[CLN-list] Bug in F_2[X] comparisson

[Remco Bloemen]

Hi Richard,

On Friday 02 November 2007 00:52:44 Richard B. Kreckel wrote:
> Hi!
>
> Remco Bloemen wrote:
> > Lately i have been implementing a few algorithms from "Modern Computer
> > Algebra". In the process of implementing Z[x] factorization i noticed the
> > following strange behavior:
> >
> >   cl_modint_ring basering = find_modint_ring(2);
> >   cl_univpoly_modint_ring ring = find_univpoly_ring(basering);
> >   cl_UP_MI x= ring->zero();
> >
> >   (x != ring->zero()) returns false (correct)
> >   (x == ring->zero()) returns false (incorrect)
> >
> > If i first compare  x == x the behavior changes:
> >
> >   (x == x) returns true (correct)
> >   (x != ring->zero()) returns true (incorrect)
> >   (x == ring->zero()) returns false (incorrect)
> >
> > Attached is a demonstrational program. I'm using cln version 1.1.13
> > compiled with gcc-4.2.2 on x86 (prescott).
> >
> > Could someone please tell me if this is fixed in cvs or being worked on?
>
> The bug is still there. I think it can only happen when x is the ring's
> zero, and only in Z/2Z and only on x86: The culprit seems to be the
> assembler function compare_loop_up in src/base/digitseq/cl_asm_i386_.cc.
> It doesn't work properly if the loop is empty.
>
> I just found that out, but it is too late to work on a patch now and I
> won't have time the next two weeks. Can you, please, try to fix this
> yourself and send a patch?
>
> Regards
>     -richy.

I have looked at the code, but i could only conclude the following: my 
assembler is even worse than my german. I'm afraid fixing assembler code is 
way over my head, otherwise i would have loved to send you a patch.

PS: I am moving to a new 64-bits laptop and i can confirm that the bug does 
not exist there.

Kind regards,
Remco