Re: [testsuite gfortran] partial fix for secnds*.f

[Tim Prince <tprince at myrealbox dot com>]

Tobias Schlüter wrote:
> Thomas Koenig wrote:
> > On Mon, 2007-05-28 at 15:28 +0200, Dominique Dhumieres wrote:
> > > Why is it so difficult to see the obvious?
> > > -      dat1 = 0.001*real (values(8)) + real (values(7)) +
> > > -     &        60.0*real (values(6)) + 3600.0* real (values(5))
> > > +      dat1 = 3600.0* real (values(5)) + 60.0*real (values(6)) +
> > > +     &       real (values(7)) + 0.001*real (values(8))
> >
> > Ouch.  Yes, that can make a difference.
> >
> > However... as a general rule, I've learned that it is better to add real
> > values from small to big (less loss of precision that way).  Would it
> > make more sense to change the order of terms in the secnds intrinsic?
>
> I think we should use parentheses to enforce the order.  Not that the 
> compiler reorders in its current incarnation, but it might at some 
> point.  I don't know of a way to enforce an order of evaluation in C 
> besides separating the addition into a series of statements or using 
> volatile.
Observance of parentheses is a requirement in the C90 and C99 standards, 
as it always was in Fortran.  According to my understanding, gcc doesn't 
break this part of the standard, even with -ffast-math.