Re: [FFmpeg-devel] libswscale.c : ff_xyz12Torgb48 expensive unaligned 16 byte accesses

From: James Almer <jamrial@gmail.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] libswscale.c : ff_xyz12Torgb48 expensive unaligned 16 byte accesses
Date: Sun, 25 May 2025 21:58:45 -0300
Message-ID: <ec30364b-2189-42b7-8048-7b6a343f775a@gmail.com> (raw)
In-Reply-To: <DM4PR21MB461911571585877FFEEE236E9265A@DM4PR21MB4619.namprd21.prod.outlook.com>

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On 5/25/2025 9:44 PM, Chitra Dey Sarkar via ffmpeg-devel wrote:
> H
> We have been profiling FFmpeg at Microsoft and have identified that ff_xyz12ToRgb48 has a high sample count ( profiled every 1ms )
> 
> It seems like ff_xyz12ToRgb48 has performance penalty for
> 
>    1.  Unaligned read and write access
>    2.  Access to xyz2rgb_matrix
>    3.  Multiplication
> 
> I would be interested in optimizing this code , wanted to check if there is an existing optimized version of this function, or any recommended approach to improve it(?
> I can move the repeated access to xyz2rgb_matrix outside the inner loop and load a full cache line at once to extract the X, Y, and Z values more efficiently-but I wanted to start by getting some initial feedback or thoughts before proceeding further

Niklas is currently working in a library rewrite, which may affect xyz 
scaling. But in any case, the obvious approach is a SIMD optimized 
implementation (SSE2, AVX2, etc) of this function.

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