cmd/gc, time: make Format 5x faster

[rsc]

A combination of changes to Format and making the compiler handle strings better can
make time.Format 5x faster than it is now.  Do that.

The experiment justifying this claim is CL 6249062, although that CL is not intended for
submission.  It works around compiler inadequacies instead of addressing them.

benchmark          old ns/op    new ns/op    delta
BenchmarkFormat         2596          507  -80.47%

[davecheney]

Comment 1:

Is Luuk's work in http://golang.org/cl/5966075/ of use here ?

[rsc]

Comment 2:

changeset:   13401:bff65816ad27
user:        Russ Cox <rsc@golang.org>
date:        Sun Jun 03 11:08:17 2012 -0400
files:       src/pkg/time/format.go src/pkg/time/time.go
src/pkg/time/time_test.go src/pkg/time/zoneinfo.go
description: time: make Format 2.7x faster
benchmark             old ns/op    new ns/op    delta
BenchmarkFormat            2495          937  -62.44%
BenchmarkFormatNow         2308          889  -61.48%
Update issue #3679.
R=r
CC=golang-dev
http://golang.org/cl/6278047

[remyoudompheng]

Comment 3:

CL 6249062 has disappeared. What was it about and why is this issue still open?

[rsc]

Comment 4:

That's too bad. Mainly it was making string manipulation a bit faster. Removing LEAQs
will help. So does special casing x = append(x, "123"...) as x = append(x, '1', '2',
'3') for short strings and also copy(x, "123"). In general I was just unhappy with the
performance of string manipulations.

[rsc]

Comment 5:

Labels changed: added size-l.

[rsc]

Comment 6:

Rob made memmove a bit faster. Let's call this done as far as Go 1.1 is concerned.

Labels changed: removed go1.1.

[bradfitz]

Comment 7:

Profiling some of the benchmarks from http://www.techempower.com/benchmarks/ recently, I
found we were spending about as much time formatting the optional HTTP "Date" response
header as we were serializing JSON. (at least after
https://golang.org/cl/9129044/)

Labels changed: added performance.

[rsc]

Comment 8:

As done as it's going to get.

Status changed to Fixed.