[m-dev.] Re: [mercury-users] tabling

[Zoltan Somogyi]

On 30-Nov-2000, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> That's true for our currently supported implementations, i.e. for `--gc
> conservative' and `--gc none', but for `--gc accurate' the implementation
> that I posted won't work; ..

Of course, but MC don't *need* it to work with --gc accurate for now
(since --gc accurate itself doesn't work yet ...).

> we'd need to use `c_pointer' rather than `int'
> for the address type, and we'd need to teach the tabling implementation
> how to table `c_pointer' values.

The problem with tabling values of type c_pointer is that Mercury doesn't know
whether any particular use of c_pointer obeys the invariant required. This
depends both on the garbage collector used and on the way the program uses
the data structures pointed to by the pointer. The first the implementation
can know about, the second it can't. The best it can do is take the
programmer's word "no, I won't be updating the data structures reachable
from the pointer after the pointer is tabled, except possibly in ways
that make no observable difference (e.g. caching hints)". I would oppose
any proposal to extend tabling to work on values containing c_pointers without
requiring the programmer to make a declaration to this effect.

> > In fact, any approach that requires the runtime to know about object(T)
> > as a special type would impose distributed fat.
> 
> `c_pointer' already needs to be handled specially, so using
> `c_pointer' rather than `int' for the address type would impose
> no additional distributed fat.

Yes, because in that case it isn't object(T) that is special, but c_pointer.

Zoltan.
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