_loop_filter_h_16_8_neon, vp9_loop_filter_v_16_8_neon and vp9_loop_filter_v_16_16_neon - note that the naming of horizonal and vertical is flipped between the libraries.)

In order to have stable, comparable numbers, the early exits in both asm versions were disabled, forcing the full filtering codepath.

Cortex A7 A8 A9 A53
vp9_loop_filter_h_16_8_neon: 597.2 472.0 482.4 415.0 libvpx vpx_lpf_vertical_16_neon: 626.0 464.5 470.7 445.0
vp9_loop_filter_v_16_8_neon: 500.2 422.5 429.7 295.0 libvpx vpx_lpf_horizontal_edge_8_neon: 586.5 414.5 415.6 383.2
vp9_loop_filter_v_16_16_neon: 905.0 784.7 791.5 546.0 libvpx vpx_lpf_horizontal_edge_16_neon: 1060.2 751.7 743.5 685.2

Our version is consistently faster on on A7 and A53, marginally slower on A8, and sometimes faster, sometimes slower on A9 (marginally slower in all three tests in this particular test run).

dd299a2 arm: vp9: Add NEON loop filters
libavcodec/arm/Makefile | 1 +
libavcodec/arm/vp9dsp_init_arm.c | 57 +++
libavcodec/arm/vp9lpf_neon.S | 770 ++++++++++++++++++++++++++++++++++++++
3 files changed, 828 insertions(+)

Upstream: git.libav.org

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