diff --git a/syncmap/go19.go b/syncmap/go19.go
new file mode 100644
index 0000000..41a5909
--- /dev/null
+++ b/syncmap/go19.go
@@ -0,0 +1,17 @@
+// Copyright 2019 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.
+
+// +build go1.9
+
+package syncmap
+
+import "sync" // home to the standard library's sync.map implementation as of Go 1.9
+
+// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map = sync.Map
diff --git a/syncmap/map.go b/syncmap/map.go
index 80e1584..4b638cb 100644
--- a/syncmap/map.go
+++ b/syncmap/map.go
@@ -1,372 +1,8 @@
-// Copyright 2016 The Go Authors. All rights reserved.
+// Copyright 2019 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 syncmap provides a concurrent map implementation.
-// It is a prototype for a proposed addition to the sync package
-// in the standard library.
-// (https://golang.org/issue/18177)
+// This was the prototype for sync.Map which was added to the standard library's
+// sync package in Go 1.9. https://golang.org/pkg/sync/#Map.
 package syncmap
-
-import (
-	"sync"
-	"sync/atomic"
-	"unsafe"
-)
-
-// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
-// It is safe for multiple goroutines to call a Map's methods concurrently.
-//
-// The zero Map is valid and empty.
-//
-// A Map must not be copied after first use.
-type Map struct {
-	mu sync.Mutex
-
-	// read contains the portion of the map's contents that are safe for
-	// concurrent access (with or without mu held).
-	//
-	// The read field itself is always safe to load, but must only be stored with
-	// mu held.
-	//
-	// Entries stored in read may be updated concurrently without mu, but updating
-	// a previously-expunged entry requires that the entry be copied to the dirty
-	// map and unexpunged with mu held.
-	read atomic.Value // readOnly
-
-	// dirty contains the portion of the map's contents that require mu to be
-	// held. To ensure that the dirty map can be promoted to the read map quickly,
-	// it also includes all of the non-expunged entries in the read map.
-	//
-	// Expunged entries are not stored in the dirty map. An expunged entry in the
-	// clean map must be unexpunged and added to the dirty map before a new value
-	// can be stored to it.
-	//
-	// If the dirty map is nil, the next write to the map will initialize it by
-	// making a shallow copy of the clean map, omitting stale entries.
-	dirty map[interface{}]*entry
-
-	// misses counts the number of loads since the read map was last updated that
-	// needed to lock mu to determine whether the key was present.
-	//
-	// Once enough misses have occurred to cover the cost of copying the dirty
-	// map, the dirty map will be promoted to the read map (in the unamended
-	// state) and the next store to the map will make a new dirty copy.
-	misses int
-}
-
-// readOnly is an immutable struct stored atomically in the Map.read field.
-type readOnly struct {
-	m       map[interface{}]*entry
-	amended bool // true if the dirty map contains some key not in m.
-}
-
-// expunged is an arbitrary pointer that marks entries which have been deleted
-// from the dirty map.
-var expunged = unsafe.Pointer(new(interface{}))
-
-// An entry is a slot in the map corresponding to a particular key.
-type entry struct {
-	// p points to the interface{} value stored for the entry.
-	//
-	// If p == nil, the entry has been deleted and m.dirty == nil.
-	//
-	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
-	// is missing from m.dirty.
-	//
-	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
-	// != nil, in m.dirty[key].
-	//
-	// An entry can be deleted by atomic replacement with nil: when m.dirty is
-	// next created, it will atomically replace nil with expunged and leave
-	// m.dirty[key] unset.
-	//
-	// An entry's associated value can be updated by atomic replacement, provided
-	// p != expunged. If p == expunged, an entry's associated value can be updated
-	// only after first setting m.dirty[key] = e so that lookups using the dirty
-	// map find the entry.
-	p unsafe.Pointer // *interface{}
-}
-
-func newEntry(i interface{}) *entry {
-	return &entry{p: unsafe.Pointer(&i)}
-}
-
-// Load returns the value stored in the map for a key, or nil if no
-// value is present.
-// The ok result indicates whether value was found in the map.
-func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
-	read, _ := m.read.Load().(readOnly)
-	e, ok := read.m[key]
-	if !ok && read.amended {
-		m.mu.Lock()
-		// Avoid reporting a spurious miss if m.dirty got promoted while we were
-		// blocked on m.mu. (If further loads of the same key will not miss, it's
-		// not worth copying the dirty map for this key.)
-		read, _ = m.read.Load().(readOnly)
-		e, ok = read.m[key]
-		if !ok && read.amended {
-			e, ok = m.dirty[key]
-			// Regardless of whether the entry was present, record a miss: this key
-			// will take the slow path until the dirty map is promoted to the read
-			// map.
-			m.missLocked()
-		}
-		m.mu.Unlock()
-	}
-	if !ok {
-		return nil, false
-	}
-	return e.load()
-}
-
-func (e *entry) load() (value interface{}, ok bool) {
-	p := atomic.LoadPointer(&e.p)
-	if p == nil || p == expunged {
-		return nil, false
-	}
-	return *(*interface{})(p), true
-}
-
-// Store sets the value for a key.
-func (m *Map) Store(key, value interface{}) {
-	read, _ := m.read.Load().(readOnly)
-	if e, ok := read.m[key]; ok && e.tryStore(&value) {
-		return
-	}
-
-	m.mu.Lock()
-	read, _ = m.read.Load().(readOnly)
-	if e, ok := read.m[key]; ok {
-		if e.unexpungeLocked() {
-			// The entry was previously expunged, which implies that there is a
-			// non-nil dirty map and this entry is not in it.
