forked from Minki/linux
02cdce53f3
addr_bit_test() is used in various places in IPv6 routing table
subsystem. It checks if the given fn_bit is set,
where fn_bit counts bits from MSB in words in network-order.
fn_bit : 0 .... 31 32 .... 64 65 .... 95 96 ....127
fn_bit >> 5 gives offset of word, and (~fn_bit & 0x1f) gives
count from LSB in the network-endian word in question.
fn_bit >> 5 : 0 1 2 3
~fn_bit & 0x1f: 31 .... 0 31 .... 0 31 .... 0 31 .... 0
Thus, the mask was generated as htonl(1 << (~fn_bit & 0x1f)).
This can be optimized by "sweezle" (See include/asm-generic/bitops/le.h).
In little-endian,
htonl(1 << bit) = 1 << (bit ^ BITOP_BE32_SWIZZLE)
where
BITOP_BE32_SWIZZLE is (0x1f & ~7)
So,
htonl(1 << (~fn_bit & 0x1f)) = 1 << ((~fn_bit & 0x1f) ^ (0x1f & ~7))
= 1 << ((~fn_bit ^ ~7) & 0x1f)
= 1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)
In big-endian, BITOP_BE32_SWIZZLE is equal to 0.
1 << ((~fn_bit ^ BITOP_BE32_SWIZZLE) & 0x1f)
= 1 << ((~fn_bit) & 0x1f)
= htonl(1 << (~fn_bit & 0x1f))
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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|---|---|---|
| .. | ||
| netfilter | ||
| addrconf_core.c | ||
| addrconf.c | ||
| addrlabel.c | ||
| af_inet6.c | ||
| ah6.c | ||
| anycast.c | ||
| datagram.c | ||
| esp6.c | ||
| exthdrs_core.c | ||
| exthdrs.c | ||
| fib6_rules.c | ||
| icmp.c | ||
| inet6_connection_sock.c | ||
| inet6_hashtables.c | ||
| ip6_fib.c | ||
| ip6_flowlabel.c | ||
| ip6_input.c | ||
| ip6_output.c | ||
| ip6_tunnel.c | ||
| ip6mr.c | ||
| ipcomp6.c | ||
| ipv6_sockglue.c | ||
| Kconfig | ||
| Makefile | ||
| mcast.c | ||
| mip6.c | ||
| ndisc.c | ||
| netfilter.c | ||
| proc.c | ||
| protocol.c | ||
| raw.c | ||
| reassembly.c | ||
| route.c | ||
| sit.c | ||
| syncookies.c | ||
| sysctl_net_ipv6.c | ||
| tcp_ipv6.c | ||
| tunnel6.c | ||
| udp_impl.h | ||
| udp.c | ||
| udplite.c | ||
| xfrm6_input.c | ||
| xfrm6_mode_beet.c | ||
| xfrm6_mode_ro.c | ||
| xfrm6_mode_transport.c | ||
| xfrm6_mode_tunnel.c | ||
| xfrm6_output.c | ||
| xfrm6_policy.c | ||
| xfrm6_state.c | ||
| xfrm6_tunnel.c | ||