Fast memset() and memcpy() functions for AVR.
The fmemset() and fmemcpy() functions compile down to unrolled loops of load/store instructions, which on the AVR 8 bit archtiecture is the fastest possible way to implement these functions. No loops are used at all, trading speed for code size.
MAX_UNROLL sets the maximum number of iterations to be unrolled, after which memset() and memcpy() will be used instead. If you increase it beyond 511 you will need to add additional power of two unrolled loops to the code.
The avr-libc functions use loops and are significantly slower, but also significantly smaller.