7.6.2.5 Aliasing By Actual Arguments

The compiler looks ahead to see how local variables are used and then makes assumptions about variables that will not change over a subprogram call. In the following example, pointers used in the subprogram defeat the compiler’s optimization strategy and the results are unpredictable. To make this work properly you need to compile with the -xalias=actual flag:

 program foo
      integer i
      call take_loc(i)
      i = 1
      print * , i
      call use_loc()
      print * , i
   end

   subroutine take_loc(i)
      integer i
      common /loc_comm/ loc_i
      loc_i = loc(i)
   end subroutine take_loc

   subroutine use_loc()
      integer vi1
      pointer (pi,vi)
      common /loc_comm/ loc_i
      pi = loc_i
      vi1 = 3
   end subroutine use_loc

take_loc takes the address of i and saves it away. use_loc uses it. This is a violation of the Fortran standard.

Compiling with the -xalias=actual flag informs the compiler that all arguments to subprograms should be considered global within the compilation unit, causing the compiler to be more cautious with its assumptions about variables appearing as actual arguments.

Programming practices like this that violate the Fortran standard should be avoided.