A built-in memset() for the hc08 and s08 ports would
1) Increase code speed
2) Reduce code size
3) Allow making the string functions standard-compliant, since the common cases would take the fast built-in path
A built-in for constant c and n at most 256 (the most common case) would a as simple as
mov ,x+, c
which is just four bytes, much less than the call overhead for a call to memset().
Some more information can be found in my post titled "A proposal for parameter c in the string functions" to sdcc-devel today.