[m-rev.] diff: allow memo tables to be reset

[Ralph Becket]

Zoltan Somogyi, Monday,  7 March 2005:
> On 07-Mar-2005, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
> > Would it be a good idea to have a version of memoing where the memo 
> > tables only last for the duration of a call?
> 
> Yes, it would. However, since such calls may be nested, it is not at all
> obvious how one could implement it without adding distributed fat.

How about converting

	:- pragma memo_transient(p/n).
	p(...) :- ...

into something like

	:- pragma foreign_code("C", "int p_depth = 0;")
	:- pragma foreign_proc("C", p_depth = (X::out),
		[will_not_call_mercury, promise_pure],
		"X = (MR_Word) &p_depth;"
	).

	p(...) :-
		impure X = get_trailed_global(p_depth),
		impure set_trailed_global(p_depth, X + 1),
		...,
		impure set_trailed_global(p_depth, X),
		( if X = 0 then
			impure reset_p_table
		  else
		  	true
		).

where get_trailed_global, set_trailed_global, and reset_p_table do the
obvious thing.

-- Ralph
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