sync/atomic: use a better first-store-in-progress marker

Unlike what the comment says, the GC can see this pointer.
Might as well make it a real pointer, even though ^uintptr(0)
isn't currently causing problems.

Removed the comment about GC not seeing the pointer.

Change-Id: I04bc1fd4848698bec6afb79bd5fda671dfc9a073
Reviewed-on: https://go-review.googlesource.com/c/go/+/241661
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Colin Arnott <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Keith Randall <[email protected]>

Author: randall77

src/sync/atomic/value.go

@@ -28,7 +28,7 @@ type ifaceWords struct {
 func (v *Value) Load() (val interface{}) {
 	vp := (*ifaceWords)(unsafe.Pointer(v))
 	typ := LoadPointer(&vp.typ)
-	if typ == nil || uintptr(typ) == ^uintptr(0) {
+	if typ == nil || typ == unsafe.Pointer(&firstStoreInProgress) {
 		// First store not yet completed.
 		return nil
 	}
@@ -39,6 +39,8 @@ func (v *Value) Load() (val interface{}) {
 	return
 }
 
+var firstStoreInProgress byte
+
 // Store sets the value of the Value to x.
 // All calls to Store for a given Value must use values of the same concrete type.
 // Store of an inconsistent type panics, as does Store(nil).
@@ -53,10 +55,9 @@ func (v *Value) Store(val interface{}) {
 		if typ == nil {
 			// Attempt to start first store.
 			// Disable preemption so that other goroutines can use
-			// active spin wait to wait for completion; and so that
-			// GC does not see the fake type accidentally.
+			// active spin wait to wait for completion.
 			runtime_procPin()
-			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
+			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
 				runtime_procUnpin()
 				continue
 			}
@@ -66,7 +67,7 @@ func (v *Value) Store(val interface{}) {
 			runtime_procUnpin()
 			return
 		}
-		if uintptr(typ) == ^uintptr(0) {
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
 			// First store in progress. Wait.
 			// Since we disable preemption around the first store,
 			// we can wait with active spinning.