[FFmpeg-devel] [PATCH 2/2] swscale/input: Avoid calls to av_pix_fmt_desc_get()

[Andreas Rheinhardt]

Michael Niedermayer:
> Hi
> 
> On Thu, Sep 08, 2022 at 04:38:11AM +0200, Andreas Rheinhardt wrote:
>> Up until now, libswscale/input.c used a macro to read
>> an input 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 six calls to av_pix_fmt_desc_get()
>> for every pair of UV pixel processed in
>> rgb64ToUV_half_c_template().
>>
>> This commit modifies these macros to ensure that isBE()
>> is evaluated at compile-time. This saved 9743B of .text
>> for me (GCC 11.2, -O3).
> 
> hmm, all these functions where supposed to be optimized out
> why where they not ?
> 
> iam asking as the code is simpler before your patch if that
> "optimization out" thing would work
> 

Why should these functions be optimized out? What would enable the
compiler to optimize them out?
(And I really don't see why this patch would make the code more
complicated.)

- Andreas