Re: User-defined integers: Asking for feedback

Hi Philipp, a concern that I've not voiced before is that I also want to
claim the "available" lowtag that you're taking, but in a way that is more
broadly applicable to SBCL as a whole. Specifically I want to revise the
implementation of immobile-symbol and compact-instance-header to work on
platforms other than x86-64 and with the address space restriction
removed.  Currently these only work on x86-64 because a space can be
forcibly mapped below 2GiB.  To lift the restriction, and represent
symbols, layouts, and perhaps other important primitive objects as
NIL-relative pointers, SBCL itself will quite likely need to use a new
lowtag or 2.  (I've never actually made progress on this; it's
theoretical.)  But when or if I do it, you can't reserve lowtags for user
code, because compressed pointers (to borrow the JVM term), i.e.
NIL-relative objects, will become more important than the accidental
discovery that you can take lowtag N for yourself.
Since I never did anything, I didn't suggest you stop trying to work on
this patch, but I'm unconvinced that your effort to land it in the main
tree is going to reach a consensus.  This is a separate question from
whether instructing random users how to take advantage of a particular
detail of the object representation is a generally good idea.
Doug

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