[FFmpeg-devel] [PATCH] libavfilter: constify filter list

[wm4]

On Tue, 30 Jan 2018 14:09:43 +0000
Mark Thompson <sw at jkqxz.net> wrote:

> On 30/01/18 07:24, Muhammad Faiz wrote:
> > Move REGISTER_FILTER to FILTER_TABLE in configure.
> > Auto generate filter extern and filter table.
> > Sort filter table, use bsearch on avfilter_get_by_name.
> > Define next pointer at filter extern, no need to initialize
> > next pointer at run time, so AVFilter can be set to const.
> > Make avfilter_register always return error.
> > Target checkasm now depends on EXTRALIBS-avformat.
> > 
> > Signed-off-by: Muhammad Faiz <mfcc64 at gmail.com>
> > ---  
> 
> I like the idea of this, but I'm not sure about some of the implementation details.
> 
> Have you considered dropping the "next" links entirely and having just the array of pointers instead?  I feel like they don't really add anything useful once we are in this form, and result in more boilerplate on every filter and some tricky generation code.
> 
> avfilter_next() would be a bit slower (since it would have to search the array, and therefore have runtime linear in the number of filters), but avfilter_get_by_name() is a lot faster because of the binary search (and is the only common use of it) so I don't think that complaint would be a strong one.

I think we considered that for libavcodec, but discarded the idea
because ffmpeg.c's use was causing noticable performance problems? (I
don't remember the details.) Also wouldn't the runtime be quadratic
rather than linear when iterating filters?

> > +const AVFilter *avfilter_next(const AVFilter *prev)
> > +{
> > +    CHECK_VALIDITY();  
> 
> Calling avfilter_next() without having called avfilter_register_all() violates the API, though?

Before this change it just returned NULL, now it returns the full list.
I think that was the intent and I don't think it's a problem, besides
keeping the old behavior would require mutable state.


> (Or is there an intent to deprecate avfilter_register_all() immediately after this?)

Hopefully.