[Bug fortran/48363] New: Recursion not converted into a loop

[Bug fortran/48363] New: Recursion not converted into a loop

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48363

           Summary: Recursion not converted into a loop
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


For the following Fortran program, the recursion could be replaced by a loop.
That's what happening for the related C program, but for the Fortran program
the recursion remains. (Tried with -O3.)

(I guess the I/O statement (_gfortran_st_write* and _gfortran_transfer_*_write)
confuse the ME, but I do not see why that should prevent the loop
transformation. Hints how to modify the FE to help the ME are welcome, too.)

! Fortran version
call x (1)
contains
  recursive subroutine x (i)
    use iso_fortran_env
    integer, value :: i
    if (mod (i, 1000000) == 0) write (error_unit,'(a,i0)')'i=', i
    call x (i+1)
  end subroutine x
end


/* C version */
#include <stdio.h>
static void
x (int i) {
  if (!(i % 1000000))
    fprintf(stderr, "i=%d\n", i);
  x(i + 1);
}
int main () {
  x (1);
}

[Bug fortran/48363] Recursion not converted into a loop

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48363

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.03.30 15:01:37
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-30 15:01:37 UTC ---
The parameter has its address taken in

  _gfortran_transfer_integer_write (&dt_parm.0, &i, 4);

thus we can't tail-recurse here (well, at least not as trivial as if
that wasn't the case).

[Bug fortran/48363] Recursion not converted into a loop

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48363

--- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> 2011-03-30 15:38:13 UTC ---
(In reply to comment #1)
> The parameter has its address taken in
>   _gfortran_transfer_integer_write (&dt_parm.0, &i, 4);

True, but the value is not modified - which the ME should know as the "fn spec"
is ".wR"

[Bug fortran/48363] Recursion not converted into a loop

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48363

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-30 15:54:28 UTC ---
(In reply to comment #2)
> (In reply to comment #1)
> > The parameter has its address taken in
> >   _gfortran_transfer_integer_write (&dt_parm.0, &i, 4);
> 
> True, but the value is not modified - which the ME should know as the "fn spec"
> is ".wR"

The parameter is not in SSA form so tail-recursion would have to insert
a store and reload the value at appropriate places.  It doesn't support that.

[Bug tree-optimization/48363] Recursion not converted into a loop

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48363

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization
   Last reconfirmed|2011-03-30 15:01:37         |2021-8-10
           Severity|normal                      |enhancement