A few months ago, support for such declarations was improved, though not fully implemented yet. At least we now get a proper error message:

hilipp@notebook6:/tmp$ /home/philipp/sdcc-trunk/sdcc/src/sdcc -I/home/philipp/sdcc-trunk/sdcc/device/include test.c
test.c:2: warning 283: function declarator with no prototype
test.c:4: warning 283: function declarator with no prototype
test.c:6: error 9: FATAL Compiler Internal Error in file 'SDCCast.c' line number '983' : Setting of register parameter vs. other parameter not yet implemented for functions without prototype. 
Contact Author with source code