[Bug fortran/30981] a ** exp fails for integer exponents if exp is "-huge()-1" (endless loop)

[burnus at gcc dot gnu dot org]

------- Comment #8 from burnus at gcc dot gnu dot org  2007-02-27 22:04 -------
> > Also isn't -huge()-1 undefined code for Fortran?
> -huge()-1 can be defined in Fortran.  [...]
> I'll also note that -pedantic will reject -huge()-1

Just for completeness: In the original example (the very long, incomplete F77
code) the problem was due to an overflow of:

        integer :: exponent
        ! temp very large or +INF
        exponent = log10(temp)

Thus as long as such -huge()-1 and (for floating point numbers) INF, NAN and
denormal numbers exists, the intrinsics have to deal with it somehow.


(In reply to comment #4)
> > With example I mean something which actually compiles and runs. Here, I have
> > two problems: 
> >       include '$(where)/amos/include/essential.ecm'
> > is missing as well as the main program which calls the routine.
> 
>   I really try to do this, but I am an "old" fortran programer and 
>   I do not know how to set +Infinity. Yes, of course, I have a case
>   that computes it and then hangs, but my code is over 500,000 lines
>   long and I did not think it is appropriate.

Well, given that you have 500,000+ lines, it was already quite short and I was
able to reproduce the problem even without the essential.ecm file. (Actually,
reproducing was easier than I initially feared.)

In any case: Thanks for reporting this bug.

[Comparison with 10.0]
>   This is not as silly as it sounds. All I care about is to handle
>   the special case of 10 (exactly). I have seen cases where the
>   equality fails, but the number is really 10.

I'm sure that the two numbers are then not exactly the same (bitwise) though;
nonetheless I'm not good at predicting when an operation will cause an
off-by-one (bit) and when not. I also failed to cook up a small example for
10.0

To create infinity etc. one can simply do:
  real :: zero, r
  zero = 0.0
  r = 1.0/zero ! => +INF
  r = zero/zero ! => NaN
  r = -1.0/zero ! => -INF


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30981