[Bug fortran/47495] New: .mod files: File modification time - Makefile build issue

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47495

           Summary: .mod files: File modification time - Makefile build
                    issue
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


Problem described by Thomas Henlich
at http://gcc.gnu.org/ml/fortran/2011-01/msg00242.html

Currently, gfortran only updates the .mod file (content and file modification
date) if the file changes.

That change has been introduced 2007 with PR 31587
and http://gcc.gnu.org/ml/fortran/2007-04/msg00331.html

The issue is the compilation cascade:

(a) If the source file changes, but the .mod file does not there should be no
    recompilation of module USErs required

(b) If the source file changes and a new .o is produced, there is no need to
    recompile the source to produce another .mod file with the same
modification
    date.

As Thomas' email shows, the current Makefile as written by "gfortran -M" cause
a contiguous recompilation of the source files:

  mymodule.o mymodule.mod: mymodule.f90
    gfortran -c mymodule.f90

will always recompile the file as "mymodule.mod" stays older than
"mymodule.f90".


On the other hand, for:
  myprogram.o: myprogram.f90 mymodule.mod
    gfortran -c myprogram.f90
the file will only be recompiled if mymodule.mod actually changes, which is
good.


The question is: How can one solve this properly? Possibly without dating back
the creation date of the .f90 file ...