FA-90437 / Garbage collector invariants / Member archive

Sliding compaction: dead objects still advance the compaction pointer · case 02

Live objects keep the gaps left by dead objects and the heap never shrinks.

Member previewVariant 2 · 3 implementations · 6 checks per implementation

Case contract

Mark-compact with sliding. objs are [start, size, fields] in arbitrary order; pointers (fields and roots) may point anywhere inside an object and belong to the object whose [start, start+size) contains them; a pointer outside every object is ["dangling-pointer", p]. Mark from the roots, assign new addresses to live objects in address order starting at base with no gaps, and relocate every pointer to new start + original offset. Return the new layout [new, old, fields], new roots and the new top.

Why this case matters

Sliding compaction must preserve address order and interior offsets while squeezing out dead space.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: interior pointers relocate with their object	`{"layout": [[100, 100, [134, null]], [124, 124, [104]]], "roots": [100], "top": 176}`	`{"layout": [[100, 100, [126, null]], [116, 124, [104]]], "roots": [100], "top": 140}`	Failed

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