Duff's Device —— 一种优化的复制方法

2011-05-19 wcdj

 

// Duff's Devicevoid send(char*to, char* from, int count){int n = (count+7)/8;switch(count%8){case 0:do { *to++ = *from++;case 7: *to++ = *from++;case 6: *to++ = *from++;case 5: *to++ = *from++;case 4: *to++ = *from++;case 3: *to++ = *from++;case 2: *to++ = *from++;case 1: *to++ = *from++;} while(--n > 0);}}

 

达夫设备的思路：
把原数组以8个 int 为单位分成若干个小组，复制的时候以小组为单位复制，即一次复制8个 int 。也就是说，在传统的memcpy函数中以一次比较运算的代价换来1个int的复制，而在达夫设备中，却能以一次比较运算的代价换来8个int的复制。而switch语句则是用来处理分组时剩下的不到8个的int（这些剩余的不是数组最后的，而是数组最开始的），很巧妙。

 

#include <cstdio>#include <cassert>void* my_send(void *to, const void *from, size_t count);int main(){char szSrc[]="12345678";char szDst[]="abcdefgh";printf("%s/n", (char *)my_send(szDst, szSrc, 8));// 12345678printf("%s/n", (char *)my_send(szSrc+1, szSrc, 1));// 1345678printf("%s/n", (char *)my_send(szSrc+1, szSrc, 2));// assertreturn 0;}void* my_send(void *to, const void *from, size_t count){char *pbTo = (char *)to;char *pbFrom = (char *)from;assert((pbFrom != NULL) && (pbTo != NULL));assert((pbTo >= pbFrom+count) || (pbFrom >= pbTo+count));if (count == 0) return to;/*while (count-- > 0) { *pbTo++ = *pbFrom++; }*/int n = (count+7)/8;switch(count%8){case 0:do {*pbTo++ = *pbFrom++;case 7: *pbTo++ = *pbFrom++;case 6: *pbTo++ = *pbFrom++;case 5: *pbTo++ = *pbFrom++;case 4: *pbTo++ = *pbFrom++;case 3: *pbTo++ = *pbFrom++;case 2: *pbTo++ = *pbFrom++;case 1: *pbTo++ = *pbFrom++;} while(--n > 0);}return to;}

 

 

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