这是sqrt函数中的一个错误(is this a bug in sqrt function)

 我创建了一个应用程序来计算64位范围内的素数，所以当我尝试使用math.h sqrt函数来计算64位数的sqrt ，我发现答案不准确，例如当输入~0ull答案应该是~0u但我得到的是0x100000000这是不正确的，所以我决定使用汇编x86语言创建我自己的版本，看看这是否是一个错误，这里是我的功能： 
 
inline unsigned prime_isqrt(unsigned long long value)
{
    const unsigned one = 1;
    const unsigned two = 2;

    __asm
    {
        test dword ptr [value+4], 0x80000000
        jz ZERO
        mov eax, dword ptr [value]
        mov ecx, dword ptr [value + 4]

        shrd eax, ecx, 1
        shr  ecx, 1
        mov  dword ptr [value],eax 
        mov  dword ptr [value+4],ecx

        fild value
        fimul two
        fiadd one
        jmp REST
ZERO: 
        fild value
REST: 
        fsqrt
        fisttp value
        mov eax, dword ptr [value]
    }
}
 
 输入是一个奇数来得到它的平方根。 当我测试我的功能相同的输入结果是一样的。 
 
 我没有得到的是为什么这些函数围绕结果或具体为什么sqrt指令围绕结果？ 

I create an application to compute the primes numbers in 64-bit range so when i tried to compute the square root of 64-bit number using sqrt function from math.h i found the answer is not accurate for example when the input is ~0ull the answer should be ~0u but the one I get is 0x100000000 which is not right, so i decided to create my own version using assembly x86 language to see if this is a bug, here is my function:
 
inline unsigned prime_isqrt(unsigned long long value)
{
    const unsigned one = 1;
    const unsigned two = 2;

    __asm
    {
        test dword ptr [value+4], 0x80000000
        jz ZERO
        mov eax, dword ptr [value]
        mov ecx, dword ptr [value + 4]

        shrd eax, ecx, 1
        shr  ecx, 1
        mov  dword ptr [value],eax 
        mov  dword ptr [value+4],ecx

        fild value
        fimul two
        fiadd one
        jmp REST
ZERO: 
        fild value
REST: 
        fsqrt
        fisttp value
        mov eax, dword ptr [value]
    }
}
 
the input is an odd number to get its square root. When i test my function with same input the result was the same.
 
What i don't get is why those functions round the result or to be specific why sqrt instruction round the result?

原文：https://stackoverflow.com/questions/10815783