Floating point failure (NaN)

[Ron Miller]

Hi,

I have spent a lot of the last week trying to track down a bug that appears
to be OS/hardware related.

The problem that I am seeing is that NaN (not a number) is mysteriously
appearing in some of my floating point variables even when it should not.

I posted a message last week, but since then I have simplified the code.

I have a code fragment that looks like the following:

    double da, db;
    while ( 1 ) {
        da = 1.0;
        db = da;
        if ( (da!=da) || (db!=db) ) {
            printf( "Found NaN\n" );
        }
    }

This will run for hours or even days on a multi-processor machine, and then
at the same time, several of the jobs will start printing out that they
found NaN (maybe 3 or 4 messages) and then go on acting normally again..
Some machines also seem more susceptible to the problem, although it seems
to eventually fail on all machines that I have tried.

My operating system / environment is:

        Redhat 6.2
        gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
        (also tried gcc version 2.95.2 19991024 (release))
        2 x 700 MHz Intel Pentium III
        100 MHz Bus Clock Speed
        2 GB RAM
        4 GB Swap Space

Changing compilers or changing compiler flags doesn't seem to fix the
problem.

It also fails with floats as well as doubles.

It seems like it might be a problem with the hardware (FPU overheating?),
but I have been able to get it to fail on machines from multiple vendors.

Has anyone else seen anything like this before?

Thanks,
Ron Miller