[Cake] an experiment with an alternate hasher
dave.taht at gmail.com
Sun Mar 26 12:00:34 EDT 2017
I am trying to see if I can get an adaquate avalanche distribution using
a hash of popcount(src),(popcount(dst),srcport,dstport, protocol, seed.
popcount is, regrettably, an sse4.2-only instruction, and this version
of the assembly routine can actually popcount up to 8 ipv6 addresses
in a row, but you typically just pass it 2. I had a great deal of fun
writing this tho I haven't got around to actually seeing how good a
resulting hash would be!
The ipv4 version does both src and dst with a single popcnt.
The startup cost to further hash this is pretty insane - 30 cycles in
http://burtleburtle.net/bob/hash/spooky.html. Xor? CRC?
4005a0: 31 c0 xor %eax,%eax
4005a2: 89 f1 mov %esi,%ecx
4005a4: 48 c1 e0 08 shl $0x8,%rax
4005a8: f3 4c 0f b8 07 popcnt (%rdi),%r8
4005ad: 48 83 c7 10 add $0x10,%rdi
4005b1: 4c 09 c0 or %r8,%rax
4005b4: f3 4c 0f b8 47 f8 popcnt -0x8(%rdi),%r8
4005ba: 4c 01 c0 add %r8,%rax
4005bd: e2 e5 loop 4005a4 <poploop7>
4005bf: c3 retq
Let's go make home routers and wifi faster! With better software!
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