#include <stdio.h>
#include <math.h>
int main () {
printf ("Double: %d %d\n", sizeof(double), sizeof(double_t)) ;
printf ("Float: %d %d\n", sizeof(float), sizeof(float_t)) ;
return 0 ;
}
provides
Double: 8 8
Float: 4 4
on both machines
> Sent: Thursday, November 14, 2024 at 3:46 AM
> From: "Michael Hennebry" <hennebry@xxxxxxxxxxxxxxxxxxxxx>
> To: "Community support for Fedora users" <users@xxxxxxxxxxxxxxxxxxxxxxx>
> Subject: Re: gcc/gsl
>
> On Wed, 13 Nov 2024, Roger Heflin wrote:
>
> > On Wed, Nov 13, 2024 at 4:25?PM Patrick Dupre <pdupre@xxxxxxx> wrote:
> >>
> >>>
> >>> How different are the values? How many significant figures match? 0? 5?
> >> relatve difference: 2.7e-8
> >> "noise" ~ 1e-35
> >> values < 2e-23
> >>
> >
> > What significant figure is it of the result? Heavy calculations are
> > sensitive to the precision of the underlying cpu calculations and one
> > cpu may pick a different underlying precision to use internally. Ie
>
> Not if both CPUs are supposed to be using IEEE 64-bit floating point.
> The 80-bit double extended registers go mostly unused lately.
> Their use in double precision arithmetic is supposed to be discoverable:
> sizeof(double) != sizeof(double_t)
>
> > if one machine takes say 3 clock cycles to do a single or a double
> > then you might as well use a double internally and if the other
> > machine takes say 3 for a single and 6 for a double then on that
> > machine the compiler optimization will choose the single (to be
> > faster) and both may produce a different answer.
>
> Not if both CPUs are supposed to be using IEEE 32-bit floating point.
> Again, discoverable: sizeof(float) != sizeof(float_t).
>
> If rounding is indeed the issue,
> 'tis most likely in the libraries, e.g.,
> one evaluates (x+y)+z and the other (x+z)+y .
> One might use exp(x)-1 and the other expm1(x) or expl(x)-1 .
>
> OP might swap libraries to test the hypothesis.
>
> --
> Michael hennebry@xxxxxxxxxxxxxxxxxxxxxx
> "SCSI is NOT magic. There are *fundamental technical
> reasons* why it is necessary to sacrifice a young
> goat to your SCSI chain now and then." -- John Woods
> --
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