Re: [FFmpeg-devel] [PATCH v2 2/2] swscale/aarch64: add hscale specializations

["Martin Storsjö" <martin@martin.st>]

On Wed, 25 May 2022, Swinney, Jonathan wrote:

> This patch adds code to support specializations of the hscale function and adds
> a specialization for filterSize == 4.
>
> ff_hscale8to15_4_neon is a complete rewrite. Since the main bottleneck here is
> loading the data from src, this data is loaded a whole block ahead and stored
> back to the stack to be loaded again with ld4. This arranges the data for most
> efficient use of the vector instructions and removes the need for completion
> adds at the end. The number of iterations of the C per iteration of the assembly
> is increased from 4 to 8, but because of the prefetching, there must be a
> special section without prefetching when dstW < 16.
>
> This improves speed on Graviton 2 (Neoverse N1) dramatically in the case where
> previously fs=8 would have been required.
>
> before: hscale_8_to_15__fs_8_dstW_512_neon: 1962.8
> after : hscale_8_to_15__fs_4_dstW_512_neon: 1220.9
>
> Signed-off-by: Jonathan Swinney <jswinney@amazon.com>
> ---
> libswscale/aarch64/hscale.S  | 172 ++++++++++++++++++++++++++++++++++-
> libswscale/aarch64/swscale.c |  40 ++++++--
> libswscale/utils.c           |   2 +-
> 3 files changed, 203 insertions(+), 11 deletions(-)
>
> diff --git a/libswscale/aarch64/hscale.S b/libswscale/aarch64/hscale.S
> index da34f1cb8d..60bcd783e7 100644
> --- a/libswscale/aarch64/hscale.S
> +++ b/libswscale/aarch64/hscale.S
> @@ -1,5 +1,7 @@
> /*
>  * Copyright (c) 2016 Clément Bœsch <clement stupeflix.com>
> + * Copyright (c) 2019-2021 Sebastian Pop <spop@amazon.com>
> + * Copyright (c) 2022 Jonathan Swinney <jswinney@amazon.com>
>  *
>  * This file is part of FFmpeg.
>  *
> @@ -20,7 +22,25 @@
>
> #include "libavutil/aarch64/asm.S"
>
> -function ff_hscale_8_to_15_neon, export=1
> +/*
> +;-----------------------------------------------------------------------------
> +; horizontal line scaling
> +;
> +; void hscale<source_width>to<intermediate_nbits>_<filterSize>_<opt>
> +;                               (SwsContext *c, int{16,32}_t *dst,
> +;                                int dstW, const uint{8,16}_t *src,
> +;                                const int16_t *filter,
> +;                                const int32_t *filterPos, int filterSize);
> +;
> +; Scale one horizontal line. Input is either 8-bit width or 16-bit width
> +; ($source_width can be either 8, 9, 10 or 16, difference is whether we have to
> +; downscale before multiplying). Filter is 14 bits. Output is either 15 bits
> +; (in int16_t) or 19 bits (in int32_t), as given in $intermediate_nbits. Each
> +; output pixel is generated from $filterSize input pixels, the position of
> +; the first pixel is given in filterPos[nOutputPixel].
> +;----------------------------------------------------------------------------- */
> +
> +function ff_hscale8to15_X8_neon, export=1
>         sbfiz               x7, x6, #1, #32             // filterSize*2 (*2 because int16)
> 1:      ldr                 w8, [x5], #4                // filterPos[idx]
>         ldr                 w0, [x5], #4                // filterPos[idx + 1]
> @@ -70,3 +90,153 @@ function ff_hscale_8_to_15_neon, export=1
>         b.gt                1b                          // loop until end of line
>         ret
> endfunc
> +
> +function ff_hscale8to15_4_neon, export=1
> +// x0  SwsContext *c (not used)
> +// x1  int16_t *dst
> +// x2  int dstW
> +// x3  const uint8_t *src
> +// x4  const int16_t *filter
> +// x5  const int32_t *filterPos
> +// x6  int filterSize
> +// x8-x15 registers for gathering src data
> +
> +// v0      madd accumulator 4S
> +// v1-v4   filter values (16 bit) 8H
> +// v5      madd accumulator 4S
> +// v16-v19 src values (8 bit) 8B
> +
> +// This implementation has 4 sections:
> +//  1. Prefetch src data
> +//  2. Interleaved prefetching src data and madd
> +//  3. Complete madd
> +//  4. Complete remaining iterations when dstW % 8 != 0
> +
> +        add                 sp, sp, #-32                // allocate 32 bytes on the stack
> +        cmp                 w2, #16                     // if dstW <16, skip to the last block used for wrapping up
> +        b.lt                2f
> +
> +        // load 8 values from filterPos to be used as offsets into src
> +        ldp                 w8, w9,  [x5]               // filterPos[idx + 0], [idx + 1]
> +        ldp                 w10, w11, [x5, 8]           // filterPos[idx + 2], [idx + 3]
> +        ldp                 w12, w13, [x5, 16]          // filterPos[idx + 4], [idx + 5]
> +        ldp                 w14, w15, [x5, 24]          // filterPos[idx + 6], [idx + 7]

The imediate offset here (8/16/24) must be preceded by a '#', otherwise it 
breaks the build with MSVC (armasm64.exe).

// Martin
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