Duff's Device —— 一种优化的复制方法

2011-05-19 wcdj

 

// Duff's Device void 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));// 12345678 printf("%s/n", (char *)my_send(szSrc+1, szSrc, 1));// 1345678 printf("%s/n", (char *)my_send(szSrc+1, szSrc, 2));// assert return 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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