酷壳上的一道面试题

又一个有趣的面试题里看到的一道题目，原因看过《深入计算机体系结构》应该就能明白。

题目如下

有两个相同功能代码如下，请在在A，B，C是什么的情况下，请给出三个原因case 1比case 2快，还有三个原因case 2会比case 1要执行的快。（不考虑编译器优化）

case 1

for (i=0; i<N; ++i){ A; B; C; }

case 2

for (i=0; i<N; ++i){ A; } for (i=0; i<N; ++i){ B; } for (i=0; i<N; ++i){ C; }

我的解法

 

int length = 10000;

int[,] a = new int[length , length ];

int[,] b = new int[length, length];

int[,] c = new int[length, length];

Stopwatch watch = new Stopwatch();

watch.Start();

for (int i = 0; i < length; i++)

{

int temp = 0;

temp = a[i, i];

temp = b[i, i];

temp = c[i, i];

}

watch.Stop();

Console.WriteLine(“case 1:” + watch.ElapsedMilliseconds);

watch.Restart();

for (int i = 0; i < length; i++)

{

int temp = 0;

temp = a[i, i];

}

for (int i = 0; i < length; i++)

{

int temp = 0;

temp = b[i, i];

}

for (int i = 0; i < length; i++)

{

int temp = 0;

temp = c[i, i];

}

watch.Stop();

Console.WriteLine(“case 2:” + watch.ElapsedMilliseconds);