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Source file src/sync/atomic/value.go

     1	// Copyright 2014 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	package atomic
     6	
     7	import (
     8		"unsafe"
     9	)
    10	
    11	// A Value provides an atomic load and store of a consistently typed value.
    12	// Values can be created as part of other data structures.
    13	// The zero value for a Value returns nil from Load.
    14	// Once Store has been called, a Value must not be copied.
    15	//
    16	// A Value must not be copied after first use.
    17	type Value struct {
    18		noCopy noCopy
    19	
    20		v interface{}
    21	}
    22	
    23	// ifaceWords is interface{} internal representation.
    24	type ifaceWords struct {
    25		typ  unsafe.Pointer
    26		data unsafe.Pointer
    27	}
    28	
    29	// Load returns the value set by the most recent Store.
    30	// It returns nil if there has been no call to Store for this Value.
    31	func (v *Value) Load() (x interface{}) {
    32		vp := (*ifaceWords)(unsafe.Pointer(v))
    33		typ := LoadPointer(&vp.typ)
    34		if typ == nil || uintptr(typ) == ^uintptr(0) {
    35			// First store not yet completed.
    36			return nil
    37		}
    38		data := LoadPointer(&vp.data)
    39		xp := (*ifaceWords)(unsafe.Pointer(&x))
    40		xp.typ = typ
    41		xp.data = data
    42		return
    43	}
    44	
    45	// Store sets the value of the Value to x.
    46	// All calls to Store for a given Value must use values of the same concrete type.
    47	// Store of an inconsistent type panics, as does Store(nil).
    48	func (v *Value) Store(x interface{}) {
    49		if x == nil {
    50			panic("sync/atomic: store of nil value into Value")
    51		}
    52		vp := (*ifaceWords)(unsafe.Pointer(v))
    53		xp := (*ifaceWords)(unsafe.Pointer(&x))
    54		for {
    55			typ := LoadPointer(&vp.typ)
    56			if typ == nil {
    57				// Attempt to start first store.
    58				// Disable preemption so that other goroutines can use
    59				// active spin wait to wait for completion; and so that
    60				// GC does not see the fake type accidentally.
    61				runtime_procPin()
    62				if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    63					runtime_procUnpin()
    64					continue
    65				}
    66				// Complete first store.
    67				StorePointer(&vp.data, xp.data)
    68				StorePointer(&vp.typ, xp.typ)
    69				runtime_procUnpin()
    70				return
    71			}
    72			if uintptr(typ) == ^uintptr(0) {
    73				// First store in progress. Wait.
    74				// Since we disable preemption around the first store,
    75				// we can wait with active spinning.
    76				continue
    77			}
    78			// First store completed. Check type and overwrite data.
    79			if typ != xp.typ {
    80				panic("sync/atomic: store of inconsistently typed value into Value")
    81			}
    82			StorePointer(&vp.data, xp.data)
    83			return
    84		}
    85	}
    86	
    87	// Disable/enable preemption, implemented in runtime.
    88	func runtime_procPin()
    89	func runtime_procUnpin()
    90	
    91	// noCopy may be embedded into structs which must not be copied
    92	// after the first use.
    93	//
    94	// See https://github.com/golang/go/issues/8005#issuecomment-190753527
    95	// for details.
    96	type noCopy struct{}
    97	
    98	// Lock is a no-op used by -copylocks checker from `go vet`.
    99	func (*noCopy) Lock() {}
   100	

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