Re: [Bug fortran/38823] New: Diagnose and treat (-2.0)**2.0 properly

[Andrew Thomas Pinski <pinskia at gmail dot com>]

On Jan 13, 2009, at 3:08 AM, "burnus at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org 
> wrote:

> Found at:
> http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/0f1d7da66fa015c2
>
>  print *, (-2.0)**2.0
>  end
> is invalid. gfortran should print a diagnostic for -std=f95/f2003/ 
> f2008 as NAG
> f95 does:
>  Error: Negative floating-point value raised to a real power
>
> Fortran 2003 in the second sentence of the second paragraph of "7.1.8
> Evaluation of Operations":
>
>  "Raising a negative-valued primary of type real to a real power is
>   prohibitted."
>
> The question is whether one needs to reject it completely or only with
> -std=f95. Steve (see thread) thinks the constant folding gets it wrong
> (-> gives "4.0").
>
> Current results:
> - Runtime and compile time evaluation (ifort, gfortran, g95):
>  -2.0**2.0 = 4.0
>  -2.0**1.9 = NaN
> - Mathematica:
>  -2^2 = 4, -2.0^2.0 = -4.0
>  2.0^1.9 = -3.73213

-2.0^1.9 will be a complex number.  Maybe we can define it as taking  
the real part. I don't know if that is better than generating a nan  
here.

Thanks,
Andrew Pinski


> --
>           Summary: Diagnose and treat (-2.0)**2.0 properly
>           Product: gcc
>           Version: 4.4.0
>            Status: UNCONFIRMED
>          Keywords: diagnostic
>          Severity: normal
>          Priority: P3
>         Component: fortran
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: burnus at gcc dot gnu dot org
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38823