in6_addr struct union with uint64_t

[James Hanley]

In a local experiment, I added the union fields __u6_addr64 and
__u6_addr128 for in the in6_addr structure to allow the compiler to
optimize access for the Interface ID of an IPv6 address as applicable using
the EUI-48/64 in IPv6 in the case of the uint64_t and to allow the compiler
to optimize access when comparing, setting, manipulating IPv6 addresses as
a whole with the GCC specific unsigned __int128. Code below modified
from inet/netinet/in.h:

#if !__USE_KERNEL_IPV6_DEFS
/* IPv6 address */
struct in6_addr
  {
    union
      {
        uint8_t __u6_addr8[16];
        uint16_t __u6_addr16[8];
        uint32_t __u6_addr32[4];
        uint64_t __u6_addr64[2]; // new
        unsigned __int128 __u6_addr128; // new
      } __in6_u;
#define s6_addr                 __in6_u.__u6_addr8
#ifdef __USE_MISC
# define s6_addr16              __in6_u.__u6_addr16
# define s6_addr32              __in6_u.__u6_addr32
# define s6_addr64              __in6_u.__u6_addr64
# define s6_addr128             __in6_u.__u6_addr128
#endif
  };
#endif /* !__USE_KERNEL_IPV6_DEFS */

First, is this useful, and is there a standard that prohibits defining
additional union members?

Second, when building, I hit an alignment issue in an assertion check in
resolv/resolv_conf.c comparing sockaddr_in and sockaddr_in6 with the
change.  I assumed I should add "__attribute__((aligned(128)))" ahead of
the struct sockaddr_in definition to force sockaddr_in to align to 128bit,
however, this did not resolve the issue. Any suggestions?

Thanks much,
-Jim
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