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// 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()