[Bug fortran/102011] New: Infinite loop in heron iteration when optimization is enabled with gfortran 10.3.0

[Bug fortran/102011] New: Infinite loop in heron iteration when optimization is enabled with gfortran 10.3.0

[Ralph-Trenkler@t-online.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102011

            Bug ID: 102011
           Summary: Infinite loop in heron iteration when optimization is
                    enabled with gfortran 10.3.0
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Ralph-Trenkler@t-online.de
  Target Milestone: ---

Created attachment 51346
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51346&action=edit
The fortran program, which does the infinite loop with compiler version 10.3.0

I wrote a function in gfortran-10.3.0, which computes the square root with the
heron iteration method. Without optimization the program is okay, but if I turn
on optimization, then it does an infinite loop. I use Kubuntu 20.04.

[Bug fortran/102011] Infinite loop in heron iteration when optimization is enabled with gfortran 10.3.0

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102011

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargl at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from kargl at gcc dot gnu.org ---
(In reply to Ralph Trenkler from comment #0)
> Created attachment 51346 [details]
> The fortran program, which does the infinite loop with compiler version
> 10.3.0
> 
> I wrote a function in gfortran-10.3.0, which computes the square root with
> the heron iteration method. Without optimization the program is okay, but if
> I turn on optimization, then it does an infinite loop. I use Kubuntu 20.04.

If I compile your program with -Wall, I get 

gfcx -o z -fcheck=all -Wall a.f90 && ./z
a.f90:11:13:

   11 |          if (abs((x2-x1)/(x1+x2)) < epsilon) exit
      |             ^
Warning: 'epsilon' may be used uninitialized [-Wmaybe-uninitialized]


Sure, enough.

      real(8), parameter :: epilson = 1.0e-15
                            ^^^^^^^

         if (abs((x2-x1)/(x1+x2)) < epsilon) exit
                                    ^^^^^^^

One of these is wrong.