[M3devel] small objects

[Tony Hosking]

I like the notion of having a TAGGED INTEGER type that is a hybrid  
ordinal/reference.

Subtyping rules for references would now be as follows:

NULL <: REF T <: REFANY
TAGGED INTEGER <: REF T <: REFANY

ROOT <: REFANY
NULL <: T OBJECT ... END <: T
TAGGED INTEGER <: T OBJECT ... END <: T (but only for T <: REFANY, not  
T <: ADDRESS)

Thus, TAGGED INTEGER is a funny sort of hybrid ordinal/reference  
type.  Values of TAGGED INTEGER are non-pointer values similar to the  
value NIL.  We can then do ISTYPE(x, TAGGED INTEGER), and NARROW(x,  
TAGGED INTEGER), and TYPECODE(TAGGED INTEGER), similarly to ISTYPE(x,  
NULL), NARROW(x, NULL), and TYPECODE(NULL).

Because TAGGED INTEGER is an ordinal we can extract the integer value  
it holds using ORD(x).
Similarly, we can construct a TAGGED INTEGER using VAL(x, TAGGED  
INTEGER).

***The only problem with this scheme is how to efficiently perform run- 
time tests for dereferencing NULL, and TAGGED INTEGER?***

So, here is a slightly less elegant alternative that is probably  
straightforward to implement.  Introduce a separate TAGGED hierarchy.

NULL <: REF T <: REFANY
NULL <: UNTRACED REF T <: ADDRESS
TAGGED INTEGER <: TAGGED REF T <: TAGGED REFANY

Note that NULL is not a subtype of TAGGED REF T or TAGGED REFANY.

ROOT <: REFANY
TAGGED ROOT <: TAGGED REFANY
NULL <: T OBJECT END <: T where T <: REFANY or T <: ADDRESS
TAGGED INTEGER <: T OBJECT ... END <: T where T <: TAGGED REFANY

Note that NULL is not a subtype of T OBJECT ... END where T <: TAGGED  
REFANY.

This way, tagged references only need a run-time test for the tag on  
dereference (we can throw an "attempt to dereference TAGGED INTEGER"  
at run time, just like we throw "attempt to dereference NIL" for  
untagged references).  This check can be a straightforward test of the  
low bit tag.  Of course, the weird thing here is now that we don't  
have a single NULL value for TAGGED references --- we have multiple  
null values VAL(x, TAGGED INTEGER).  Instead of asking "x == NIL" we  
must ask "ISTYPE(x, TAGGED INTEGER)".

Of course, because TAGGED INTEGER is an ordinal, we can also perform  
the usual ordinal operations like INC(x), DEC(x), wherever x is typed  
TAGGED INTEGER.


On 6 Apr 2009, at 11:44, Rodney M. Bates wrote:

> I spent quite a bit of time playing with a more general system where
> there is a set of "tagged" types, with (an implementation-defined
> subrange of) INTEGER and a reference type both being a subtype of a
> tagged type.  It might have been tolerable, if it weren't for the
> interaction with object subtypes and opaque types, which quickly
> gets deep into a deep linguistic tar pit.
>
> Tony's approach is much simpler and would serve what I see as the
> need.  It does sacrifice any static checking of what reference type
> is being tagged, and will also require an extra runtime ISTYPE/ 
> TYPECASE.
>
> Would INTEGER and REFANY be assignable to TAGGED, or would there
> also need to be an explicit conversion in that direction too, say
> VAL(x, TAGGED)?  I think I favor the explicit conversion here.  It
> is a bit inconsistent with the usual Modula-3 assignability  
> philosophy,
> but not requiring the explicit conversion already gets your toes  
> into tar.
>
> We would have to have something more like ISTYPE, though, which will
> tell which type the value belongs to without risking a runtime error,
> which VAL would do.
>
> An intermediate approach might be to have a set of tagged types
> constructed by, say, TAGGED T, where I is a reference type, but
> with no subtype relations on them at all, thus still requiring
> the explicit conversions and checks.
>
> I do want to say, I _really_ want this capability somehow.  I have
> an ADT module whose internal data structure, for full generality,
> needs to have two heap objects (the second because it has nonstatic
> size) and 11 total words counting the orginal pointer, in the case of
> values that would fit directly  into the "pointer" word.  Moreover,
> these small cases are in the great majority.
>
> Besides the 11-to-one increase in actual occupied space, there
> is extra time for allocation, periodic tracing, and collection, space
> loss due to heap fragmentation, and time/space tradeoff loss due to
> reduced locality of reference.  So sometimes, it would be a big
> performance gain if we could do it.
>
>
> Tony Hosking wrote:
>> It is a much more pervasive hack than I would be comfortable with
>> since it touches on the compiler (for all the type operations) as  
>> well
>> as the run-time system in ways that are pretty gross.  I would much
>> rather have a new TAGGED builtin.  ISTYPE would not work on it since
>> that only works on references.  The only thing you need is a way to
>> extract the value -- we could overload VAL(x, T) where x can be a
>> TAGGED and T can be REFANY or INTEGER.
>>