[m-dev.] foreign types and tag bits

[Ian MacLarty]

On 18 Feb 2006, at 10:45, Fergus Henderson wrote:

> On 17-Feb-2006, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
>> 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.
>
> It will get boxed.
>
> But the same will happen even if you use a Mercury type such as
> mercury_t (below) rather than foreign_t, even though mercury_t
> is going to be represented as a word-aligned pointer.
>
> 	:- type mercury_t ---> mercury_t(int, int, int, int, int).
>
>> 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?
>
> It wouldn't be sufficient.  Currently the compiler doesn't take 
> advantage
> of tag bits in those kinds of situations.  Modifying the compiler to
> do so would unfortunately be hard to implement.  (It requires modifying
> code generation and RTTI, amongst other things.)
>

Oh well.  I can still achieve the same effect by checking the tag bits 
of my foreign type with C code, though I was hoping to move the 
discrimination between an old and the latest version of a version array 
into a Mercury discriminated union which would make coding it a bit 
easier.

Ian.

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