sync: add fast paths to WaitGroup

benchmark                                        old ns/op    new ns/op    delta
BenchmarkWaitGroupUncontended                        93.50        33.60  -64.06%
BenchmarkWaitGroupUncontended-2                      44.30        16.90  -61.85%
BenchmarkWaitGroupUncontended-4                      21.80         8.47  -61.15%
BenchmarkWaitGroupUncontended-8                      12.10         4.86  -59.83%
BenchmarkWaitGroupUncontended-16                      7.38         3.35  -54.61%
BenchmarkWaitGroupAddDone                            58.40        33.70  -42.29%
BenchmarkWaitGroupAddDone-2                         293.00        85.80  -70.72%
BenchmarkWaitGroupAddDone-4                         243.00        51.10  -78.97%
BenchmarkWaitGroupAddDone-8                         236.00        52.20  -77.88%
BenchmarkWaitGroupAddDone-16                        215.00        43.30  -79.86%
BenchmarkWaitGroupAddDoneWork                       826.00       794.00   -3.87%
BenchmarkWaitGroupAddDoneWork-2                     450.00       424.00   -5.78%
BenchmarkWaitGroupAddDoneWork-4                     277.00       220.00  -20.58%
BenchmarkWaitGroupAddDoneWork-8                     440.00       116.00  -73.64%
BenchmarkWaitGroupAddDoneWork-16                    569.00        66.50  -88.31%
BenchmarkWaitGroupWait                               29.00         8.04  -72.28%
BenchmarkWaitGroupWait-2                             74.10         4.15  -94.40%
BenchmarkWaitGroupWait-4                            117.00         2.30  -98.03%
BenchmarkWaitGroupWait-8                            111.00         1.31  -98.82%
BenchmarkWaitGroupWait-16                           104.00         1.27  -98.78%
BenchmarkWaitGroupWaitWork                          802.00       792.00   -1.25%
BenchmarkWaitGroupWaitWork-2                        411.00       401.00   -2.43%
BenchmarkWaitGroupWaitWork-4                        210.00       199.00   -5.24%
BenchmarkWaitGroupWaitWork-8                        206.00       105.00  -49.03%
BenchmarkWaitGroupWaitWork-16                       334.00        54.40  -83.71%

R=rsc
CC=golang-dev
https://golang.org/cl/4672050

src/pkg/sync/atomic/asm_386.s

@@ -85,3 +85,12 @@ addloop:
 	MOVL	BX, retlo+12(FP)
 	MOVL	CX, rethi+16(FP)
 	RET
+
+TEXT ·LoadInt32(SB),7,$0
+	JMP	·LoadUint32(SB)
+
+TEXT ·LoadUint32(SB),7,$0
+	MOVL	addrptr+0(FP), AX
+	MOVL	0(AX), AX
+	MOVL	AX, ret+4(FP)
+	RET

src/pkg/sync/atomic/asm_amd64.s

@@ -57,3 +57,13 @@ TEXT ·AddUint64(SB),7,$0
 	ADDQ	AX, CX
 	MOVQ	CX, ret+16(FP)
 	RET
+
+TEXT ·LoadInt32(SB),7,$0
+	JMP	·LoadUint32(SB)
+
+TEXT ·LoadUint32(SB),7,$0
+	MOVQ	addrptr+0(FP), AX
+	MOVL	0(AX), AX
+	MOVL	AX, ret+8(FP)
+	RET
+

src/pkg/sync/atomic/asm_linux_arm.s

@@ -83,3 +83,16 @@ TEXT ·AddInt64(SB),7,$0
 
 TEXT ·AddUint64(SB),7,$0
 	B	·armAddUint64(SB)
+
+TEXT ·LoadInt32(SB),7,$0
+	B	·LoadUint32(SB)
+
+TEXT ·LoadUint32(SB),7,$0
+	MOVW	addrptr+0(FP), R2
+loadloop1:
+	MOVW	0(R2), R0
+	MOVW	R0, R1
+	BL	cas<>(SB)
+	BCC	loadloop1
+	MOVW	R0, val+4(FP)
+	RET

