Hi, On Sat, Mar 12, 2016 at 9:03 AM, Ganesh Ajjanagadde <gajja...@gmail.com> wrote:
> 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. Please submit a new patch, this is not a minor cosmetic change. Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
