Activate -mrecip with -ffast-math?

[Richard Guenther]

On 6/18/07, Brooks Moses <brooks.moses@codesourcery.com> wrote:
> Giovanni Bajo wrote:
> > Both our goals are legitimate. But that's not the point. The point is
> > what -ffast-math semantically means (the simplistic list of suboptions
> > activated by it is of couse unsufficiente because it doesn't explain how
> > to behave in face of new options, like -mrecip). My proposal is:
> >
> > "-ffast-math activates all the mathematical-related optimizations that
> > improves code speed while destroying floating point accuracy."
>
> I don't think that's a workable proposal.  If it is taken literally, it
> means that the optimization of converting all floating-point arithmetic
> to no-ops and replacing all references to floating-point variables with
> zeros is allowed (and would be appropriate under this option).
>
> And, personally, I don't think that documentation is of use if it can't
> be taken reasonably literally.  There's a line between what's acceptable
> and what's not, and regardless of where exactly it is, the documentation
> needs to fairly clearly indicate its location.

I agree.  'destroying floating point accuracy' is too broad and
discuraging.  Even if in some cases this is exactly what happens - the
error we introduce (if you define it as difference of result with and without
-ffast-math) is essentially unbound.  Still in most 'regular' cases we
preserve accuracy quite well or even improve it (for some other metric
of accuracy).

This is really a hard to solve communication problem.

OTOH, if we start to produce NaN for sqrt(0.0) that is of course simply
'wrong', not inaccurate ;)

Richard.