あ、ついでに無知なQZに教えといてやるよ

http://stackoverflow.com/questions/2397984/undefined-unspecified-and-implementation-defined-behavior

§1.3.10 [defns.impl.defined]
implementation-defined behavior
behavior, for a well-formed program construct and correct data, that depends on the implementation
and that each implementation documents

これが処理系依存だ