rsqrtfast

float rsqrtf(const float v){
    union {
        float vh;
        int i0;
    };

    union {
        float vr;
        int i1;
    };

    vh = v * 0.5f;
    i1 = 0x5f3759df - (i0 >> 1);
    return vr * (1.5f - vh * vr * vr);
}