sync/atomic: remove unnecessary race instrumentation in Value

It is left from the time when Value was implemented in assembly.
Now it is implemented in Go and race detector understands Go.
In particular the atomic operations must provide
all necessary synchronization.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews, khr, rsc
https://golang.org/cl/145880043

src/sync/atomic/norace.go

-// Copyright 2014 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build !race
-
-package atomic
-
-import "unsafe"
-
-const raceenabled = false
-
-func raceAcquire(addr unsafe.Pointer) {
-}
-
-func raceReleaseMerge(addr unsafe.Pointer) {
-}

src/sync/atomic/race.go

-// Copyright 2014 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build race
-
-package atomic
-
-import (
-	"runtime"
-	"unsafe"
-)
-
-const raceenabled = true
-
-func raceAcquire(addr unsafe.Pointer) {
-	runtime.RaceAcquire(addr)
-}
-
-func raceReleaseMerge(addr unsafe.Pointer) {
-	runtime.RaceReleaseMerge(addr)
-}

src/sync/atomic/value.go

@@ -35,9 +35,6 @@ func (v *Value) Load() (x interface{}) {
 	xp := (*ifaceWords)(unsafe.Pointer(&x))
 	xp.typ = typ
 	xp.data = data
-	if raceenabled {
-		raceAcquire(unsafe.Pointer(v))
-	}
 	return
 }
 
@@ -48,9 +45,6 @@ func (v *Value) Store(x interface{}) {
 	if x == nil {
 		panic("sync/atomic: store of nil value into Value")
 	}
-	if raceenabled {
-		raceReleaseMerge(unsafe.Pointer(v))
-	}
 	vp := (*ifaceWords)(unsafe.Pointer(v))
 	xp := (*ifaceWords)(unsafe.Pointer(&x))
 	for {