[Bug fortran/107753] gfortran returns NaN in complex divisions (x+x*I)/(x+x*I) and (x+x*I)/(x-x*I)

["sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753

--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Fri, Nov 18, 2022 at 11:24:29PM +0000, sgk at troutmask dot
apl.washington.edu wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
> 
> --- Comment #5 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
> On Fri, Nov 18, 2022 at 10:05:21PM +0000, kargl at gcc dot gnu.org wrote:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
> > 
> > --- Comment #4 from kargl at gcc dot gnu.org ---
> > (In reply to anlauf from comment #3)
> > > I guess the reporter assumes that gcc uses a clever algorithm like Smith's
> > > to handle such extreme cases of complex division.  Not sure if that one is
> > > available by some compilation flag, and I think it would impact performance.
> > > 
> > > In any case, if the reporter wants to get robust results and in a portable
> > > way, I would advise him to change/fix his algorithm accordingly.  It appears
> > > that a few other compilers behave here like gfortran.
> > 
> > It's likely coming from the middle-end where gcc.info has
> > the option
> > 
> > '-fcx-fortran-rules'
> >      Complex multiplication and division follow Fortran rules.  Range
> >      reduction is done as part of complex division, but there is no
> >      checking whether the result of a complex multiplication or division
> >      is 'NaN + I*NaN', with an attempt to rescue the situation in that
> >      case.
> 
> Does anyone know what is meant by "Fortran rules"?  F66 does not
> have any particular algorithm specified.  I'll look at F77 shortly.
> 

I add the subroutine

   subroutine ohno
      complex(dp), parameter :: a = cmplx(huge(1.d0),huge(1.d0),dp)
      complex(dp), parameter :: b = a / a
      write(*,*) a
      write(*,*) b
   end subroutine ohno 


% gfortran -o z a.f90 && ./z
   (1.79769313486231571E+308,1.79769313486231571E+308)
                              (NaN,0.0000000000000000)
   (1.79769313486231571E+308,1.79769313486231571E+308)
               (1.0000000000000000,0.0000000000000000)

The last two lines are from ohno.