FA-90469 / Garbage collector invariants / Member archive

Free-list sweep: split block keeps the allocated address · case 04

The next allocation is handed the same address again.

Member previewVariant 4 · 3 implementations · 6 checks per implementation

Case contract

blocks are [address, size, marked] covering the heap in any order. Sweep in address order: unmarked blocks become free and physically adjacent free blocks coalesce. Then serve requests first-fit in address order: round the request up to 8 bytes; split the chosen block when the remainder is at least 16 bytes (the allocation takes the low end), otherwise hand out the whole block; no fit yields None. Return the swept free list (as it was right after sweeping), the allocation addresses and the final free list.

Why this case matters

Sweepers and free-list allocators must keep block boundaries exact or they hand out overlapping memory.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: sweep, split and exhaust	`{"alloc": [4112, 4112, 4168, 4168, null], "free": [], "swept": [[4112, 48], [4168, 64]]}`	`{"alloc": [4112, 4136, 4168, 4208, null], "free": [], "swept": [[4112, 48], [4168, 64]]}`	Failed

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