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

[Ganesh Ajjanagadde]

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?

>
> Ronald
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