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

[Michael Niedermayer <michael@niedermayer.cc>]

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On Thu, Sep 08, 2022 at 11:44:51PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > On Thu, Sep 08, 2022 at 09:38:51PM +0200, Andreas Rheinhardt wrote:
> >> 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?
> > 
> > Going back into the past, there was
> > 6b0768e2021b90215a2ab55ed427bce91d148148
> > 
> > before this the code certainly did get optimized out, it was just
> > #define isBE(x) ((x)&1)
> > 
> > thats simple and clean code btw
> 
> I don't really consider such magic numbers to be clean.

no, but that wasnt what i meant
what i meant was more that if you can structure an identifer so that it
itself can provide alot of the used information from within its structure, 
that has advanatges over requiring a descriptor table lookup.
I didnt mean to suggest that this should use hardcoded litteral numbers
and be undocumented and untested.

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No human being will ever know the Truth, for even if they happen to say it
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