runtime: optimize some hash lookups.
When comparing strings, check these (in order):
- length mismatch => not equal
- string pointer equal => equal
- if length is short:
- memeq on body
- if length is long:
- compare first&last few bytes, if different => not equal
- save entry as a possible match
- after checking every entry, if there is only one possible
match, use memeq on that entry. Otherwise, fallback to hash.
benchmark old ns/op new ns/op delta
BenchmarkSameLengthMap 43 4 -89.77%
Fixes #5194.
Update #3885.
R=golang-dev, bradfitz, khr, rsc
CC=golang-dev
https://golang.org/cl/
12128044