C++ Div3、Sqrt 函数高性能实现（带汇编指令集）

均采用魔法数字（Magic Number）实现，一个是经典求平方根函数所使用的魔法数字：0x5f375a86、0x5f3759df。

float Sqrt(float x) noexcept { /* 0x5f3759df */
    float xhalf = 0.5f * x;
    int32_t i = *(int32_t*)&x;
    i = 0x5f375a86 - (i >> 1);
    x = *(float*)&i;
    x = x * (1.5f - xhalf * x * x);
    return x;
}

unsigned int Div3(unsigned int i) noexcept {
    // AT&T:
    // movl    $2863311531, %edx
    // imulq   %rcx, %rdx
    // shrq    $33, %rdx

    // INTEL:
    // mov     edx, 2863311531
    // imul    rdx, rcx
    // shr     rdx, 33

    unsigned long long n = static_cast(i) * 2863311531;
    unsigned int r = static_cast(n >> 33);
    return r;
}

unsigned long long Div3(unsigned long long i) noexcept {
    // INTEL:
    // mov     rax, -6148914691236517205; 
    // mul     rcx
    // shr     rdx, 1

    int64_t rax = -6148914691236517205;
    int64_t rdx = (rax * i) >> 1;
    return rdx;
}