[Bug fortran/72798] New: Module (.mod) file changes even when interface does not

["townsend at astro dot wisc.edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72798

            Bug ID: 72798
           Summary: Module (.mod) file changes even when interface does
                    not
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: townsend at astro dot wisc.edu
  Target Milestone: ---

Created attachment 39054
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39054&action=edit
Test case demonstrating problem

In a big project I'm involved in, we take advantage of the fact that gfortran's
.mod files don't change across a recompilation, if the public interface of the
module is unchanged. This allows us to avoid very time-consuming compilation
cascades.

However, I've run into a case where this desirable behavior doesn't seem to
work. If I compile the attached module ('gfortran -c test_mod.f90') with and
without the print statement commented out, I get .mod files which not only
differ in contents, but also in size.

Is this correct behavior? If so, how might I go about judging whether the
public interface of a module has or hasn't changed?