[Bug fortran/88342] Possible bug with IEEE_POSITIVE_INF and -ffpe-trap=overflow

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88342

--- Comment #2 from kargl at gcc dot gnu.org ---
(In reply to kargl from comment #1)
> (In reply to Matt Thompson from comment #0)
> > All,
> > 
> > A colleague of mine encountered an issue with 8.2.0 (but it's also in 7.3.0
> > at least). We believe it might be a bug since our reading of the Standard
> > seems to make it legal.
> > 
> > Namely if this program:
> > 
> > PROGRAM test
> >    USE, INTRINSIC :: ieee_arithmetic
> >    IMPLICIT NONE
> >    REAL :: inf
> >    inf = IEEE_VALUE(inf,  IEEE_POSITIVE_INF)
> > END PROGRAM test
> > 
> > is compiled with -ffpe-trap=overflow the code FPEs:
> 
> gfortran's IEEE modules and the -ffpe-trap option are independent
> of each other.  As the manual states, -ffpe-trap is a *debugging*
> option, and will enabling traps on floating point exceptions.  You
> specifically requested an overflow trap.  What do you expect to
> happen?
> 
> The likely correct solution is to manipulate the halting 
> behavior with a combination of IEEE_GET_HALTING_MODE and
> IEEE_SET_HALTING_MODE.  In fact, F2018, 17.11.6 contains
> this exact example.
> 
>    Example. To store the halting mode for IEEE_OVERFLOW, do a
>    calculation without halting, and restore the halting mode later:
> 
>    USE, INTRINSIC :: IEEE_ARITHMETIC
>    LOGICAL HALTING
>    ...
>    CALL IEEE_GET_HALTING_MODE (IEEE_OVERFLOW, HALTING) ! Store halting mode
>    CALL IEEE_SET_HALTING_MODE (IEEE_OVERFLOW, .FALSE.) ! No halting
>    ... ! calculation without halting
>    CALL IEEE_SET_HALTING_MODE (IEEE_OVERFLOW, HALTING) ! Restore halting mode
> 
> whether or not these functions works is something that I
> haven't investigated.

Just tested.

program test
   use, intrinsic :: ieee_arithmetic
   implicit none
   real :: inf
   logical h
   call ieee_get_halting_mode(ieee_overflow, h)       ! store halting mode
   call ieee_set_halting_mode(ieee_overflow, .false.) ! no halting
   inf = ieee_value(inf, ieee_positive_inf)
   print *, inf
   call ieee_set_halting_mode(ieee_overflow, h)       ! restore halting mode
end program test

% gfcx -o z a.f90 && ./z
         Infinity
% gfcx -o z -ffpe-trap=overflow a.f90 && ./z
         Infinity

This seems to give the desired behavior.