Hello Markus.

Since the variable is not returned through the routine the preprocessor assumes it does nothing so it removes it.

If there was an actual use inside the routine that wouldn't happen.

If you don't use the variable for doing something other than just changing its value, like sendind it through the serial port the preprocessor will always remove it to relief the mcu from executing useless instructions.

If you declare the variable as a global variable that will surely work since a global variable is accessed by many routines and therefore by removing any references to it, that would cause you many problem (program not functioning).

You should try to ask for more help at the SDCC's mailing list. You can locate more info on the mailing list at the SDCC homepage.