code review 12128044: runtime: optimize some hash lookups.

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 issue 5194.
Update issue 3885.

[khr, 2013-07-30 23:11:46]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://khr%40golang.org@code.google.com/p/go/

[bradfitz, 2013-07-30 23:45:02]

LGTM

Did any of the other benchmark numbers change?

[khr1, 2013-07-30 23:47:42]

Everything else was in the noise.


On Tue, Jul 30, 2013 at 4:45 PM, <bradfitz@golang.org> wrote:

> LGTM
>
> Did any of the other benchmark numbers change?
>
>
>
https://codereview.appspot.**com/12128044/<https://codereview.appspot.com/121...
>

[rsc, 2013-07-31 00:22:19]

LGTM

[khr, 2013-07-31 04:40:00]

*** Submitted as https://code.google.com/p/go/source/detail?r=2faab4336bf1 ***

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 issue 5194.
Update issue 3885.

R=golang-dev, bradfitz, khr, rsc
CC=golang-dev
https://codereview.appspot.com/12128044