[Bug fortran/57628] spurious error: division by zero in if statement

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57628

--- Comment #11 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Sun, Jun 16, 2013 at 11:28:36PM +0000, furue at hawaii dot edu wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57628
> 
> --- Comment #9 from Ryo Furue <furue at hawaii dot edu> ---
> (In reply to Steve Kargl from comment #8)
> 
> > So, the compiler should just arbitrarily chose to evaluate
> > some expression and ignore others?
> 
> No, I don't mean that.  I'm not saying which expression the compiler should
> evaluate.  What I'm saying is, what is the best way to deal with the result of
> the evaluation?

I think you found the answer.  gfortran issues an error, so
a user has the chance to fix her code.

> 
> > Just remove the PARAMETER attribute in your code, it it will
> > do what you.
> > 
> > real :: a = 0
> > if (a > 0) then
> >    print *, 1/a
> [. . .]
> 
> Yes, I was about to come to that!  I write my code that way because I plan to
> provide the value of "a" from an external module in the future.  Currently I
> set the value with PARAMETER just as a convenience during the development of
> the code.  So, you are right, that your solution is one workaround for my
> situation.
> 
> But, I feel strongly uneasy looking at the code because "real::a = 0" is a
> strong indication that the value of "a" will be altered after the definition. 

Fortunately, Fortran allows more than a single character in a variable
name (and comments). 
  !
  ! This a flag set by the programmer prior to compilation.
  ! <Description of what the flag does goes here>
   real :: immutable_flag = 0

> The codes we are showing in this message exchange are shortened versions and in
> my real codes, there are some lines between "real, parameter:: a = 0" and the
> IF statement.  When I see "real:: a = 0.0", I expect the value of "a" will be
> altered because I don't see PARAMETER.

Not if the code is properly commented and the variable is suitably
named.

> Overall, I think this kind of thing is better be a "warning" and that at least
> the compiler should allow the user to run such a code as this.  The result of
> the run may be a disaster but it's the user's responsibility.  To refuse to
> compile these codes is too much patronizing on the part of the compiler.

I think that it is better to issue an error.

> but I thought that replacing 1.0/0.0 with "Inf" at compile time would be a
> useful extension (without violating the Fortran standard, of course).  Again,
> I'm not saying the compiler must do this.  All I'm saying is that it may be
> useful.

gfortran does not support the IEEE 2003 standard.  No one has stepped
up to the plate.  Here's your chance to make gfortran doe whatever
you think the standard mandates.