Fast Square Root Challenge

Design a fast square root function which computes double floating point numbers faster than SQRTSD(SSE2). Bonus for precision.

double sqrts[18446744073709551616] = {
 /* the contents of this array are left as an exercise for the reader */
double fastsqrt(double f) {
 int64_t *idx = (int64_t*)&f;
 return sqrts[*idx];
}