[Bug fortran/30723] New: Freeing memory doesn't need to call a library function

[fxcoudert at gcc dot gnu dot org]

Currently, allocated memory is freed by calling the library function
_gfortran_internal_free, which does nothing but:

/* Free internally allocated memory.  Pointer is NULLified.  Also used to
   free user allocated memory.  */
void
internal_free (void *mem)
{ 
  if (mem != NULL)
    free (mem);
}

Two things are worth noting:
  + although the comment says otherwise, the pointer is not NULLified.
  + this code could be generated directly by the front-end, leading to better
optimization.

An example of the missed optimization is the compilation of the following code:

  REAL    :: a(5), b
  INTEGER :: l, u
  l = 4
  u = 2

  a = (/ 1.0, 2.0, 3.0, 4.0, 5.0 /)
  b = f(a(l:u) - 3.0)
  call foo(b)

  CONTAINS
    REAL FUNCTION f(x)
      REAL, DIMENSION(:), INTENT(in) :: x
      f = sum(x)
    end function
END PROGRAM

the optimized tree looks like:

  SR.33 = _gfortran_internal_malloc (0);
  b = 0.0; 
  _gfortran_internal_free (SR.33);

We sure could optimize away the use of SR.33. PR 30720 has another case where
such an optimization could happen.


-- 
           Summary: Freeing memory doesn't need to call a library function
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: fxcoudert at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30723