float Q_rsqrt( float number ){　　long i;　　float x2, y;　　const float threehalfs = 1.5F;　　x2 = number * 0.5F;　　y = number;　　i = * ( long * ) &y; // evil floating point bit level hacking　　i = 0x5f3759df - ( i >> 1 ); // what the fuck?　　y = * ( float * ) &i;　　y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration　　// y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed　　#ifndef Q3_VM　　#ifdef __linux__　　assert( !isnan(y) ); // bk010122 - FPE?　　#endif　　#endif　　return y;}

 

float InvSqrt(float x){    float xhalf = 0.5f*x;    int i = *(int*)&x; // get bits for floating VALUE    i = 0x5f375a86- (i>>1); // gives initial guess y0    x = *(float*)&i; // convert bits BACK to float    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy    return x;}

参考：

http://www.matrix67.com/data/InvSqrt.pdf

http://www.guokr.com/post/90718/