1 files changed, 85 insertions, 0 deletions
diff --git a/src/sync/atomic/value.go b/src/sync/atomic/value.go
new file mode 100644
index 000000000..ab3aa1128
--- /dev/null
+++ b/src/sync/atomic/value.go
@@ -0,0 +1,85 @@
+// 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.
+
+package atomic
+
+import (
+	"unsafe"
+)
+
+// A Value provides an atomic load and store of a consistently typed value.
+// Values can be created as part of other data structures.
+// The zero value for a Value returns nil from Load.
+// Once Store has been called, a Value must not be copied.
+type Value struct {
+	v interface{}
+}
+
+// ifaceWords is interface{} internal representation.
+type ifaceWords struct {
+	typ  unsafe.Pointer
+	data unsafe.Pointer
+}
+
+// Load returns the value set by the most recent Store.
+// It returns nil if there has been no call to Store for this Value.
+func (v *Value) Load() (x interface{}) {
+	vp := (*ifaceWords)(unsafe.Pointer(v))
+	typ := LoadPointer(&vp.typ)
+	if typ == nil || uintptr(typ) == ^uintptr(0) {
+		// First store not yet completed.
+		return nil
+	}
+	data := LoadPointer(&vp.data)
+	xp := (*ifaceWords)(unsafe.Pointer(&x))
+	xp.typ = typ
+	xp.data = data
+	return
+}
+
+// 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).
+func (v *Value) Store(x interface{}) {
+	if x == nil {
+		panic("sync/atomic: store of nil value into Value")
+	}
+	vp := (*ifaceWords)(unsafe.Pointer(v))
+	xp := (*ifaceWords)(unsafe.Pointer(&x))
+	for {
+		typ := LoadPointer(&vp.typ)
+		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.
+			runtime_procPin()
+			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
+				runtime_procUnpin()
+				continue
+			}
+			// Complete first store.
+			StorePointer(&vp.data, xp.data)
+			StorePointer(&vp.typ, xp.typ)
+			runtime_procUnpin()
+			return
+		}
+		if uintptr(typ) == ^uintptr(0) {
+			// First store in progress. Wait.
+			// Since we disable preemption around the first store,
+			// we can wait with active spinning.
+			continue
+		}
+		// First store completed. Check type and overwrite data.
+		if typ != xp.typ {
+			panic("sync/atomic: store of inconsistently typed value into Value")
+		}
+		StorePointer(&vp.data, xp.data)
+		return
+	}
+}
+
+// Disable/enable preemption, implemented in runtime.
+func runtime_procPin()
+func runtime_procUnpin()