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Date: Fri, 30 May 2025 10:08:18 +0300 (EEST)
From: =?ISO-8859-15?Q?Martin_Storsj=F6?= <martin@martin.st>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
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Subject: Re: [FFmpeg-devel] [PATCH 2/2] swscale: Neon rgb_to_yuv_half
process 16 pixels at a time
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On Thu, 29 May 2025, Martin Storsj=F6 wrote:
> On Tue, 27 May 2025, Dmitriy Kovalenko wrote:
>
>> This patches integrates so called double bufferring when we are loading
>> 2 batch elements at a time and then processing them in parallel. On the
>> moden arm processors especially Apple Silicon it gives a visible
>> benefit, for subsampled pixel processing it is especially nice because
>> it allows to read elements w/ 2 instructions and write with a single one
>> (which is usually the slowest part).
>> =
>> Including the previous patch in a stack on macbook pro m4 max =
>> rgb_to_yuv_half
>> in checkasm goes up 2x of the c version
>> ---
>> libswscale/aarch64/input.S | 332 ++++++++++++++++++++++++++++++++++---
>> 1 file changed, 309 insertions(+), 23 deletions(-)
>> =
>> diff --git a/libswscale/aarch64/input.S b/libswscale/aarch64/input.S
>> index ee8eb24c14..59d66d0022 100644
>> --- a/libswscale/aarch64/input.S
>> +++ b/libswscale/aarch64/input.S
>> @@ -194,40 +194,94 @@ function ff_\fmt_rgb\()ToUV_half_neon, export=3D1
>> ldp w12, w13, [x6, #20] // w12: bu, w13: rv
>> ldp w14, w15, [x6, #28] // w14: gv, w15: bv
>> 4:
>> - cmp w5, #8
>> rgb_set_uv_coeff half=3D1
>> - b.lt 2f
>> -1: // load 16 pixels and prefetch memory for the next block
>> +
>> + cmp w5, #16
>> + b.lt 2f // Go directly to scala=
r =
>> if < 16
>> +
>> +1:
>> .if \element =3D=3D 3
>> - ld3 { v16.16b, v17.16b, v18.16b }, [x3], #48
>> - prfm pldl1strm, [x3, #48]
>> + ld3 { v16.16b, v17.16b, v18.16b }, [x3], #48 // Fi=
rst =
>> 16 pixels
>> + ld3 { v26.16b, v27.16b, v28.16b }, [x3], #48 // =
>> Second 16 pixels
>> + prfm pldl1keep, [x3, #96]
>> .else
>> - ld4 { v16.16b, v17.16b, v18.16b, v19.16b }, [x3], #=
64
>> - prfm pldl1strm, [x3, #64]
>> + ld4 { v16.16b, v17.16b, v18.16b, v19.16b }, [x3], #=
64 =
>> // First 16 pixels
>> + ld4 { v26.16b, v27.16b, v28.16b, v29.16b }, [x3], #=
64 =
>> // Second 16 pixels
>> + prfm pldl1keep, [x3, #128]
>> .endif
>> =
>> + // **Sum adjacent pixel pairs**
>> .if \alpha_first
>> - uaddlp v21.8h, v19.16b // v21: summed b pairs
>> - uaddlp v20.8h, v18.16b // v20: summed g pairs
>> - uaddlp v19.8h, v17.16b // v19: summed r pairs
>> + uaddlp v21.8h, v19.16b // Block 1: B sums
>> + uaddlp v20.8h, v18.16b // Block 1: G sums
>> + uaddlp v19.8h, v17.16b // Block 1: R sums
>> + uaddlp v31.8h, v29.16b // Block 2: B sums
>> + uaddlp v30.8h, v28.16b // Block 2: G sums
>> + uaddlp v29.8h, v27.16b // Block 2: R sums
>> .else
>> - uaddlp v19.8h, v16.16b // v19: summed r pairs
>> - uaddlp v20.8h, v17.16b // v20: summed g pairs
>> - uaddlp v21.8h, v18.16b // v21: summed b pairs
>> + uaddlp v19.8h, v16.16b // Block 1: R sums
>> + uaddlp v20.8h, v17.16b // Block 1: G sums
>> + uaddlp v21.8h, v18.16b // Block 1: B sums
>> + uaddlp v29.8h, v26.16b // Block 2: R sums
>> + uaddlp v30.8h, v27.16b // Block 2: G sums
>> + uaddlp v31.8h, v28.16b // Block 2: B sums
>> .endif
>> =
>> - mov v22.16b, v6.16b // U first half
>> - mov v23.16b, v6.16b // U second half
