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

[Ronald S. Bultje]

Hi,

On Sat, Mar 12, 2016 at 9:03 AM, Ganesh Ajjanagadde <gajjanag at gmail.com>
wrote:

> 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.


Please submit a new patch, this is not a minor cosmetic change.

Ronald