Re: round4 performance

[Xavier-François Roblot <roblot@euler.univ-lyon1.fr>]

On Wed, 2004-05-05 at 17:36, Bill Allombert wrote:
> On Tue, May 04, 2004 at 04:17:06PM +0200, Xavier-François Roblot wrote:
> > Hi developers,
> > 
> > On Wed, 2004-04-28 at 04:36, Igor Schein wrote:
> > > Hi,
> > > 
> > > here's a polynomial on which round2 heavily outperforms round4:
> > > 
> > > x^64 + 528*x^60 + 422640*x^56 + 154189440*x^52 + 46085461920*x^48 + 86643136
> > > 30464*x^44 + 1067417121457152*x^40 + 76273480007101440*x^36 + 29307987576360
> > > 23040*x^32 + 31785192406564024320*x^28 + 729662629421496287232*x^24 - 866453
> > > 9928316858220544*x^20 + 114361270118934673858560*x^16 - 51757447983683584779
> > > 87840*x^12 + 47264303406943521096007680*x^8 - 774599638688652156020195328*x^
> > > 4 + 7585622913396815983510880256
> > 
> > I have committed a big patch that attempts to make round4 perform better
> > with this kind of polynomial. There are some improvements but still it
> > is significantly slower than round2... Also, I added some garbage
> > collecting so at least the stack necessary should now be reasonable. In
> > any case, the changes I have made still require a lot of tunings (and
> > also probably debugging!), so any feedback is welcome.
> 
> Apparently, now round4 is faster than round2 by 10% on this example.
> (maybe not if we look only at the prime 3, though).
> 
> I wonder if fastvalpos() could not be changed to compute 
> newton sums iteratively and stop as soon as the criterium apply
> and return 0, instead of precomputing a fixed set of sums.
> 
> Cheers,
> Bill.

Well, I have modified update_alpha (after Karim pointed out a strange
behavior in this function) and that kind of miraculously speed up
dramatically that example!... As you will see, the computing time is now
very reasonable and it runs with a small stack too (I hope the result is
still correct though, I haven't checked yet). Igor, Karim and I still
have some ideas for improvements for nilord but you need some new bad
polynomials to test them. Please send me your worst examples!

Take care,

Xavier