>> - mov v24.16b, v6.16b // V first half
>> - mov v25.16b, v6.16b // V second half
>> -
>> - rgb_to_uv_interleaved_product v19, v20, v21, v0, v1, v2, v3, v4=
, =
>> v5, v22, v23, v24, v25, v16, v17, #10
>> + // init accumulatos for both blocks
>> + mov v7.16b, v6.16b // U_low
>> + mov v8.16b, v6.16b // U_high
>> + mov v9.16b, v6.16b // V_low
>> + mov v10.16b, v6.16b // V_high
>> + mov v11.16b, v6.16b // U_low
>> + mov v12.16b, v6.16b // U_high
>> + mov v13.16b, v6.16b // V_low
>> + mov v14.16b, v6.16b // V_high
>> +
>> + smlal v7.4s, v0.4h, v19.4h // U +=3D ru * r (0-3)
>> + smlal v9.4s, v3.4h, v19.4h // V +=3D rv * r (0-3)
>> + smlal v11.4s, v0.4h, v29.4h // U +=3D ru * r (0-3)
>> + smlal v13.4s, v3.4h, v29.4h // V +=3D rv * r (0-3)
>> +
>> + smlal2 v8.4s, v0.8h, v19.8h // U +=3D ru * r (4-7)
>> + smlal2 v10.4s, v3.8h, v19.8h // V +=3D rv * r (4-7)
>> + smlal2 v12.4s, v0.8h, v29.8h // U +=3D ru * r (4-7)
>> + smlal2 v14.4s, v3.8h, v29.8h // V +=3D rv * r (4-7)
>> +
>> + smlal v7.4s, v1.4h, v20.4h // U +=3D gu * g (0-3)
>> + smlal v9.4s, v4.4h, v20.4h // V +=3D gv * g (0-3)
>> + smlal v11.4s, v1.4h, v30.4h // U +=3D gu * g (0-3)
>> + smlal v13.4s, v4.4h, v30.4h // V +=3D gv * g (0-3)
>> +
>> + smlal2 v8.4s, v1.8h, v20.8h // U +=3D gu * g (4-7)
>> + smlal2 v10.4s, v4.8h, v20.8h // V +=3D gv * g (4-7)
>> + smlal2 v12.4s, v1.8h, v30.8h // U +=3D gu * g (4-7)
>> + smlal2 v14.4s, v4.8h, v30.8h // V +=3D gv * g (4-7)
>> +
>> + smlal v7.4s, v2.4h, v21.4h // U +=3D bu * b (0-3)
>> + smlal v9.4s, v5.4h, v21.4h // V +=3D bv * b (0-3)
>> + smlal v11.4s, v2.4h, v31.4h // U +=3D bu * b (0-3)
>> + smlal v13.4s, v5.4h, v31.4h // V +=3D bv * b (0-3)
>> +
>> + smlal2 v8.4s, v2.8h, v21.8h // U +=3D bu * b (4-7)
>> + smlal2 v10.4s, v5.8h, v21.8h // V +=3D bv * b (4-7)
>> + smlal2 v12.4s, v2.8h, v31.8h // U +=3D bu * b (4-7)
>> + smlal2 v14.4s, v5.8h, v31.8h // V +=3D bv * b (4-7)
>> +
>> + sqshrn v16.4h, v7.4s, #10 // U (0-3)
>> + sqshrn v17.4h, v9.4s, #10 // V (0-3)
>> + sqshrn v22.4h, v11.4s, #10 // U (0-3)
>> + sqshrn v23.4h, v13.4s, #10 // V (0-3)
>> +
>> + sqshrn2 v16.8h, v8.4s, #10 // U (0-7)
>> + sqshrn2 v17.8h, v10.4s, #10 // V (0-7)
>> + sqshrn2 v22.8h, v12.4s, #10 // U (0-7)
>> + sqshrn2 v23.8h, v14.4s, #10 // V (0-7)
>> =
>> - str q16, [x0], #16 // store dst_u
>> - str q17, [x1], #16 // store dst_v
>> + stp q16, q22, [x0], #32 // Store all 16 U values
>> + stp q17, q23, [x1], #32 // Store all 16 V values
>> =
>> - sub w5, w5, #8 // width -=3D 8
>> - cmp w5, #8 // width >=3D 8 ?
>> + sub w5, w5, #16 // width -=3D 16
>> + cmp w5, #16 // width >=3D 16 ?
>> b.ge 1b
>> cbz w5, 3f // No pixels left? Exit
>> =
>> @@ -459,3 +513,235 @@ endfunc
>> =
>> DISABLE_DOTPROD
>> #endif
>> +
>> +.macro rgbToUV_half_neon_double fmt_bgr, fmt_rgb, element, alpha_first=
=3D0
>> +function ff_\fmt_bgr\()ToUV_half_neon_double, export=3D1
>> + cbz w5, 9f // exit immediately if =
>> width is 0
>> + cmp w5, #16 // check if we have at =
>> least 16 pixels
>> + b.lt _ff_\fmt_bgr\()ToUV_half_neon
>
> Also, with that fixed, this fails to properly back up and restore registe=
rs =
> v8-v15; checkasm doesn't notice this on macOS, but on Linux and windows, =
> checkasm has a call wrapper which does detect such issues.
This comment is still unaddressed, checkasm still fails on Linux and =
Windows.
// Martin
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