Fold in some startup randomness to make the hash vary across
different runs. This helps prevent attackers from choosing
keys that all map to the same bucket.
Also, reorganize the hash a bit. Move the *m1 multiply to after
the xor of the current hash and the message. For hash quality
it doesn't really matter, but for DDOS resistance it helps a lot
(any processing done to the message before it is merged with the
random seed is useless, as it is easily inverted by an attacker).
Update #9365
Change-Id: Ib19968168e1bbc541d1d28be2701bb83e53f1e24
Reviewed-on: https://go-review.googlesource.com/2344
Reviewed-by: Ian Lance Taylor <iant@golang.org>
For arm and powerpc, as well as x86 without aes instructions.
Contains a mixture of ideas from cityhash and xxhash.
Compared to our old fallback on ARM, it's ~no slower on
small objects and up to ~50% faster on large objects. More
importantly, it is a much better hash function and thus has
less chance of bad behavior.
Fixes#8737
benchmark old ns/op new ns/op delta
BenchmarkHash5 173 181 +4.62%
BenchmarkHash16 252 212 -15.87%
BenchmarkHash64 575 419 -27.13%
BenchmarkHash1024 7173 3995 -44.31%
BenchmarkHash65536 516940 313173 -39.42%
BenchmarkHashStringSpeed 300 279 -7.00%
BenchmarkHashBytesSpeed 478 424 -11.30%
BenchmarkHashInt32Speed 217 207 -4.61%
BenchmarkHashInt64Speed 262 231 -11.83%
BenchmarkHashStringArraySpeed 609 631 +3.61%
Change-Id: I0a9335028f32b10ad484966e3019987973afd3eb
Reviewed-on: https://go-review.googlesource.com/1360
Reviewed-by: Russ Cox <rsc@golang.org>
