Using absolute-difference-accumulate does use twice the amount of
absolute-difference instructions, but avoids the need for the
uaddl and add instructions, reducing the total number of instructions
by 3.
These can be interleaved in the rest of the calculation, to avoid
tight dependencies at the end. Unfortunately, this is marginally
slower on Cortex A53, but faster on A72 and A73.
Before: Cortex A53 A72 A73 Graviton 3
pix_abs_0_3_neon: 175.7 109.2 92.0 41.2
After:
pix_abs_0_3_neon: 179.7 96.7 87.5 41.2
---
libavcodec/aarch64/me_cmp_neon.S | 32 +++++++++++---------------------
1 file changed, 11 insertions(+), 21 deletions(-)
diff --git a/libavcodec/aarch64/me_cmp_neon.S b/libavcodec/aarch64/me_cmp_neon.S
index 0ae23d8922..89546869fb 100644
--- a/libavcodec/aarch64/me_cmp_neon.S
+++ b/libavcodec/aarch64/me_cmp_neon.S
@@ -124,6 +124,9 @@ function ff_pix_abs16_xy2_neon, export=1
add v26.8h, v30.8h, v2.8h // add up 0..7, using pix2
+ pix2+1 values from pix3 above
add v27.8h, v31.8h, v3.8h // add up 8..15, using
pix2 + pix2+1 values from pix3 above
+ uabdl v24.8h, v1.8b, v23.8b // absolute difference
0..7, i=0
+ uabdl2 v23.8h, v1.16b, v23.16b // absolute difference
8..15, i=0
+
ld1 {v21.16b}, [x5], x3 // load pix3
ld1 {v20.16b}, [x1], x3 // load pix1
@@ -137,6 +140,9 @@ function ff_pix_abs16_xy2_neon, export=1
rshrn v28.8b, v28.8h, #2 // shift right 2 0..7
(rounding shift right)
rshrn2 v28.16b, v29.8h, #2 // shift right 2 8..15
+ uabal v24.8h, v16.8b, v26.8b // absolute difference
0..7, i=1
+ uabal2 v23.8h, v16.16b, v26.16b // absolute difference
8..15, i=1
+
uaddl v2.8h, v21.8b, v22.8b // pix3 + pix3+1 0..7
uaddl2 v3.8h, v21.16b, v22.16b // pix3 + pix3+1 8..15
add v30.8h, v4.8h, v2.8h // add up 0..7, using pix2
+ pix2+1 values from pix3 above
@@ -144,33 +150,17 @@ function ff_pix_abs16_xy2_neon, export=1
rshrn v30.8b, v30.8h, #2 // shift right 2 0..7
(rounding shift right)
rshrn2 v30.16b, v31.8h, #2 // shift right 2 8..15
- // Averages are now stored in these registers:
- // v23, v16, v28, v30
- // pix1 values in these registers:
- // v1, v16, v17, v20
- // available:
- // v4, v5, v7, v18, v19, v24, v25, v27, v29, v31
+ uabal v24.8h, v17.8b, v28.8b // absolute difference
0..7, i=2
+ uabal2 v23.8h, v17.16b, v28.16b // absolute difference
8..15, i=2
sub w4, w4, #4 // h -= 4
- // Using absolute-difference instructions instead of
absolute-difference-accumulate allows
- // us to keep the results in 16b vectors instead of widening values
with twice the instructions.
- // This approach also has fewer data dependencies, allowing better
instruction level parallelism.
- uabd v4.16b, v1.16b, v23.16b // absolute difference
0..15, i=0
- uabd v5.16b, v16.16b, v26.16b // absolute difference
0..15, i=1
- uabd v6.16b, v17.16b, v28.16b // absolute difference
0..15, i=2
- uabd v7.16b, v20.16b, v30.16b // absolute difference
0..15, i=3
+ uabal v24.8h, v20.8b, v30.8b // absolute difference
0..7, i=3
+ uabal2 v23.8h, v20.16b, v30.16b // absolute difference
8..15, i=3
cmp w4, #4 // loop if h >= 4
- // Now add up all the values in each vector, v4-v7 with widening adds
- uaddl v19.8h, v4.8b, v5.8b
- uaddl2 v18.8h, v4.16b, v5.16b
- uaddl v4.8h, v6.8b, v7.8b
- uaddl2 v5.8h, v6.16b, v7.16b
- add v4.8h, v4.8h, v5.8h
- add v4.8h, v4.8h, v18.8h
- add v4.8h, v4.8h, v19.8h
+ add v4.8h, v23.8h, v24.8h
uaddlv s4, v4.8h // finish adding up
accumulated values
add d0, d0, d4 // add the value to the
top level accumulator
--
2.25.1
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