我有一个简单的C#和C++代码,可以计算点积的总和。
C#代码如下:
using System;
namespace DotPerfTestCS
{
class Program
{
struct Point3D
{
public double X, Y, Z;
public Point3D(double x, double y, double z)
{
X = x;
Y = y;
Z = z;
}
}
static void RunTest()
{
unchecked
{
const int numPoints = 100000;
const int numIters = 100000000;
Point3D[] pts = new Point3D[numPoints];
for (int i = 0; i < numPoints; i++) pts[i] = new Point3D(i, i + 1, i + 2);
var begin = DateTime.Now;
double sum = 0.0;
var u = new Point3D(1, 2, 3);
for (int i = 0; i < numIters; i++)
{
var v = pts[i % numPoints];
sum += u.X * v.X + u.Y * v.Y + u.Z * v.Z;
}
var end = DateTime.Now;
Console.WriteLine("Sum: {0} Time elapsed: {1} ms", sum, (end - begin).TotalMilliseconds);
}
}
static void Main(string[] args)
{
for (int i = 0; i < 5; i++) RunTest();
}
}
}
而且C++是
#include <iostream>
#include <vector>
#include <time.h>
using namespace std;
typedef struct point3d
{
double x, y, z;
point3d(double x, double y, double z)
{
this->x = x;
this->y = y;
this->z = z;
}
} point3d_t;
double diffclock(clock_t clock1,clock_t clock2)
{
double diffticks=clock1-clock2;
double diffms=(diffticks*10)/CLOCKS_PER_SEC;
return diffms;
}
void runTest()
{
const int numPoints = 100000;
const int numIters = 100000000;
vector<point3d_t> pts;
for (int i = 0; i < numPoints; i++) pts.push_back(point3d_t(i, i + 1, i + 2));
auto begin = clock();
double sum = 0.0, dum = 0.0;
point3d_t u(1, 2, 3);
for (int i = 0; i < numIters; i++)
{
point3d_t v = pts[i % numPoints];
sum += u.x * v.x + u.y * v.y + u.z * v.z;
}
auto end = clock();
cout << "Sum: " << sum << " Time elapsed: " << double(diffclock(end,begin)) << " ms" << endl;
}
int main()
{
for (int i = 0; i < 5; i++) runTest();
return 0;
}
C#版本(启用优化的Release x86版本,x64更慢)的输出结果为:
Sum: 30000500000000 Time elapsed: 551.0299 ms
Sum: 30000500000000 Time elapsed: 551.0315 ms
Sum: 30000500000000 Time elapsed: 552.0294 ms
Sum: 30000500000000 Time elapsed: 551.0316 ms
Sum: 30000500000000 Time elapsed: 550.0315 ms
使用C++(默认的VS2010发布版本设置)会产生如下结果:
Sum: 3.00005e+013 Time elapsed: 4.27 ms
Sum: 3.00005e+013 Time elapsed: 4.27 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms
我本来期望C#代码会慢一些,但是慢了130倍对我来说太多了。请问有人能解释一下发生了什么吗?
编辑
我不是C++程序员,只是从互联网上随便找了一个diffclock代码而没有真正检查它是否正确。
使用std::difftime,C++的结果是
Sum: 3.00005e+013 Time elapsed: 457 ms
Sum: 3.00005e+013 Time elapsed: 452 ms
Sum: 3.00005e+013 Time elapsed: 451 ms
Sum: 3.00005e+013 Time elapsed: 451 ms
Sum: 3.00005e+013 Time elapsed: 451 ms
这似乎是正确的。
DateTime.Now
,建议使用Stopwatch
类。尽管我怀疑这是造成差异的原因。 - DeCaf