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

[Hendrik Leppkes]

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.
>
> 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.

- Hendrik