[m-dev.] foreign types and tag bits

[Ian MacLarty]

Hello,

Suppose I have a type like the following:

:- type t
	--->	f1(foreign_t)
	;	f2(foreign_t).

where foreign_t is a foreign type.

Suppose also that foreign_t is a word aligned C pointer.

Will the compiler be able to use the tag bits of the C pointer to
implement the type t, or will the foreign type argument end up getting
boxed.

I think they will have to be boxed, since the two current assertions on
foreign_type declarations (can_pass_as_mercury_type and stable) don't
give enough information to know whether the tag bits can be used.

I think there should be another assertion `is_aligned_pointer' which
will allow the tag bits to be used.

Does anyone see any problems with this?  Would it be hard to implement?

The reason I'm asking is I've having a look at making version_array's
faster and it occured to me that I could use the tag bits to decide if
the version array is the latest version or not, which would mean
version_array lookups would be as efficient as array looks (for the
latest version), since you'd only need to dereference one pointer.
Currently you must first dereference the foreign_type pointer to find
out if the va is the latest version, and then you must dereference the
array pointer.

Ian.

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