float InvSqrt(float x)
{
    float xhalf = 0.5f*x;
    int i = *(int*)&x;       // get bits for floating value
    i = 0x5f3759df - (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;
}

转自：https://www.zhihu.com/question/37692782/answer/73323805