On Fri, Mar 11, 2016 at 9:30 AM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > Hi, > > On Thu, Mar 10, 2016 at 9:23 PM, Ganesh Ajjanagadde <gajja...@gmail.com> > wrote: > >> On Thu, Mar 10, 2016 at 8:56 AM, Ronald S. Bultje <rsbul...@gmail.com> >> wrote: >> > Hi, >> > >> > On Thu, Mar 10, 2016 at 2:37 AM, Reimar Döffinger < >> reimar.doeffin...@gmx.de> >> > wrote: >> > >> >> On 10.03.2016, at 03:06, Ganesh Ajjanagadde <gajja...@gmail.com> wrote: >> >> >> >> > On Wed, Mar 9, 2016 at 2:16 AM, Reimar Döffinger >> >> > <reimar.doeffin...@gmx.de> wrote: >> >> >> On 08.03.2016, at 04:48, Ganesh Ajjanagadde <gajja...@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 > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
