keyword that not save interrupt context

To be sure I understand correctly, I will restate what you want.  You want to write an interrupt function in C.  The return sequence must be the one used for interrupts.  No matter what registers are modified, none of them are to be preserved.  I presume this description is correct.

There is no compiler feature which supports that.  Such a function has to be written in hand-coded assembly.

Thanks and regards,

-George