FA-90790 / Garbage collector invariants / Member archive

TAMS: watermark aliases the live allocation top · case 05

TAMS moves with allocation, so objects allocated during marking must be marked or are freed.

Member previewVariant 5 · 3 implementations · 6 checks per implementation

Case contract

Concurrent marking with per-region top-at-mark-start (TAMS). alloc r size bump-allocates in region r (addresses r*region_size onward; None if it would pass the region end). start snapshots every region's current top as its TAMS (regions never allocated in have TAMS at their bottom) and clears marks. mark addr marks an object. end frees, in address order, every object below its region's TAMS that is not marked; objects at or above TAMS were allocated during marking and are live. Return allocation addresses and the freed list of each end.

Why this case matters

Snapshot collectors treat objects allocated during marking as live using a per-region watermark.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: objects above TAMS are implicitly live	`[0, 30, 100, 55, 200, [30, 55, 100, 200]]`	`[0, 30, 100, 55, 200, [30, 100]]`	Failed

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