[FFmpeg-devel] [PATCH 2/3] avcodec/aacsbr: Add comment about possibly optimization in sbr_dequant()

[Michael Niedermayer]

On Sat, Dec 12, 2015 at 01:08:01PM -0500, Ganesh Ajjanagadde wrote:
> On Sat, Dec 12, 2015 at 12:58 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Fri, Dec 11, 2015 at 12:09:57PM -0500, Ganesh Ajjanagadde wrote:
> >> On Fri, Dec 11, 2015 at 11:36 AM, Andreas Cadhalpun
> >> <andreas.cadhalpun at googlemail.com> wrote:
> >> > On 11.12.2015 17:21, Ganesh Ajjanagadde wrote:
> >> >> On Fri, Dec 11, 2015 at 11:16 AM, Andreas Cadhalpun
> >> >> <andreas.cadhalpun at googlemail.com> wrote:
> >> >>> On 19.11.2015 14:17, Michael Niedermayer wrote:
> >> >>>> From: Michael Niedermayer <michael at niedermayer.cc>
> >> >>>>
> >> >>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> >> >>>> ---
> >> >>>>  libavcodec/aacsbr.c |    1 +
> >> >>>>  1 file changed, 1 insertion(+)
> >> >>>>
> >> >>>> diff --git a/libavcodec/aacsbr.c b/libavcodec/aacsbr.c
> >> >>>> index d1e3a91..e014646 100644
> >> >>>> --- a/libavcodec/aacsbr.c
> >> >>>> +++ b/libavcodec/aacsbr.c
> >> >>>> @@ -73,6 +73,7 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
> >> >>>>  {
> >> >>>>      int k, e;
> >> >>>>      int ch;
> >> >>>> +    //TODO: Replace exp2f(0.5*x) by a LUT, the inputs are all integer and have a small range
> >> >>>>
> >> >>>>      if (id_aac == TYPE_CPE && sbr->bs_coupling) {
> >> >>>>          float alpha      = sbr->data[0].bs_amp_res ?  1.0f :  0.5f;
> >> >>>>
> >> >>>
> >> >>> This shouldn't hurt, with or without the clarification requested by Ganesh.
> >> >>
> >> >> I am doing related work cleaning up and optimizing usages of slow libm
> >> >> functions such as pow and exp2. Do you know the exact possible range
> >> >> of the inputs x, and if so, can it be added to the comment? That will
> >> >> be very helpful for me to come up with a patch. Thanks.
> >> >
> >> > The exp2f expressions are:
> >> > exp2f(sbr->data[0].env_facs_q[e][k] * alpha + 7.0f);
> >> > exp2f((pan_offset - sbr->data[1].env_facs_q[e][k]) * alpha);
> >> > exp2f(NOISE_FLOOR_OFFSET - sbr->data[0].noise_facs_q[e][k] + 1);
> >> > exp2f(12 - sbr->data[1].noise_facs_q[e][k]);
> >> > exp2f(alpha * sbr->data[ch].env_facs_q[e][k] + 6.0f);
> >> > exp2f(NOISE_FLOOR_OFFSET - sbr->data[ch].noise_facs_q[e][k]);
> >> >
> >> > Here alpha is 1 or 0.5, pan_offset 12 or 24 and NOISE_FLOOR_OFFSET is 6.
> >> > After patch 3 of this series, env_facs_q is in the range from 0 to 127 and
> >> > noise_facs_q is already limited to the range from 0 to 30.
> >> >
> >> > So x should always be in the range -300..300, or so.
> >>
> >> Very good, thanks a lot.
> >>
> >> Based on the above range, my idea is to not even use a LUT, but use
> >> something like exp2fi followed by multiplication by M_SQRT2 depending
> >> on even or odd.
> >
> > conditional operations can due to branch misprediction be potentially
> > rather slow
> 
> I think it will still be far faster than exp2f, and in the absence of
> hard numbers, I view this a far better approach than a large (~300
> element) lut. Of course, the proof and extent of this will need to
> wait for actual benches.

alternatively one could do a
if (x+A < (unsigned)B)
    LUT[x+A]
else
    exp2whatever(x)

the range in practice should be much smaller than +-300

also the LUT can possibly be shared between codecs

or that code could be in a exp_sqrt2i() or something

just some random ideas...

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Complexity theory is the science of finding the exact solution to an
approximation. Benchmarking OTOH is finding an approximation of the exact
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