FA-90109 / Bytecode virtual machines / Member archive

Structured control: branch depth counted from the outermost label · case 04

br_if 1 inside a loop jumps back to the loop instead of leaving the enclosing block, so loops never end.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

WebAssembly-style structured control over a value stack. block k / loop / end nest; each block or loop pushes a label recording its operand height and arity (block: k result values, loop: 0). br d targets the d-th enclosing label counting outward from 0; the branch keeps the top arity values, discards operands down to the label height, then for a block pops that label and continues after its end, for a loop keeps the loop label and continues at the first instruction inside the loop. br_if pops a condition and branches when it is non-zero. Other ops: const v, add, sub, eqz, get k, set k, drop. Each instruction costs one fuel. Return status (done/fuel/trap), stack and locals.

Why this case matters

Label-depth branching is the core of structured bytecode interpreters.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: counted loop exits through outer block	`{"locals": [4, 5], "stack": [], "status": "done"}`	`{"locals": [0, 15], "stack": [], "status": "done"}`	Failed

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