An annoying way to copy a region. Don't try this anymore.

test.c

#include <sys/types.h>

void copy (char *dst, char *src, size_t size) {
    if (size == 0) {
        return ;
    }
    size_t sz = size / 4 ;
    switch (size % 4) {
        while (sz-- != 0) {
    default:
            *dst++ = *src++ ;
    case 3:
            *dst++ = *src++ ;
    case 2:
            *dst++ = *src++ ;
    case 1:
            *dst++ = *src++ ;
        }
    }
}