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

["Martin Storsjö" <martin@martin.st>]

On Wed, 25 May 2022, Martin Storsjö wrote:

> 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).

Sorry - just for clarity, this of course holds for both these ldp here, 
but also all other ldp/stp cases in the function.

// Martin
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