danluu knows much more about the subject matter than me. Meanwhile...

If I take the main.cpp from http://www.strchr.com/media/crc32_popcnt.zip that he comparing and paste it into a new, default, VC2010 project. Then compile it as a Release build, then the mixed disassembly for the inner loop of POPCNT_HardwareSubbuN() looks like

		    cnt += __popcnt(*(DWORD*)buf) +
			       __popcnt(*(DWORD*)(buf + sizeof(DWORD) )) +
			       __popcnt(*(DWORD*)(buf + sizeof(DWORD) * 2 )) +
			       __popcnt(*(DWORD*)(buf + sizeof(DWORD) * 3 ));
    00E71BE0  popcnt      ebx,dword ptr [edx+8]  
    00E71BE5  popcnt      esi,dword ptr [edx+0Ch]  
    00E71BEA  add         esi,ebx  
    00E71BEC  popcnt      ebx,dword ptr [edx+4]  
    00E71BF1  add         esi,ebx  
    00E71BF3  popcnt      ebx,dword ptr [edx]  
    00E71BF7  add         eax,ebx  
    00E71BF9  add         eax,esi  
		    buf += sizeof(DWORD) * 4;
    00E71BFB  add         edx,10h  
    00E71BFE  dec         ecx  
    00E71BFF  jne         POPCNT_HardwareUnrolled+50h (0E71BE0h)  

which looks to me to be as good or better than his inline asm

    00000001000013c3        popcntq %r10, %r10
    00000001000013c8        addq    %r10, %rcx
    00000001000013cb        popcntq %r11, %r11
    00000001000013d0        addq    %r11, %r9
    00000001000013d3        popcntq %r14, %r14
    00000001000013d8        addq    %r14, %r8
    00000001000013db        popcntq %rbx, %rbx

And does not have the odd

    000000010000133e        movl    %ecx, %r10d
    0000000100001341        movl    %edx, %r11d
    0000000100001344        movl    %eax, %r14d
    0000000100001347        movl    %r8d, %ebx

He was complaining about.

Maybe there's some compiler issue about arrays that makes using "int cnt[4];" as the accumulator induce the seemingly extraneous movls?