[FFmpeg-devel] [PATCH] avcodec/aac_tablegen: speed up table initialization

[wm4]

On Fri, 27 Nov 2015 13:51:57 +0100
Hendrik Leppkes <h.leppkes at gmail.com> wrote:

> On Fri, Nov 27, 2015 at 1:48 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> > On Fri, Nov 27, 2015 at 7:05 AM, wm4 <nfxjfg at googlemail.com> wrote:  
> >> On Fri, 27 Nov 2015 06:42:21 -0500
> >> Ganesh Ajjanagadde <gajjanagadde at gmail.com> wrote:
> >>  
> >>> On Fri, Nov 27, 2015 at 5:35 AM, Rostislav Pehlivanov
> >>> <atomnuker at gmail.com> wrote:  
> >>> > LGTM, but could you leave (just comment it out) the old code in there
> >>> > so it's a little easier to follow?  
> >>> >>         //ff_aac_pow2sf_tab[i] = pow(2, (i - POW_SF2_ZERO) / 4.0);
> >>> >>         //ff_aac_pow34sf_tab[i] = pow(ff_aac_pow2sf_tab[i], 3.0/4.0);  
> >>> >
> >>> > The accuracy increase is always nice.  
> >>>
> >>> Done and pushed. Thanks.
> >>> BTW, do you or others think that the new performance figures are
> >>> sufficient to justify getting rid of config_hardcoded_tables and
> >>> associated ifdefry and C file here?  
> >>
> >> I think this would be a very good goal. I never understood why there
> >> has to be both, and the complication caused by it is terrible.  
> >
> > I definitely agree that the complication is terrible, experiencing it
> > first hand with some patches currently under review. I was initially
> > skeptical of the need for both myself, but am now neutral about it,
> > see https://ffmpeg.org/pipermail/ffmpeg-devel/2015-November/183932.html
> > for some links about the rationale for this thing and my own thoughts
> > on it - I am sure a computer systems person can explain it far better.
> > However, for this to be valid, at least some basic tests need to be
> > done, e.g what is the library size before and after
> > --enable-hardcoded-tables?
> >
> > The trouble is that at the moment packagers and other clients (vlc,
> > mpv, chromium?) have generally followed the defaults, so if we move to
> > "hardcoded by default", clients can and perhaps should accuse us of
> > library bloat. I hence personally prefer doing it all at runtime, but
> > it will likely lead to bikeshedding. Maybe we can actually use our new
> > "developer committee" for a poll on this. A simple library size test
> > should be revealing in this respect.
> >  
> >>
> >> I'm not sure if dynamic generation was already the default, but in any
> >> case the init call should be protected with ff_thread_once.  
> >
> > Dynamic is in the sense that --enable-hardcoded-tables is not done by
> > default. I got the numbers by running ffplay on some test file.

Well, I meant switching each case that can be configured to either
hardcoded or dynamic generation only, depending on its effect on file
size.

> >
> > As for the ff_thread_once, did not know that. I am not comfortable
> > with threading, but if you send a patch for this, I can understand it
> > better for future.
> >  
> 
> aac init should already be thread safe, Derek sent patches to that effect.

aacenc.c calls it unconditionally.