1 files changed, 194 insertions, 0 deletions
diff --git a/src/sync/atomic/value.go b/src/sync/atomic/value.go
new file mode 100644
index 0000000..a57b08a
--- /dev/null
+++ b/src/sync/atomic/value.go
@@ -0,0 +1,194 @@
+// 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 any
+}
+
+// efaceWords is interface{} internal representation.
+type efaceWords 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() (val any) {
+	vp := (*efaceWords)(unsafe.Pointer(v))
+	typ := LoadPointer(&vp.typ)
+	if typ == nil || typ == unsafe.Pointer(&firstStoreInProgress) {
+		// First store not yet completed.
+		return nil
+	}
+	data := LoadPointer(&vp.data)
+	vlp := (*efaceWords)(unsafe.Pointer(&val))
+	vlp.typ = typ
+	vlp.data = data
+	return
+}
+
+var firstStoreInProgress byte
+
+// Store sets the value of the Value v to val.
+// 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(val any) {
+	if val == nil {
+		panic("sync/atomic: store of nil value into Value")
+	}
+	vp := (*efaceWords)(unsafe.Pointer(v))
+	vlp := (*efaceWords)(unsafe.Pointer(&val))
+	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.
+			runtime_procPin()
+			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
+				runtime_procUnpin()
+				continue
+			}
+			// Complete first store.
+			StorePointer(&vp.data, vlp.data)
+			StorePointer(&vp.typ, vlp.typ)
+			runtime_procUnpin()
+			return
+		}
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
+			// 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 != vlp.typ {
+			panic("sync/atomic: store of inconsistently typed value into Value")
+		}
+		StorePointer(&vp.data, vlp.data)
+		return
+	}
+}
+
+// Swap stores new into Value and returns the previous value. It returns nil if
+// the Value is empty.
+//
+// All calls to Swap for a given Value must use values of the same concrete
+// type. Swap of an inconsistent type panics, as does Swap(nil).
+func (v *Value) Swap(new any) (old any) {
+	if new == nil {
+		panic("sync/atomic: swap of nil value into Value")
+	}
+	vp := (*efaceWords)(unsafe.Pointer(v))
+	np := (*efaceWords)(unsafe.Pointer(&new))
+	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(&firstStoreInProgress)) {
+				runtime_procUnpin()
+				continue
+			}
+			// Complete first store.
+			StorePointer(&vp.data, np.data)
+			StorePointer(&vp.typ, np.typ)
+			runtime_procUnpin()
+			return nil
+		}
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
+			// 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 != np.typ {
+			panic("sync/atomic: swap of inconsistently typed value into Value")
+		}
+		op := (*efaceWords)(unsafe.Pointer(&old))
+		op.typ, op.data = np.typ, SwapPointer(&vp.data, np.data)
+		return old
+	}
+}
+
+// CompareAndSwap executes the compare-and-swap operation for the Value.
+//
+// All calls to CompareAndSwap for a given Value must use values of the same
+// concrete type. CompareAndSwap of an inconsistent type panics, as does
+// CompareAndSwap(old, nil).
+func (v *Value) CompareAndSwap(old, new any) (swapped bool) {
+	if new == nil {
+		panic("sync/atomic: compare and swap of nil value into Value")
+	}
+	vp := (*efaceWords)(unsafe.Pointer(v))
+	np := (*efaceWords)(unsafe.Pointer(&new))
+	op := (*efaceWords)(unsafe.Pointer(&old))
+	if op.typ != nil && np.typ != op.typ {
+		panic("sync/atomic: compare and swap of inconsistently typed values")
+	}
+	for {
+		typ := LoadPointer(&vp.typ)
+		if typ == nil {
+			if old != nil {
+				return false
+			}
+			// 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(&firstStoreInProgress)) {
+				runtime_procUnpin()
+				continue
+			}
+			// Complete first store.
+			StorePointer(&vp.data, np.data)
+			StorePointer(&vp.typ, np.typ)
+			runtime_procUnpin()
+			return true
+		}
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
+			// 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 != np.typ {
+			panic("sync/atomic: compare and swap of inconsistently typed value into Value")
+		}
+		// Compare old and current via runtime equality check.
+		// This allows value types to be compared, something
+		// not offered by the package functions.
+		// CompareAndSwapPointer below only ensures vp.data
+		// has not changed since LoadPointer.
+		data := LoadPointer(&vp.data)
+		var i any
+		(*efaceWords)(unsafe.Pointer(&i)).typ = typ
+		(*efaceWords)(unsafe.Pointer(&i)).data = data
+		if i != old {
+			return false
+		}
+		return CompareAndSwapPointer(&vp.data, data, np.data)
+	}
+}
+
+// Disable/enable preemption, implemented in runtime.
+func runtime_procPin() int
+func runtime_procUnpin()