memmove

if (d==s) return d;
if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);

s - d - n mod p <= -2n mod p

s - d -n < 0   
s - d -n <= -2n 

   s < d + n
s  <= d -n

s|     |d    |

0 <= s - d -n <= p - 2n
s - d >= n
|     |d    |s