The Z80 and some derivatives support complex instructions, such as ldir, cpir, ... . SDCC only emits these instructions for functions in the standard library. Thus, e.g. copying one array into another is more efficient when using memcpy() than by using a a user-written loop.
Depending on the target, code generation options and the parameters to the call, SDCC emits ldir for memcpy(), ldir or lsidr for memset(), ldi for strcpy(), ldi for strncpy(). Other library functions use the complex instructions as well, but for those, function calls are generated.