Re: -ffast-math and gfortran

[Steve Kargl <sgk at troutmask dot apl dot washington dot edu>]

On Sun, Jun 04, 2006 at 08:41:42PM -0400, Iwan Kawrakow wrote:
> 
> To come back to your example:
> 
> program h
>   complex z
>   z = cmplx(1.e-25,1.e-25)
>   call div(z,z)
> end program h
> subroutine div(a,b)
>   complex a, b, c
>   c = a / b
>   print *, c
> end subroutine div
> 
> gfortran h.f90 && ./a.out
> (  1.000000    ,  0.000000    )
> gfortran -ffast-math  h.f90 && ./a.out
> (           NaN,           NaN)
> gfortran -fno-math-errno -funsafe-math-optimizations -fno-trapping-math -ffinite-math-only -fno-rounding-math -fno-signaling-nans 
> h.f90 && ./a.out
> (  1.000000    ,  0.000000    )
> The last line uses all flags set by -ffast-math except fcx-limited-range. 

Yes, it's a simple example that leads one to test individual
flags (if she knows of their existence).  With a much more
complex program with numerous data paths -ffast-math may
give one a false sense of security.

I note you avoided the most important question from my previous
email.  What potential unsafe transformations does -ffast-math
trigger?  -ffast-math isn't documented!  Toon was going to
document all of these transformations until he realized the size
of the job.

Go ahead use -ffast-math.  I'll continue to recommend that gfortran
users should avoid it.

-- 
Steve