[Bug fortran/84848] [8 Regression] FAIL: gfortran.dg/coarray/event_3.f08 -fcoarray=single -O2 -latomic execution test

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84848

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |burnus at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org,
                   |                            |vehre at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This sounds like UB (use of uninitialized var), though I know next to nothing
about type(event_type), but looking around I see various examples in Fortran
pdfs that use type(event_type) and never initialize it in any special way.
So, based on that the bug is likely in the -fcoarray=single support of
event_type that we fail to zero initialize the vars we create for it, or, if
some initialization is required from users, in the testcase.

In any case, on x86_64-linux with -fcoarray=single I see in the *.original
dump:
exchange ()
{
  integer(kind=4) cnt;

  (integer(kind=4)) x = (integer(kind=4)) x + 1;
  (integer(kind=4)) x = (integer(kind=4)) x + 1;
  cnt = (integer(kind=4)) x;
  if (cnt != 2)
    {
      _gfortran_error_stop_numeric (1, 0);
    }
  L.1:;
  (integer(kind=4)) x = (integer(kind=4)) x + -2;
}

(this is fine, it only uses the x from the parent function) and
global_event ()
{
  static void exchange (void);
  integer(kind=4) x;

  exchange ();
}

As x is automatic variable and is not initialized, the x++ statements invoke
(from middle-end POV) UB by incrementing uninitialized variable.