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? > > 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
