data.length opcode

[SoniEx2]

we should have data.length opcode

(it should return 0 for dropped segments ofc)

[SoniEx2]

for context: memory.init (already) needs to know the data segment length so it can trap, but trapping isn't particularly useful for programs. a data.length would expose the length in an useful way (non-trapping).

a hypothetical single-pass compiler could use a data.length opcode for generated data segments, since data segments come after code segments. such data.length opcode also happens to be oddly convenient when hand-writing wasm/wat.