FA-90098 / Bytecode virtual machines / Member archive

Upvalues: reads decide by slot existence instead of open state · case 03

A closure reads whatever local now occupies its old slot.

Member previewVariant 3 · 3 implementations · 7 checks per implementation

Case contract

A register/stack VM with open and closed upvalues. ["local", v] pushes a slot; ["set", s, v] and ["get", s] access slots; ["closure", id, [slots]] captures slots, reusing the existing open upvalue for a slot so closures share variables; ["uset"/"uget", id, k, (v)] access the k-th upvalue, going to the stack slot while open and to the upvalue's own cell once closed; ["close", level] closes every open upvalue whose slot >= level (copying the slot value) and truncates the stack to level. Accessing a vanished slot appends "dangling-slot" and stops.

Why this case matters

Closure capture and upvalue closing are subtle lifetime transitions in register-based VMs.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: new local reusing a closed slot	`[8, 8, 0, 80, 80]`	`[8, 8, 0, 80, 0]`	Failed

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