[RFC] Generic population count function

[Julian Elischer]

On 11/14/12 4:46 PM, Jung-uk Kim wrote:
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> I implemented generic population count function.  Please see the
> attachment.  It is also available from here:
>
> http://people.freebsd.org/~jkim/bitcount.diff
>
> The idea is to make use of CPU-supported population count instructions
> if available and the compiler supports them (i.e., clang), especially
> for larger than 32-bit data.  The patch also has use cases for the new
> function (i.e., counting number of bits in IPv6 address[*]).
>
> Any objection?

there are more efficient algorithms than the one you show in bitcount.h

see the bit-twidling site: 
http://graphics.stanford.edu/~seander/bithacks.html



>
> Jung-uk Kim
>
> * PS: BTW, I am not sure whether this is correct:
>
> - --- sys/netpfil/ipfw/ip_fw_table.c
> +++ sys/netpfil/ipfw/ip_fw_table.c
> @@ -720,11 +717,10 @@ dump_table_xentry_extended(struct radix_node *rn,
>          switch (tbl->type) {
>   #ifdef INET6
>          case IPFW_TABLE_CIDR:
> - -               /* Count IPv6 mask */
> - -               v = (uint32_t *)&n->m.mask6.sin6_addr;
> - -               for (i = 0; i < sizeof(struct in6_addr) / 4; i++, v++)
> - -                       xent->masklen += bitcount32(*v);
> - -               memcpy(&xent->k, &n->a.addr6.sin6_addr, sizeof(struct
> in6_addr));
> +               xent->masklen += bitcount(&n->m.mask6.sin6_addr,
> +                   sizeof(struct in6_addr));
> +               memcpy(&xent->k, &n->a.addr6.sin6_addr,
> +                   sizeof(struct in6_addr));
>                  break;
>   #endif
>          case IPFW_TABLE_INTERFACE:
>
> Is xent->masklen initialized to a non-zero value somewhere, i.e.,
> shouldn't we just use '=' instead of '+=' here?
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