On 13 November 2015 at 21:41, Jean-Pierre Flori <jpfl...@gmail.com> wrote:
> > > On Friday, November 13, 2015 at 12:34:41 PM UTC-8, Bill Hart wrote: >> >> >> >> On 13 November 2015 at 19:10, Jean-Pierre Flori <jpf...@gmail.com> wrote: >> >>> >>> >>> On Friday, November 13, 2015 at 9:40:27 AM UTC-8, Bill Hart wrote: >>>> >>>> Not in the published divide-and-conquer algorithm, no. >>>> >>>> There was a bug in the implementation of the new basecase algorithm, >>>> which was not published, nor very important. On most machines it was >>>> switched off. And even on the machines where it was used (with the >>>> exception of itanium where it seemed to be used a lot), it was only used >>>> for *very* small operands. >>>> >>>> To be verbose, do you mean the basecase schoolbook multiprecision >>> division using 3by2 small division using a new 1 word precomputed inverse >>> is broken? >>> >> >> Yes it was a 3by2 version using a new 1 word precomputed inverse >> (unrelated to the 3by2 division originally used in MPIR and GMP). >> > So do you still trust the 3by2 algorithm in your preprint? > I don't have any reason to not trust it. I believe this was just an implementation bug. If you look at the most recent commit to the MPIR repository from me, you'll see why I think that. https://github.com/wbhart/mpir/commit/8435273a1ae6c033bec43fc54966bf121b73cf62 > >> >>> So is the code from BDSNT broken as wel? >>> https://github.com/wbhart/bsdnt/blob/master/helper.c#L186 >>> https://github.com/wbhart/bsdnt/blob/master/nn_quadratic.c#L240 >>> >>> >> Doubtful. The implementation in MPIR was much, much more complex. >> >> The algorithm itself is quite straightforward. >> >> On the other hand, bsdnt does not have many users, so if bugs exist >> there, it is much less likely they will be detected, so beware. >> >> Yup, I did an implementation myself for another project using your > precomputed reciprocal and a naive schoolbook long division. > I tested the values mentioned on http://trac.sagemath.org/ticket/19280 > and get a correct remainder. > Good to know. Thanks. > Note that I don't store the quotient but it should be correct as well. > If you are aware of any other values triggered wrong answer in MPIR, I'd > be more than happy to pass them to my implementation. > I'm not aware of other values that triggered it, though there may well have been others. This was certainly a rare corner case. As I said, I had tested the original implementation on a variety of inputs for hours. Bill. > -- > You received this message because you are subscribed to the Google Groups > "mpir-devel" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to mpir-devel+unsubscr...@googlegroups.com. > To post to this group, send email to mpir-devel@googlegroups.com. > Visit this group at http://groups.google.com/group/mpir-devel. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "mpir-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to mpir-devel+unsubscr...@googlegroups.com. To post to this group, send email to mpir-devel@googlegroups.com. Visit this group at http://groups.google.com/group/mpir-devel. For more options, visit https://groups.google.com/d/optout.
