#1824: Change locatives representation

[Chicken Trac]

#1824: Change locatives representation
----------------------------+---------------------------------
 Reporter:  sjamaan         |                 Owner:  sjamaan
     Type:  enhancement     |                Status:  assigned
 Priority:  minor           |             Milestone:  6.0.0
Component:  core libraries  |               Version:  5.3.0
 Keywords:                  |  Estimated difficulty:  medium
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 As pointed out [https://lists.nongnu.org/archive/html/chicken-
 hackers/2023-06/msg00029.html here], we can change the representation of
 locatives such that they take up less space, and we only have to traverse
 the live '''weak''' locatives for update during GC.

 The idea is to drop the extra "object" slot, and store it in the pointer
 slot instead. When the locative is weak, set {{{C_SPECIALBLOCK_BIT}}}.
 When it is strong, do not set it (so that the object is referenced
 normally). For weak locatives, we traverse the live ones using the
 "chained" approach and update the object.

 We don't need to do the pointer recalculation on every GC.  Instead, we
 calculate the pointer on-the-fly during {{{locative_ref}}}.

 NOTE: Perhaps we also want to expose a version of locative-ref that
 doesn't error out when the object has been collected, but returns {{{bwp-
 object}}} instead? This means handling of such locatives is a bit simpler
 and doesn't require exception handling.

-- 
Ticket URL: <https://bugs.call-cc.org/ticket/1824>
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