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

["burnus at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from burnus at gcc dot gnu dot org  2007-02-27 19:35 -------
Hi,

> > Could you post an example?

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. And as always:
The smaller the better.

In addition, please attach such long files, they clutter the bug report page
quite a bit if they are put into the comment field.

>       if ( n_normal .ge. 10.-eps .and. n_normal .le. 10+eps ) then
>
>   PS. Notice the "eps" in the last segment of code. This is 
>   really a bug also. It originally was 
>       If ( n_normal .eq. 10. ) then
>   But in some cases it missed the proper branch.

If you do:
  n_normal = 10.0
then  if(n_normal == 10.0)  should work. If you do
  n_normal = 500.0/100.0*2.0
then it might not work due to rounding errors and representation problems of a
decimal number as binary number. Therefore, one should almost always use
something like  if ( abs(n_normal - 10.0) < eps),  where eps is sensibly
chosen, it could be e.g. eps = epsilon(n_normal), but this might be even too
small. For irrational numbers such as 3.14159 it is more obvious that one
should use, e.g. (abs(myNumber - 3.14159) < 1e-4) instead of  "if (myNumber ==
3.14159)".


Back to the problem:

The calculation goes only into an endless loop if the exponent is
"-huge(integer)-1".

Test case:
      program test
      implicit none
      print *, 5.0**(-huge(i)-1)
      end program

The problem is that in pow_r4_i4 the following operation is done:

      if (n < 0)
        {
          n = -n;

The problem is: For n == -huge(n) - 1 exists no positive number. Thus
 n == -n == -2147483648  (for integer(4))


-- 

burnus at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
   Last reconfirmed|0000-00-00 00:00:00         |2007-02-27 19:35:04
               date|                            |
            Summary|Program "Hangs"             |a ** exp fails for integer
                   |                            |exponents if exp is "-
                   |                            |huge()-1" (endless loop)


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