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sync: improve Once fast path

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Use atomic.LoadUint32(&done) instead of
atomic.AddInt32(&done, 0) on fast path.

benchmark            old ns/op    new ns/op    delta
BenchmarkOnce            13.40         7.26  -45.82%
BenchmarkOnce-2          22.90         4.04  -82.36%
BenchmarkOnce-4          25.60         2.16  -91.56%
BenchmarkOnce-8          25.80         1.38  -94.65%
BenchmarkOnce-16         24.40         1.33  -94.55%

(on HP Z600, 2 x Intel Xeon E5620, 8 HT cores, 2.4 GHz)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4787041
This commit is contained in:
Dmitriy Vyukov 2011-07-21 11:30:14 -04:00 committed by Russ Cox
parent d6ed1b70ad
commit d2ab2b9f42
1 changed files with 3 additions and 3 deletions
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6
src/pkg/sync/once.go
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@ -11,7 +11,7 @@ import (
// Once is an object that will perform exactly one action.
type Once struct {
m Mutex
done int32
done uint32
}
// Do calls the function f if and only if the method is being called for the
@ -30,7 +30,7 @@ type Once struct {
// Do to be called, it will deadlock.
//
func (o *Once) Do(f func()) {
if atomic.AddInt32(&o.done, 0) == 1 {
if atomic.LoadUint32(&o.done) == 1 {
return
}
// Slow-path.
@ -38,6 +38,6 @@ func (o *Once) Do(f func()) {
defer o.m.Unlock()
if o.done == 0 {
f()
atomic.CompareAndSwapInt32(&o.done, 0, 1)
atomic.CompareAndSwapUint32(&o.done, 0, 1)
}
}