[FFmpeg-devel] [PATCH] adpcm: Reset the ssd back to zero more often

[Jason Garrett-Glaser]

On Sat, Nov 20, 2010 at 3:11 PM, Martin Storsj? <martin at martin.st> wrote:
> On Sat, 20 Nov 2010, Michael Niedermayer wrote:
>
>> On Sat, Nov 20, 2010 at 08:59:25AM +0200, Martin Storsj? wrote:
>> > On Sat, 20 Nov 2010, Michael Niedermayer wrote:
>> >
>> > > On Thu, Nov 18, 2010 at 04:01:31PM +0200, Martin Storsjo wrote:
>> > > > If using very large trellis sizes (e.g. -trellis 15), the frontier
>> > > > is so large that the difference between the best and the worst
>> > > > trellis node in the frontier is large enough to cause wraparound.
>> > > >
>> > >
>> > > > Resetting at (1<<20) is enough to avoid the issue at -trellis 16
>> > >
>> > > have you come to this conclusion by proof or by simply changing the
>> > > threshold and seeing no problems?
>> >
>> > By changing the threshold until there were no more problems with that
>> > particular sample - sorry, I should have mentioned that.
>> >
>> > I think it can't be proven how often it needs to be reset. In the worst
>> > pathological case, the best trellis node has ssd 0 while the worst one has
>> > a ssd of 65535*65535 added in each generation, overflowing even if we'd
>> > subtract the best node's ssd each round.
>> >
>> > The attached patch should avoid the issue properly regardless of how
>> > pathologically bad case it is, giving a small but tolerable slowdown. Do
>> > you think that is enough, or should that one be combined with this
>> > previous patch, resetting it to 0 more often (or perhaps even every round)
>> > to avoid this issue happening at all?
>>
>> have you tried to change ssd to 64bit ?
>> maybe its faster
>
> In 64 bit mode, changing ssd to 64 bit is a bit faster (current code 61
> sec, adding the wraparound check 64 sec, 64 bit ssd 62 sec), but in 32 bit
> mode, it's quite a bit slower (current code 69.6 sec, with wraparound
> check 69.8 sec, 64 bit ssd 79.6 sec), so I wouldn't suggest that solution.

Have you tried limiting the amount of 64-bit math that needs to be
done?  Usually with 64-bit counters in trellises, you can get away
with doing 64-bit in relatively few parts of the process.

x264's uses 64-bit, for reference.

Dark Shikari