[mercury-users] Exceptions and unique modes

[Peter Ross]

On Thu, Feb 22, 2001 at 10:56:50PM +1100, Peter Schachte wrote:
> On Thu, Feb 22, 2001 at 12:26:46PM +0100, Peter Ross wrote:
> > You would think that the best canditate for reuse of that memory is the
> > next state.  Unfortunately it is not quite that easy as the following
> > type indicates.
> > 
> > :- type t
> >     --->    f(int)
> >     ;       g(int, int).
> 
> I think a more interesting case would be something like this:
> 
> 	:- pred swap_stuff(type1::di, type1::uo, type2::di, type2::uo).
> 
> 	swap_stuff(f(A,B,C), f(D,E,C), g(D,E,F), g(A,B,F)).
> 
> It's natural to want the compiler to cleverly reuse the term passed as the
> first and third arguments, and pass them back as the second and fourth,
> respectively, just swapping the first two arguments of each term.  But a
> good structure reuse optimization would instead pass the firstand third
> arguments back as the fourth and second respectively, swapping just the
> third arguments.
> 
Yes that would be nice, and it is one of the things Nancy and I are
looking at for structure reuse.  However I think that compared to the
massive gain you get from not having extra garbage the relative payoff
of this extra optimization will not be hugely significant, and I would
be willing to pay it for the ability to use one name to refer to unique
objects.

Pete
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