sync: allow inlining the Once.Do fast path

Using Once.Do is now extremely cheap because the fast path is just an inlined
atomic load of a variable that is written only once and a conditional jump.
This is very beneficial for Once.Do because, due to its nature, the fast path
will be used for every call after the first one.

In a attempt to mimize code size increase, reorder the fields so that the
pointer to Once is also the pointer to Once.done, that is the only field used
in the hot path. This allows to use more compact instruction encodings or less
instructions in the hot path (that is inlined at every callsite).

name     old time/op  new time/op  delta
Once     4.54ns ± 0%  2.06ns ± 0%  -54.59%  (p=0.000 n=19+16)
Once-4   1.18ns ± 0%  0.55ns ± 0%  -53.39%  (p=0.000 n=15+16)
Once-16  0.53ns ± 0%  0.17ns ± 0%  -67.92%  (p=0.000 n=18+17)

linux/amd64 bin/go 14675861 (previous commit 14663387, +12474/+0.09%)

Change-Id: Ie2708103ab473787875d66746d2f20f1d90a6916
Reviewed-on: https://go-review.googlesource.com/c/go/+/152697
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/sync/once.go

@@ -10,8 +10,13 @@ import (
 
 // Once is an object that will perform exactly one action.
 type Once struct {
-	m    Mutex
+	// done indicates whether the action has been performed.
+	// It is first in the struct because it is used in the hot path.
+	// The hot path is inlined at every call site.
+	// Placing done first allows more compact instructions on some architectures (amd64/x86),
+	// and fewer instructions (to calculate offset) on other architectures.
 	done uint32
+	m    Mutex
 }
 
 // Do calls the function f if and only if Do is being called for the
@@ -33,10 +38,13 @@ type Once struct {
 // without calling f.
 //
 func (o *Once) Do(f func()) {
-	if atomic.LoadUint32(&o.done) == 1 {
-		return
+	if atomic.LoadUint32(&o.done) == 0 {
+		// Outlined slow-path to allow inlining of the fast-path.
+		o.doSlow(f)
 	}
-	// Slow-path.
+}
+
+func (o *Once) doSlow(f func()) {
 	o.m.Lock()
 	defer o.m.Unlock()
 	if o.done == 0 {

test/inline_sync.go

@@ -31,3 +31,10 @@ func small6() { // ERROR "can inline small6"
 	// the Lock fast path should be inlined
 	mutex.Lock() // ERROR "inlining call to sync\.\(\*Mutex\)\.Lock" "&sync\.m\.state escapes to heap"
 }
+
+var once *sync.Once
+
+func small7() { // ERROR "can inline small7"
+        // the Do fast path should be inlined
+        once.Do(small5) // ERROR "inlining call to sync\.\(\*Once\)\.Do" "&sync\.o\.done escapes to heap"
+}