为什么 C# double 算法似乎比 long 算法更快？

Question

以下代码令人惊讶的输出显示 double 运算比 long 运算快几乎 100%：

Test_DivOperator Float arithmetic measured time: 15974.5024 ms.

Test_DivOperator Integer arithmetic measured time: 28548.183 ms.

使用的构建设置是.Net4.5 C# 5.0（平台目标：x64）

使用的硬件是Intel Core i5-2520M (运行 Windows7 64Bit)

注意：使用的运算符（这里是除法）确实会影响结果，除法最大化这个观察

const int numOfIterations = 1; //this value takes memory access out of the game
const int numOfRepetitions = 500000000; //CPU bound application
Random rand = new Random();
double[] Operand1 = new double[numOfIterations];
double[] Operand2 = new double[numOfIterations];
double[] Operand3 = new double[numOfIterations];

long[] Int64Operand1 = new long[numOfIterations];
long[] Int64Operand2 = new long[numOfIterations];
long[] Int64Operand3 = new long[numOfIterations];

for (int i = 0; i < numOfIterations; i++)
{
    Operand1[i]=(rand.NextDouble() * 100);
    Operand2[i]=(rand.NextDouble() * 80);
    Operand3[i]=(rand.NextDouble() * 17);
    Int64Operand1[i] = (long)Operand1[i];
    Int64Operand2[i] = (long)Operand2[i]+1;
    Int64Operand3[i] = (long)Operand3[i]+1;
}

double[] StdResult = new double[numOfIterations];
long[] NewResult = new long[numOfIterations];

TimeSpan begin = Process.GetCurrentProcess().TotalProcessorTime;

for (int j = 0; j < numOfRepetitions; j++)
{
    for (int i = 0; i < numOfIterations; i++)
    {
        double result = Operand1[i] / Operand2[i];
        result = result / Operand3[i];
        StdResult[i]=(result);
    }

}

TimeSpan end = Process.GetCurrentProcess().TotalProcessorTime;
Console.WriteLine("Test_DivOperator Float arithmetic measured time: " + (end - begin).TotalMilliseconds + " ms.");

begin = Process.GetCurrentProcess().TotalProcessorTime;

for (int j = 0; j < numOfRepetitions; j++)
{
    for (int i = 0; i < numOfIterations; i++)
    {
        long result =    Int64Operand1[i] / Int64Operand2[i];
        result = result / Int64Operand3[i];
        NewResult[i]=(result);
    }

}

end = Process.GetCurrentProcess().TotalProcessorTime;
Console.WriteLine("Test_DivOperator Integer arithmetic measured time: " + (end - begin).TotalMilliseconds + " ms.");

Answer 1

这并不意外。 64 位整数除法就是这么慢。

您的处理器是 Sandy Bridge，观察 table 的延迟和吞吐量，64 位 idiv 的延迟比 divsd 高得多，吞吐量差得多。

其他微体系结构显示出类似的差异。

做实际的数学计算，每次迭代 2.8548183E10ns / 500000000 = 57ns，频率为 3.2GHz，大约 183 个周期，有两个除法和一些额外的开销，所以这并不奇怪。

对于双打，它达到了 32ns，102 个周期，实际上比我预期的要多。

为什么 C# double 算法似乎比 long 算法更快？

Why C# arithmetic on double appears to be faster than arithmetic on long?

c#

floating-point

double

performance

long-integer