FA-90287 / Bytecode virtual machines / Member archive

Wordcode prefixes: prefix leaks into following instructions · case 02

Every instruction after a prefixed one gets an inflated argument.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

Two-byte wordcode (opcode, arg). EXTENDED_ARG (144) prefixes build wider arguments: each prefix ORs its byte into an accumulator that is then shifted left 8; the real instruction ORs its own byte in and resets the accumulator. Each decoded instruction reports [start of its first prefix, own offset, name, full arg, jump target in bytes]: JUMP_FORWARD (110) targets the next instruction plus 2*arg, JUMP_ABSOLUTE (113) targets 2*arg. A prefix at the end reports ["dangling-extended-arg", start]. A trailing odd byte is ignored.

Why this case matters

Instruction prefixes change argument width and jump targets in wordcode interpreters.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: prefixed absolute jump	`[[0, 0, "LOAD_CONST", 2, null], [2, 4, "JUMP_ABSOLUTE", 260, 520], [6, 6, "RETURN_VALUE", 256, null]]`	`[[0, 0, "LOAD_CONST", 2, null], [2, 4, "JUMP_ABSOLUTE", 260, 520], [6, 6, "RETURN_VALUE", 0, null]]`	Failed

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