FA-90118 / Bytecode virtual machines / Member archive

Structured control: branching to a loop pops the loop label · case 03

After a continue-style branch the label stack is one level short, so later branches exit the wrong construct.

Member previewVariant 3 · 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": [2, 7], "stack": [], "status": "done"}`	`{"locals": [0, 10], "stack": [], "status": "done"}`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.