src/pkg/sync/atomic/atomic_test.go

@@ -308,6 +308,46 @@ func TestCompareAndSwapUintptr(t *testing.T) {
 	}
 }
 
+func TestLoadInt32(t *testing.T) {
+	var x struct {
+		before int32
+		i      int32
+		after  int32
+	}
+	x.before = magic32
+	x.after = magic32
+	for delta := int32(1); delta+delta > delta; delta += delta {
+		k := LoadInt32(&x.i)
+		if k != x.i {
+			t.Fatalf("delta=%d i=%d k=%d", delta, x.i, k)
+		}
+		x.i += delta
+	}
+	if x.before != magic32 || x.after != magic32 {
+		t.Fatalf("wrong magic: %#x _ %#x != %#x _ %#x", x.before, x.after, magic32, magic32)
+	}
+}
+
+func TestLoadUint32(t *testing.T) {
+	var x struct {
+		before uint32
+		i      uint32
+		after  uint32
+	}
+	x.before = magic32
+	x.after = magic32
+	for delta := uint32(1); delta+delta > delta; delta += delta {
+		k := LoadUint32(&x.i)
+		if k != x.i {
+			t.Fatalf("delta=%d i=%d k=%d", delta, x.i, k)
+		}
+		x.i += delta
+	}
+	if x.before != magic32 || x.after != magic32 {
+		t.Fatalf("wrong magic: %#x _ %#x != %#x _ %#x", x.before, x.after, magic32, magic32)
+	}
+}
+
 // Tests of correct behavior, with contention.
 // (Is the function atomic?)
 //
@@ -537,3 +577,65 @@ func TestHammer64(t *testing.T) {
 		}
 	}
 }
+
+func hammerLoadInt32(t *testing.T, uval *uint32) {
+	val := (*int32)(unsafe.Pointer(uval))
+	for {
+		v := LoadInt32(val)
+		vlo := v & ((1 << 16) - 1)
+		vhi := v >> 16
+		if vlo != vhi {
+			t.Fatalf("LoadInt32: %#x != %#x", vlo, vhi)
+		}
+		new := v + 1 + 1<<16
+		if vlo == 1e4 {
+			new = 0
+		}
+		if CompareAndSwapInt32(val, v, new) {
+			break
+		}
+	}
+}
+
+func hammerLoadUint32(t *testing.T, val *uint32) {
+	for {
+		v := LoadUint32(val)
+		vlo := v & ((1 << 16) - 1)
+		vhi := v >> 16
+		if vlo != vhi {
+			t.Fatalf("LoadUint32: %#x != %#x", vlo, vhi)
+		}
+		new := v + 1 + 1<<16
+		if vlo == 1e4 {
+			new = 0
+		}
+		if CompareAndSwapUint32(val, v, new) {
+			break
+		}
+	}
+}
+
+func TestHammerLoad(t *testing.T) {
+	tests := [...]func(*testing.T, *uint32){hammerLoadInt32, hammerLoadUint32}
+	n := 100000
+	if testing.Short() {
+		n = 10000
+	}
+	const procs = 8
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(procs))
+	for _, tt := range tests {
+		c := make(chan int)
+		var val uint32
+		for p := 0; p < procs; p++ {
+			go func() {
+				for i := 0; i < n; i++ {
+					tt(t, &val)
+				}
+				c <- 1
+			}()
+		}
+		for p := 0; p < procs; p++ {
+			<-c
+		}
+	}
+}

src/pkg/sync/atomic/doc.go

@@ -56,6 +56,12 @@ func AddUint64(val *uint64, delta uint64) (new uint64)
 // AddUintptr atomically adds delta to *val and returns the new value.
 func AddUintptr(val *uintptr, delta uintptr) (new uintptr)
 
+// LoadInt32 atomically loads *addr.
+func LoadInt32(addr *int32) (val int32)
+
+// LoadUint32 atomically loads *addr.
+func LoadUint32(addr *uint32) (val uint32)
+
 // Helper for ARM.  Linker will discard on other systems
 func panic64() {
 	panic("sync/atomic: broken 64-bit atomic operations (buggy QEMU)")

src/pkg/sync/waitgroup.go

@@ -4,7 +4,10 @@
 
 package sync
 
-import "runtime"
+import (
+	"runtime"
+	"sync/atomic"
+)
 
 // A WaitGroup waits for a collection of goroutines to finish.
 // The main goroutine calls Add to set the number of
@@ -28,8 +31,8 @@ import "runtime"
 // 
 type WaitGroup struct {
 	m       Mutex
-	counter int
-	waiters int
+	counter int32
+	waiters int32
 	sema    *uint32
 }
 
@@ -48,19 +51,19 @@ type WaitGroup struct {
 // Add adds delta, which may be negative, to the WaitGroup counter.
 // If the counter becomes zero, all goroutines blocked on Wait() are released.
 func (wg *WaitGroup) Add(delta int) {
-	wg.m.Lock()
-	if delta < -wg.counter {
-		wg.m.Unlock()
+	v := atomic.AddInt32(&wg.counter, int32(delta))
+	if v < 0 {
 		panic("sync: negative WaitGroup count")
 	}
-	wg.counter += delta
-	if wg.counter == 0 && wg.waiters > 0 {
-		for i := 0; i < wg.waiters; i++ {
+	if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
+		return
+	}
+	wg.m.Lock()
+	for i := int32(0); i < wg.waiters; i++ {
 		runtime.Semrelease(wg.sema)
 	}
 	wg.waiters = 0
 	wg.sema = nil
-	}
 	wg.m.Unlock()
 }
 
