[M3devel] small objects

[Mika Nystrom]

You probably mean something like this:

http://www.async.caltech.edu/~mika/interp/

Note that this code, as it stands, probably does not work (if we are
to trust what the implementor of the garbage collector has said).

    Mika


hendrik at topoi.pooq.com writes:
>There are many times I want to express data which could be efficiently 
>coded as the disjoing union of (small) integer and pointer to object.  
>The pointer-to-object is used in the case where tho objects are big;  
>the (small) integer for the more common case where the objects are 
>small.
>
>High-level languages seem to pe quite paranoid about admitting thise 
>kind of data into the fold, except maybe for Lisp systems, which have 
>been doing this from time immemorial.  (I believe CAML does this, too).  
>These languages use it internally, and manage to (mostly) hide it from 
>the user.
>
>The X toolkit uses this trick too -- there's a constant somewhere, and 
>if an integer is less than this constant, it's passed to an X toolkit 
>function as an integer; otherwise by reference.  The idea there is that 
>there's a range of addresses of storage that can never be used as 
>parameters for the X toolkit functions (presumably because of hardware 
>or OS limitations), and that the bit patterns that are unavailable for 
>addresses can be used as small integers.
>
>Now the semantics of such a union, efficiently coded, are quite clear.  
>There's a range of numbers that can be packed unamiguously into 
>pointers, and if your integer can be so packed, you do it; 
>otherwise you use a reference to sime kind of INTEGER object 
>elsewhere.  There are operations for packing integers and object 
>pointers into such words, and others for unpacking them (complete with 
>type-test).  The actual physical representation can be machine- or 
>implemetation dependent -- you could do a bit of shifting and pack 
>integers into words with the low bit set (if pointers to objects are 
usually aligned in some way, the integers will stand out as being 
>unalinged)  Or you could use an uppoer bound on "small" integers, as C 
>does.  And on a machine where such packing is impossible (for whatever 
>reason) you could simply set the upper bound of (the absolute alue 
>of) such packable integers to be zero, so there wouldn't be any.
>
>Is there any way such a thing can be done in Modula 3?  Remember -- I do 
>want the garbage collector to be aware of such conventions and do proper 
>tracing on the pointers?
>
>(I suspect the answer is "no".  But would be a pity.)
>
>-- hendrik