Here is how it should be done with PADDB: http://pastebin.com/MY9tENpW http://pastebin.com/MY9tENpW This is 20% faster than the author's version on my computer:

Here is how it should be done with PADDB: http://pastebin.com/MY9tENpW http://pastebin.com/MY9tENpW This is 20% faster than the author's version on my computer: 0.210 sec vs. 0.260 sec to process 1GiB. The tight loop is simple: 400710: 66 0f fc 04 07 paddb (%rdi,%rax,1),%xmm0 400715: 48 83 c0 10 add $0x10,%rax 400719: 48 39 c6 cmp %rax,%rsi 40071c: 77 f2 ja 400710 <sum_array+0x10> Compare this to the author's complex version: 400720: 66 0f 6f 14 07 movdqa (%rdi,%rax,1),%xmm2 400725: 48 83 c0 10 add $0x10,%rax 400729: 48 39 c6 cmp %rax,%rsi 40072c: 66 0f 6f c2 movdqa %xmm2,%xmm0 400730: 66 0f 68 d4 punpckhbw %xmm4,%xmm2 400734: 66 0f 60 c4 punpcklbw %xmm4,%xmm0 400738: 66 0f f5 d1 pmaddwd %xmm1,%xmm2 40073c: 66 0f f5 c1 pmaddwd %xmm1,%xmm0 400740: 66 0f fe c3 paddd %xmm3,%xmm0 400744: 66 0f fe c2 paddd %xmm2,%xmm0 400748: 66 0f 6f d8 movdqa %xmm0,%xmm3 40074c: 77 d2 ja 400720 <sum_array+0x20>