学步园

参考“平凡的程序员”的博客文章，自己做了一定修改。

传送门http://blog.csdn.net/feixiaoxing/article/list/2

注意：我这里讨论的是ARM平台上的嵌入式C语言，与PC机上的运行结果有一定出入。其中有些结果是完全相反的。

编译环境：Fedora14  arm-linux-gcc 4.4.3

程序运行环境：mini2440  Linux2.6.32

编程规则1：二维数据先内层，再外层

理由：系统中二维数组采用行续优先的存储方式存储

main.c

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <netinet/in.h>

int getTickCount()
{
    struct timeval current;
    int currentTime;
    gettimeofday(&current, NULL);
    currentTime = current.tv_sec * 1000000 + current.tv_usec;
    return currentTime;
}

int main()
{
    int beginTime, endTime;
    int i, j;
    int data[1000][1000];

    beginTime = getTickCount();
    for (i = 0; i < 1000; i++)
        for (j = 0; j < 1000; j++)
            data[i][j] = 12;
    endTime = getTickCount();
    printf("duration = %d \n", endTime - beginTime);

    beginTime = getTickCount();
    for (i = 0; i < 1000; i++)
        for (j = 0; j < 1000; j++)
            data[j][i] = 12;
    endTime = getTickCount();
    printf("duration = %d \n", endTime - beginTime);

}

运行结果：

[root@ShiGuang /home]# ./arm-test 
duration = 131362 
duration = 628395 
[root@ShiGuang /home]#

编程规则2：尽量少使用乘除，多使用移位运算

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <netinet/in.h>

int getTickCount()
{
    struct timeval current;
    int currentTime;
    gettimeofday(¤t, NULL);
    currentTime = current.tv_sec * 1000000 + current.tv_usec;
    return currentTime;
}

int main()
{
    int beginTime, endTime;
    int i, j;
    int data1 = 65536;
    int data2 = 65536;
    int result;

    beginTime = getTickCount();
    for (i = 0; i < 1000; i++)
        for (j = 0; j < 1000; j++)
            result = data1 / 16 + data2 * 4;
    endTime = getTickCount();
    printf("duration = %d \n", endTime - beginTime);

    beginTime = getTickCount();
    for (i = 0; i < 1000; i++)
        for (j = 0; j < 1000; j++)
            result = data1 >> 4 + data2 << 2;
    endTime = getTickCount();
    printf("duration = %d \n", endTime - beginTime);

}

运行结果：

[root@ShiGuang /home]# ./arm-test 
duration = 62228 
duration = 52170 
[root@ShiGuang /home]#