Re: [FFmpeg-devel] [PATCH v2 2/2] swscale/output: Don't call av_pix_fmt_desc_get() in a loop

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v2 2/2] swscale/output: Don't call av_pix_fmt_desc_get() in a loop
Date: Mon, 19 Sep 2022 16:34:35 +0200
Message-ID: <20220919143435.GI2088045@pb2> (raw)
In-Reply-To: <AS8P250MB0744F1C0E18E9DA96E8354148F489@AS8P250MB0744.EURP250.PROD.OUTLOOK.COM>

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On Fri, Sep 16, 2022 at 04:55:39PM +0200, Andreas Rheinhardt wrote:
> Up until now, libswscale/output.c used a macro to write
> an output pixel which involved a call to av_pix_fmt_desc_get()
> to find out whether the input pixel format is BE or LE
> despite this being known at compile-time (there are templates
> per pixfmt). Even worse, these calls are made in a loop,
> so that e.g. there are eight calls to av_pix_fmt_desc_get()
> for every pixel processed in yuv2rgba64_X_c_template()
> for 64bit RGB formats.
> 
> This commit modifies these macros to ensure that isBE()
> is evaluated at compile-time. This saved 41184B of .text
> for me (GCC 11.2, -O3). Of course, it also improved performance.
> E.g. ffmpeg_g -f lavfi -i testsrc2,format=yuva420p -pix_fmt rgba64le \
> -threads 1  -t 1:00  -f null - (which uses yuv2rgba64le_X_c,
> which is an invocation of yuv2rgba64_X_c_template() mentioned above),
> performance improved from 95589 to 41387 decicycles for one call
> to yuv2packedX; for the be variant the numbers went down from
> 76087 to 43024 decicycles.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libswscale/output.c | 100 +++++++++++++++++++++++++-------------------
>  1 file changed, 58 insertions(+), 42 deletions(-)

This looks alot better than before

thx

PS: i still think that broader support for compile time evaluation of 
"pure" functions would be usefull. Ideally with minimal mess on the source
side, more on the build tool side

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When you are offended at any man's fault, turn to yourself and study your
own failings. Then you will forget your anger. -- Epictetus

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