> No, it's literally undecidable in principle whether every bit of assembler correctly restores some register R. For any given bit of inline assembler, it's quite likely to be trivial.

Exactly. And if it's non-trivial to decide something that basic, you're doing it wrong. Saving and restoring all the registers is always an option. Only saving and restoring some of them is an optimization which must be shown to be sound.