@@ -71,12 +74,20 @@ func (wg *WaitGroup) Done() {
 
 // Wait blocks until the WaitGroup counter is zero.
 func (wg *WaitGroup) Wait() {
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		return
+	}
 	wg.m.Lock()
-	if wg.counter == 0 {
+	atomic.AddInt32(&wg.waiters, 1)
+	// This code is racing with the unlocked path in Add above.
+	// The code above modifies counter and then reads waiters.
+	// We must modify waiters and then read counter (the opposite order)
+	// to avoid missing an Add.
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		atomic.AddInt32(&wg.waiters, -1)
 		wg.m.Unlock()
 		return
 	}
-	wg.waiters++
 	if wg.sema == nil {
 		wg.sema = new(uint32)
 	}

src/pkg/sync/waitgroup_test.go

@@ -5,7 +5,9 @@
 package sync_test
 
 import (
+	"runtime"
 	. "sync"
+	"sync/atomic"
 	"testing"
 )
 
@@ -58,3 +60,106 @@ func TestWaitGroupMisuse(t *testing.T) {
 	wg.Done()
 	t.Fatal("Should panic")
 }
+
+func BenchmarkWaitGroupUncontended(b *testing.B) {
+	type PaddedWaitGroup struct {
+		WaitGroup
+		pad [128]uint8
+	}
+	const CallsPerSched = 1000
+	procs := runtime.GOMAXPROCS(-1)
+	N := int32(b.N / CallsPerSched)
+	c := make(chan bool, procs)
+	for p := 0; p < procs; p++ {
+		go func() {
+			var wg PaddedWaitGroup
+			for atomic.AddInt32(&N, -1) >= 0 {
+				runtime.Gosched()
+				for g := 0; g < CallsPerSched; g++ {
+					wg.Add(1)
+					wg.Done()
+					wg.Wait()
+				}
+			}
+			c <- true
+		}()
+	}
+	for p := 0; p < procs; p++ {
+		<-c
+	}
+}
+
+func benchmarkWaitGroupAddDone(b *testing.B, localWork int) {
+	const CallsPerSched = 1000
+	procs := runtime.GOMAXPROCS(-1)
+	N := int32(b.N / CallsPerSched)
+	c := make(chan bool, procs)
+	var wg WaitGroup
+	for p := 0; p < procs; p++ {
+		go func() {
+			foo := 0
+			for atomic.AddInt32(&N, -1) >= 0 {
+				runtime.Gosched()
+				for g := 0; g < CallsPerSched; g++ {
+					wg.Add(1)
+					for i := 0; i < localWork; i++ {
+						foo *= 2
+						foo /= 2
+					}
+					wg.Done()
+				}
+			}
+			c <- foo == 42
+		}()
+	}
+	for p := 0; p < procs; p++ {
+		<-c
+	}
+}
+
+func BenchmarkWaitGroupAddDone(b *testing.B) {
+	benchmarkWaitGroupAddDone(b, 0)
+}
+
+func BenchmarkWaitGroupAddDoneWork(b *testing.B) {
+	benchmarkWaitGroupAddDone(b, 100)
+}
+
+func benchmarkWaitGroupWait(b *testing.B, localWork int) {
+	const CallsPerSched = 1000
+	procs := runtime.GOMAXPROCS(-1)
+	N := int32(b.N / CallsPerSched)
+	c := make(chan bool, procs)
+	var wg WaitGroup
+	wg.Add(procs)
+	for p := 0; p < procs; p++ {
+		go wg.Done()
+	}
+	for p := 0; p < procs; p++ {
+		go func() {
+			foo := 0
+			for atomic.AddInt32(&N, -1) >= 0 {
+				runtime.Gosched()
+				for g := 0; g < CallsPerSched; g++ {
+					wg.Wait()
+					for i := 0; i < localWork; i++ {
+						foo *= 2
+						foo /= 2
+					}
+				}
+			}
+			c <- foo == 42
+		}()
+	}
+	for p := 0; p < procs; p++ {
+		<-c
+	}
+}
+
+func BenchmarkWaitGroupWait(b *testing.B) {
+	benchmarkWaitGroupWait(b, 0)
+}
+
+func BenchmarkWaitGroupWaitWork(b *testing.B) {
+	benchmarkWaitGroupWait(b, 100)
+}