FA-90698 / Garbage collector invariants / Member archive

Block offset table: back-skips always point one card back · case 03

Lookups deep inside large objects take one hop per card and the table encoding differs from the contract.

Member previewVariant 3 · 3 implementations · 6 checks per implementation

Case contract

A contiguous heap of [start, size] objects is covered by cards of the given size. For each card the table stores the offset of the first object that starts inside the card; a card with no object start that is covered by an object from card c stores -(k - c), a direct back-skip to that card. To find the object containing address q: from q's card follow back-skips; if the card's first start lies after q, move to the previous card; then walk objects forward until one contains q (an entry that does not land on an object start yields corrupt-table). Return the table and the owner start for each query.

Why this case matters

Card scanning needs to locate object starts quickly; a stale or wrong table misparses the heap.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: owners across spanning objects	`{"owners": [64, 184, 384], "table": [0, 0, 36, -1, -1, -1, 0, -1]}`	`{"owners": [64, 184, 384], "table": [0, 0, 36, -1, -2, -3, 0, -1]}`	Failed

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