Re: [Bug-gsl] Re: infinite loop in gsl_eigen_symm

[Brian Gough]

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/