[m-dev.] For review: Add implementation of reference types (global heap)

[Peter Schachte]

> Index: runtime/mercury_memory.c
> ===================================================================
> RCS file: /home/mercury1/repository/mercury/runtime/mercury_memory.c,v
> retrieving revision 1.8
> diff -u -r1.8 mercury_memory.c

...

> +
> +	global_heap_zone = create_zone("global_heap", 1, global_heap_size,
> +		next_offset(), global_heap_zone_size, default_handler);
> +	global_hp = global_heap_zone->min;

Maybe this zone should only be created on demand.  Perhaps set
global_heap_zone = NULL, and put something like

    if (global_heap_zone == NULL) {
	global_heap_zone = create_zone("global_heap", 1, global_heap_size,
		next_offset(), global_heap_zone_size, default_handler);
	global_hp = global_heap_zone->min;
    }

in the code for making permanent copies.  I guess it depends on the
cost of allocating a big chunk of memory you'll never need vs. the
cost of an extra test for every permanent copy.

> NEW FILE: extras/references/nb_reference.m
> ===================================================================

...

> :- pragma inline(new_nb_reference/2).
> :- pragma c_code(new_nb_reference(X::in, Ref::out), will_not_call_mercury, "
> 	incr_hp(Ref, 1);
> 	save_transient_registers();
> 	*(Word *)Ref = MR_make_partially_permanent(X, (Word *) TypeInfo_for_T,
> 			(Word *) Ref);
> 	restore_transient_registers();
> ").

Are the save_ and restore_transient_registers(); calls really
necessary here?  MR_make_partially_permanent() already does this, so I
wouldn't have thought this could would have to do it too.  Similarly
for update/2.


> NEW FILE: extras/references/scoped_update.m
> ===================================================================

This file has a lot of commented-out debugging code.  I think it would
be better to write a macro or two, defined insides #ifdefs, that do
the printing, so that the printing can be turned on and off without
editing the file.


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