Using -mcall-prologues generates several problems on ATMEGA2561
One gets hundreds errors like
G:\ATMEGA/usart.h(831,1): internal error: out of range error
Yet the project is buildign itself OK.
The binary itself hangs easily when calling those functions.
apparently for all functions above 128KB limit.
same story in STUDIO4.
This option is an important size saver, therefore it is likely to be used when there are no alternatives, that is, exactly for 256KB devices...