FA-90799 / Garbage collector invariants / Member archive

Oop maps: array length word traced as an element · case 04

The length of an object array is treated as a reference and the last element is skipped.

Member previewVariant 4 · 3 implementations · 6 checks per implementation

Case contract

Precise tracing with per-class reference maps. Objects are word lists: words 0-1 are the header; object arrays keep their length in word 2 and elements in words 3..3+len-1 (anything after is padding); primitive arrays contain no references. Instance classes list [offset, count] runs of reference words and inherit all superclass runs. A reference word of 0 is null; a nonzero reference that is not an object id is reported as [object, word]. Return the reachable ids and the bad references.

Why this case matters

Tracing must follow exactly the reference words described by type metadata, no more and no less.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: trace through fields, arrays and subclasses	`{"bad": [[102, 2]], "live": [100, 101, 102, 103]}`	`{"bad": [], "live": [100, 101, 102, 103, 104, 107]}`	Failed

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