[Discuss] Test for positive gives wacko results for underflowed values for the gcc compiler suite (including the gcc compiler, and g77 and gfortran compilers)

[Alan W. Irwin]

Here is a simpler test

int main(void) {
    double x=1.e-305, xscale=1.e+305;
    double y;
    int greater;
    greater = (x/xscale) > 0.e0;
    printf("%5.1d\n",greater);
    y = x/xscale;
    greater = y > 0.e0;
    printf("%5.1d\n", greater);
}

with the result

     1
     0

So the fundamental question is why are the results for (x/xscale) > 0.e0;
and y > 0.e0; different?

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state implementation
for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
Linux Links project (loll.sf.net); and the Linux Brochure Project
(lbproject.sf.net).
__________________________

Linux-powered Science
__________________________