[Ffmpeg-cvslog] r8474 - trunk/libavcodec/resample2.c
Michael Niedermayer
michaelni
Thu Mar 22 12:55:38 CET 2007
Hi
On Thu, Mar 22, 2007 at 01:24:23PM +0200, Uoti Urpala wrote:
> On Thu, 2007-03-22 at 11:47 +0100, Michael Niedermayer wrote:
> > On Wed, Mar 21, 2007 at 10:50:06PM -0500, Rich Felker wrote:
> > > This is not trivial and most likely an illegal optimization for the
> > > compiler to make. As soon as you go to floating point, algebraic
> > > identities simply fail to hold in the general case, and
> > > "optimizations" using them will make code do the wrong things in
> > > corner cases. You may not care about the corner cases, but the
> > > compiler can't know that. So floating point optimization of this sort
> > > always needs to be done explicitly by the coder.
> >
> > hmm
> >
> > for(i=1; i<50; i++){
> > t *= i;
> > v += pow(x*x/4, i)/(t*t);
> >
> > factoring the x*x/4 out doesnt need any algebraic identities
>
> Does your gcc not factor that out then?
i didnt check that specific part, just that the code before and after all
optimizations (which you cliped away from the mail) differed in speed by
a huge factor
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Everything should be made as simple as possible, but not simpler.
-- Albert Einstein
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