src/sync/atomic/value.go

// 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 [Value.Load].
// Once [Value.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.
			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.
			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()