CheckSum计算的优化

  最近在纠结于大数据的checksum的计算,算法如下:

 1 unsigned long cs_cal(const unsigned char *buff, unsigned long size)
 2 {
 3     unsigned long cs;
 4 
 5     cs = 0;
 6     while (size--)
 7         cs += buff[size];
 8 
 9     return cs;
10 }

  当文件大于4G后这种计算的龟速就很明显了,在网上看到大牛说MMX指令对于数据计算相当不俗,于是改成下面代码(bug:不对齐部分没有处理,这部分消耗很小,对于效率影响不大):

 1 unsigned long cs_cal(const unsigned char *buff, unsigned long size)
 2 {
 3     unsigned long cs;
 4     unsigned long cs_2[2];
 5     unsigned long cnt;
 6 
 7     cnt = size / 2;
 8 __asm {
 9     mov    eax, 0
10     movd    mm2, eax
11     mov    ecx, cnt
12     mov    esi, buff
13 cnt_label:
14     mov     al, byte ptr[esi]
15     movd    mm0, eax
16     inc    esi
17 
18     mov     al, byte ptr[esi]
19     movd    mm1, eax
20     inc    esi
21 
22     punpckldq mm0, mm1
23     
24     paddd mm2, mm0
25 
26     dec    ecx
27     jnz    cnt_label
28 
29     movq    qword ptr [cs_2], mm2
30 
31 
32     emms
33 }
34     cs = cs_2[0] + cs_2[1];
35 #endif
36     return cs;
37 }

  测试1G的内存数据发现速度近快了2倍,MMX的威力果然不是吹的.

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