At Wed, 22 Aug 2007 17:30:18 +0200,
Andries E. Brouwer wrote:
> > In a program that has called gsl_eigen_symm() successfully 10^9 times,
> > I find that on one specific matrix this routine hangs in an infinite loop.
> > Trying this same matrix on a different architecture, all is well.
> >
> > The machine with infinite loop is an x86_64:
> > Intel(R) Core(TM)2 CPU 6300 @ 1.86GHz
> >
> > A test program that loops is given below.
>
> Investigating a bit closer, I see in symm.c:gsl_eigen_symm()
> a loop while (b > 0) { ... } that hangs (flipflops between two states).
> If I change the test
> if (sd[b - 1] == 0.0 || ...
> into
> if ((sd[b - 1] > -5e-188 && sd[b-1] < 5e-188) || ...
> then all is fine.
>
> So, it seems gsl has assumptions about the arithmetic on very small
> numbers that are false on this particular machine.
Thanks for the bug report. Can you send me a complete list of the
compiler version, operating system version and options used to compile
GSL so I can try to reproduce the problem.
I expect it is caused by different underflow behaviour with some
recent 64bit or SSE instructions -- I haven't been able to reproduce
it compiling with -msse2 -fpmath=sse on a 32-bit machine.
--
Brian Gough
GNU Scientific Library -
http://www.gnu.org/software/gsl/
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