inefficiencies when creating the .opt file

[Christopher Rodd SPEIRS]

Hi all,
	I was just looking at the code for making the .opt files (with
the intention of adding termination information to the files).  I think
that in some cases the datestamp of the .opt file may be changed
unnecessarily, causing unnecessary recompilation of other files.  The
reason is as follows:

currently the code is:
mercury_compile__maybe_write_optfile(MakeOptInt, HLDS0, HLDS) -->
	....
		intermod__write_optfile(HLDS0, HLDS1),
		( { IntermodArgs = yes } ->
                        mercury_compile__frontend_pass_2_by_phases(
                                HLDS1, HLDS2, FoundModeError),
                        ( { FoundModeError = no } ->
                                mercury_compile__maybe_polymorphism(HLDS2,
                                        Verbose, Stats, HLDS3),
                                mercury_compile__maybe_unused_args(HLDS3,
                                        Verbose, Stats, HLDS)
                	....
In mercury_compile__maybe_unused_args, information about unused_args is
annotated to the end of the .opt file

in intermod__write_optfile(HLDS0, HLDS1), the code is essentially

intermod__write_optfile(HLDS0, HLDS1) :-
	write all the data to the .opt.tmp file
	call update_interface(.opt file)


So, if the compiler is run when intermodule unused args are on, 
then the optfile created by  intermod__write_optfile will not contain
the unusedargs information, so update_interface will update the .opt
file, and then mercury_compile__maybe_unused_args will annotate its
information to the end of the .opt file, possibly making it exactly the
same as it was before the compiler was started.


Chris