Such cases are rare bugs in a small amount of code — mostly just compilers and hand-written assembly. Preemptively saving and restoring all registers in all ca

Such cases are rare bugs in a small amount of code — mostly just compilers and hand-written assembly. Preemptively saving and restoring all registers in all callers would appreciably slow down every single function call on every device in the world using that ABI. The cumulative cost would be astronomical.