> This function efficiently counts the number of bits in a block of
> memory.
Would it be worth the annoying build- and run-time machinery to detect
and use the -msse4.2 __builtin_popcount() gcc intrinsic?
> +static unsigned int popcount32(unsigned int w)
> +{
> + unsigned int res = w - ((w >> 1) & 0x55555555);
> + res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
> + res = (res + (res >> 4)) & 0x0F0F0F0F;
> + res = res + (res >> 8);
> + return (res + (res >> 16)) & 0x000000FF;
> +}
40042b: 89 c2 mov %eax,%edx
400432: d1 ea shr %edx
400434: 81 e2 55 55 55 55 and $0x55555555,%edx
40043a: 29 d0 sub %edx,%eax
40043c: 89 c2 mov %eax,%edx
40043e: c1 e8 02 shr $0x2,%eax
400441: 81 e2 33 33 33 33 and $0x33333333,%edx
400447: 25 33 33 33 33 and $0x33333333,%eax
40044c: 01 d0 add %edx,%eax
40044e: 89 c2 mov %eax,%edx
400450: c1 ea 04 shr $0x4,%edx
400453: 01 d0 add %edx,%eax
400455: 25 0f 0f 0f 0f and $0xf0f0f0f,%eax
40045a: 89 c2 mov %eax,%edx
40045c: c1 ea 08 shr $0x8,%edx
40045f: 01 d0 add %edx,%eax
400461: 89 c6 mov %eax,%esi
400463: c1 ee 10 shr $0x10,%esi
400468: 0f b6 f0 movzbl %al,%esi
__builtin_popcount():
40047e: f3 0f b8 f0 popcnt %eax,%esi
- z
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