Callee保存，调用者传递使用过的寄存器？

In compiler design, why instead of having a caller or callee register saving arrangement, couldn't the caller pass its list of used registers (that it would push in case of a caller saving arrangement) to the callee so that the callee can compare its list of used registers to the registers used by the caller. Then only the registers that really need to be pushed, would be pushed. Am I missing something?In compiler design, why instead of having a cal