From 93dde6b0e6c0de50a47f9dc5f3ac7205c36742aa Mon Sep 17 00:00:00 2001
From: Dmitriy Vyukov <dvyukov@google.com>
Date: Tue, 28 Jun 2011 09:43:01 -0400
Subject: [PATCH] sync: add fast path to Once The implementation does not grab
 the lock, if Once is already initalized. Benchmark results on HP Z600 (2 x
 Xeon E5620, 8 HT cores, 2.40GHz) are as follows: benchmark                   
                     old ns/op    new ns/op    delta sync_test.BenchmarkOnce  
                           187.00        14.00  -92.51%
 sync_test.BenchmarkOnce-2                           909.00        21.40 
 -97.65% sync_test.BenchmarkOnce-4                          3684.00       
 20.90  -99.43% sync_test.BenchmarkOnce-8                          5987.00    
    23.00  -99.62% sync_test.BenchmarkOnce-16                         5051.00 
       21.60  -99.57%

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4641066
---
 src/pkg/sync/once.go | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/pkg/sync/once.go b/src/pkg/sync/once.go
index b6f5f5a8726..447b71dcb7a 100644
--- a/src/pkg/sync/once.go
+++ b/src/pkg/sync/once.go
@@ -4,10 +4,14 @@
 
 package sync
 
+import (
+	"sync/atomic"
+)
+
 // Once is an object that will perform exactly one action.
 type Once struct {
 	m    Mutex
-	done bool
+	done int32
 }
 
 // Do calls the function f if and only if the method is being called for the
@@ -26,10 +30,14 @@ type Once struct {
 // Do to be called, it will deadlock.
 //
 func (o *Once) Do(f func()) {
+	if atomic.AddInt32(&o.done, 0) == 1 {
+		return
+	}
+	// Slow-path.
 	o.m.Lock()
 	defer o.m.Unlock()
-	if !o.done {
-		o.done = true
+	if o.done == 0 {
 		f()
+		atomic.CompareAndSwapInt32(&o.done, 0, 1)
 	}
 }