FA-89920 / Bytecode virtual machines / Member archive

Stack-depth verifier: negative targets index from the end · case 05

A branch to -1 verifies the last instruction instead of being rejected.

Member previewVariant 5 · 3 implementations · 10 checks per implementation

Case contract

code is a list of [op, arg]. Stack effects (required, delta): push (0,+1), pop (1,-1), dup (1,+1), dup_x1 (2,+1), swap (2,0), add (2,-1), ifeq (1,-1; successors fallthrough and arg), goto (0,0; only arg), return (1,-1; none). Dataflow from index 0 with depth 0: report ["underflow", i], ["falls-off", i] for a successor outside the code, ["inconsistent", s] when a merge sees two depths, else ["ok", max depth]. Unreachable code is never examined.

Why this case matters

Bytecode verifiers must agree exactly on stack effects and control-flow successors.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
negative branch target	`["ok", 1]`	`["falls-off", 9]`	Failed

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