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-rw-r--r--src/sync/atomic/value.go86
1 files changed, 86 insertions, 0 deletions
diff --git a/src/sync/atomic/value.go b/src/sync/atomic/value.go
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+++ b/src/sync/atomic/value.go
@@ -0,0 +1,86 @@
+// 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.
+// The zero value for a Value returns nil from Load.
+// Once Store has been called, a Value must not be copied.
+//
+// A Value must not be copied after first use.
+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()