[FFmpeg-devel] [PATCH] lavc/aacenc_utils: replace powf(x, y) by expf(logf(x), y)

[Ganesh Ajjanagadde]

On Fri, Mar 11, 2016 at 9:30 AM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> Hi,
>
> On Thu, Mar 10, 2016 at 9:23 PM, Ganesh Ajjanagadde <gajjanag at gmail.com>
> wrote:
>
>> On Thu, Mar 10, 2016 at 8:56 AM, Ronald S. Bultje <rsbultje at gmail.com>
>> wrote:
>> > Hi,
>> >
>> > On Thu, Mar 10, 2016 at 2:37 AM, Reimar Döffinger <
>> Reimar.Doeffinger at gmx.de>
>> > wrote:
>> >
>> >> On 10.03.2016, at 03:06, Ganesh Ajjanagadde <gajjanag at gmail.com> wrote:
>> >>
>> >> > On Wed, Mar 9, 2016 at 2:16 AM, Reimar Döffinger
>> >> > <Reimar.Doeffinger at gmx.de> wrote:
>> >> >> On 08.03.2016, at 04:48, Ganesh Ajjanagadde <gajjanag at gmail.com>
>> wrote:
>> >> >>
>> >> >>> +                    nzl += expf(logf(s / ethresh) * nzslope);
>> >> >>
>> >> >> Shouldn't log2f/exp2f be faster?
>> >> >> log2f at least has CPU support on x86 AFAICT.
>> >> >
>> >> > I had tested this, and no, though it is still faster than powf.
>> >> >
>> >> > It still seems to rely on libm, note that we don't use -ffast-math and
>> >> > a look at
>> >> https://github.com/lattera/glibc/tree/master/sysdeps/x86_64/fpu
>> >> > as well seems to say no. Problem is, GNU people like to prioritize
>> >> > "correctly rounded" behavior over fast, reasonably accurate code,
>> >> > sometimes to ludicruous degrees.
>> >> >
>> >> > Personally, I don't know why we don't use -ffast-math, not many seem
>> >> > to care that heavily on strict IEEE semantics. Maybe it leads to too
>> >> > much variation across platforms?
>> >>
>> >> You lose some guarantees. In particular, the compiler will assume NaNs
>> do
>> >> not happen and you cannot predict which code path (after a comparison
>> for
>> >> example) they take.
>> >> But some code for either security or correctness reasons needs them to
>> be
>> >> handled a certain way.
>> >> I guess in theory you could try to make sure fisnan is used in all those
>> >> cases, but then you need to find them, and I think if you take
>> -ffast-math
>> >> description literally there is no guarantee that even fisnan continues
>> to
>> >> work... I am also not sure none of the code relies on order of
>> operations
>> >> to get the precision it needs.
>> >> So it is simply too dangerous.
>> >> Some more specific options might be possible to use though (but I think
>> >> even full -ffast-math gains you almost nothing? Does it even help
>> here?).
>>
>> Yes, sorry, I meant some specific things from -ffast-math. I checked
>> configure, most of the unambiguously clear ones are already being
>> turned on. As such, it seems ok.
>>
>> >
>> >
>> > One could also consider writing some customized assembly (calling the
>> > relevant instructions instead of C wrappers) in cases where it is
>> > speed-sensitive. It's sort of the inverse of what Ganesh is suggesting, I
>> > guess, maybe some more effort involved but it can't be that much. You
>> could
>> > even use av_always_inline functions and inline assembly to call the
>> > relevant instruction and otherwise keep things in C. That's identical to
>> > what -ffast-math does but turns on only when specifically calling the new
>> > API function name...
>>
>> So seems like everything wrt this patch is fine, right?
>
>
> Not really. Your patch still does two things, and I don't like the explicit
> exp(log(a)*b).

Well, both are needed for the speedup. Without the 2.0 check, there is
a speed regression. I don't understand why it is "two things" in that
case.

> What I'm thinking is that you should have a static inline
> function, let's call it fast_pow(a, b), which can internally (in the C
> version) be implemented as exp+log. Just as you found for pow, we might
> find that for exp/log, the system lib is not very optimized and we can do
> it faster ourselves by doing whatever -ffast-math is doing for these
> functions. Those would be more specifically optimized and that would be
> part of the fast_pow implementation. This way, the code in aacenc remains
> easy to follow and the optimization is accessible for other parts of ffmpeg
> also.

Ok, changed locally.

>
> Ronald
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