FA-90430 / Garbage collector invariants / Member archive

Cheney copy: exactly filling to-space reported as overflow · case 05

A heap whose survivors fit to-space exactly fails with out-of-memory.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

Breadth-first semispace copying. heap maps from-space address -> {size, fields}. Roots are copied in order, then a scan pointer walks to-space copying each referenced object exactly once (forwarding addresses are reused) and rewriting fields to to-space addresses. Copies are bump-allocated from base with sizes rounded up to 8 bytes; if a copy would end beyond limit, return ["out-of-memory", copies so far]. Return new roots, the to-space layout [new, old, fields] and the final free pointer.

Why this case matters

Copying collectors rely on forwarding pointers and the scan/free invariant to preserve sharing.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
to-space exactly full	`["out-of-memory", 3]`	`{"free": 1072, "layout": [[1000, 400, [1024]], [1024, 200, [1048]], [1048, 300, [1056, null]], [1056, 100, [1024, 1048]]], "roots": [1000]}`	Failed

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