Benchmarks:

benchmark              old ns/op    new ns/op    delta
BenchmarkHash8Bytes          762          674  -11.55%
BenchmarkHash1K             8791         7375  -16.11%
BenchmarkHash8K            65094        54881  -15.69%

benchmark               old MB/s     new MB/s  speedup
BenchmarkHash8Bytes        10.50        11.86    1.13x
BenchmarkHash1K           116.48       138.84    1.19x
BenchmarkHash8K           125.85       149.27    1.19x

R=dave, rsc, iant
CC=golang-dev
https://golang.org/cl/6820096