On Tue, 2004-06-08 at 18:10, Ennals, Robert wrote:
> [snip]

> While a GC does need to have some way to distinguish between pointers
> and data, this is not the only way that this can be done. Another
> approach (used by GHC) is to dispense with tag bits and instead store a
> bitmask for each datatype and each stack layout, indicating which fields
> are pointers.
> 
> I don't know much about the relative advantages/disadvantages of the two
> approaches, but there is definitely a design space to be explored, and
> it is definitely possible to have native 32 bit floats in a garbage
> collected language.

Felix uses an array of offsets to locate pointers.
And in Felix all types are fully expanded (except in sums
due to a design stupidity in C++).

A disadvantage of the offsets approach is that
Felix pointers and C pointers can't be mixed
in polymorphic functions operating on pointer to T:
the distinction is entirely static. In Ocaml, the
check is done at run time -- meaning less code bloat
and a small performance hit.

Expanded types provide superior efficiency
in some cases, and of course include boxing (pointers)
as a special case and so subsume systems that only box.

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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