Actually, register tells the compiler that the variable does not alias with anything else in the program (not even char's).
That can be exploited by modern compilers in a variety of situations, and can help the compiler quite a bit in complex code - in simple code the compilers can figure this out on their own.
Otherwise, it serves no purpose and is not used for register allocation. It does not usually incur performance degradation to specify it, as long as your compiler is modern enough.