-			m.dirty[key] = e
-		}
-		e.storeLocked(&value)
-	} else if e, ok := m.dirty[key]; ok {
-		e.storeLocked(&value)
-	} else {
-		if !read.amended {
-			// We're adding the first new key to the dirty map.
-			// Make sure it is allocated and mark the read-only map as incomplete.
-			m.dirtyLocked()
-			m.read.Store(readOnly{m: read.m, amended: true})
-		}
-		m.dirty[key] = newEntry(value)
-	}
-	m.mu.Unlock()
-}
-
-// tryStore stores a value if the entry has not been expunged.
-//
-// If the entry is expunged, tryStore returns false and leaves the entry
-// unchanged.
-func (e *entry) tryStore(i *interface{}) bool {
-	p := atomic.LoadPointer(&e.p)
-	if p == expunged {
-		return false
-	}
-	for {
-		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
-			return true
-		}
-		p = atomic.LoadPointer(&e.p)
-		if p == expunged {
-			return false
-		}
-	}
-}
-
-// unexpungeLocked ensures that the entry is not marked as expunged.
-//
-// If the entry was previously expunged, it must be added to the dirty map
-// before m.mu is unlocked.
-func (e *entry) unexpungeLocked() (wasExpunged bool) {
-	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
-}
-
-// storeLocked unconditionally stores a value to the entry.
-//
-// The entry must be known not to be expunged.
-func (e *entry) storeLocked(i *interface{}) {
-	atomic.StorePointer(&e.p, unsafe.Pointer(i))
-}
-
-// LoadOrStore returns the existing value for the key if present.
-// Otherwise, it stores and returns the given value.
-// The loaded result is true if the value was loaded, false if stored.
-func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
-	// Avoid locking if it's a clean hit.
-	read, _ := m.read.Load().(readOnly)
-	if e, ok := read.m[key]; ok {
-		actual, loaded, ok := e.tryLoadOrStore(value)
-		if ok {
-			return actual, loaded
-		}
-	}
-
-	m.mu.Lock()
-	read, _ = m.read.Load().(readOnly)
-	if e, ok := read.m[key]; ok {
-		if e.unexpungeLocked() {
-			m.dirty[key] = e
-		}
-		actual, loaded, _ = e.tryLoadOrStore(value)
-	} else if e, ok := m.dirty[key]; ok {
-		actual, loaded, _ = e.tryLoadOrStore(value)
-		m.missLocked()
-	} else {
-		if !read.amended {
-			// We're adding the first new key to the dirty map.
-			// Make sure it is allocated and mark the read-only map as incomplete.
-			m.dirtyLocked()
-			m.read.Store(readOnly{m: read.m, amended: true})
-		}
-		m.dirty[key] = newEntry(value)
-		actual, loaded = value, false
-	}
-	m.mu.Unlock()
-
-	return actual, loaded
-}
-
-// tryLoadOrStore atomically loads or stores a value if the entry is not
-// expunged.
-//
-// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
-// returns with ok==false.
-func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
-	p := atomic.LoadPointer(&e.p)
-	if p == expunged {
-		return nil, false, false
-	}
-	if p != nil {
-		return *(*interface{})(p), true, true
-	}
-
-	// Copy the interface after the first load to make this method more amenable
-	// to escape analysis: if we hit the "load" path or the entry is expunged, we
-	// shouldn't bother heap-allocating.
-	ic := i
-	for {
-		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
-			return i, false, true
-		}
-		p = atomic.LoadPointer(&e.p)
-		if p == expunged {
-			return nil, false, false
-		}
-		if p != nil {
-			return *(*interface{})(p), true, true
-		}
-	}
-}
-
-// Delete deletes the value for a key.
-func (m *Map) Delete(key interface{}) {
-	read, _ := m.read.Load().(readOnly)
-	e, ok := read.m[key]
-	if !ok && read.amended {
-		m.mu.Lock()
-		read, _ = m.read.Load().(readOnly)
-		e, ok = read.m[key]
-		if !ok && read.amended {
-			delete(m.dirty, key)
-		}
-		m.mu.Unlock()
-	}
-	if ok {
-		e.delete()
-	}
-}
-
-func (e *entry) delete() (hadValue bool) {
-	for {
-		p := atomic.LoadPointer(&e.p)
-		if p == nil || p == expunged {
-			return false
-		}
-		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
-			return true
-		}
-	}
-}
-
-// Range calls f sequentially for each key and value present in the map.
-// If f returns false, range stops the iteration.
-//
-// Range does not necessarily correspond to any consistent snapshot of the Map's
-// contents: no key will be visited more than once, but if the value for any key
-// is stored or deleted concurrently, Range may reflect any mapping for that key
-// from any point during the Range call.
-//
-// Range may be O(N) with the number of elements in the map even if f returns
-// false after a constant number of calls.
-func (m *Map) Range(f func(key, value interface{}) bool) {
-	// We need to be able to iterate over all of the keys that were already
-	// present at the start of the call to Range.
-	// If read.amended is false, then read.m satisfies that property without
-	// requiring us to hold m.mu for a long time.
-	read, _ := m.read.Load().(readOnly)
-	if read.amended {
-		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
-		// (assuming the caller does not break out early), so a call to Range
-		// amortizes an entire copy of the map: we can promote the dirty copy
-		// immediately!
-		m.mu.Lock()
-		read, _ = m.read.Load().(readOnly)
-		if read.amended {
-			read = readOnly{m: m.dirty}
-			m.read.Store(read)
-			m.dirty = nil
-			m.misses = 0
-		}
-		m.mu.Unlock()
-	}
-
-	for k, e := range read.m {
-		v, ok := e.load()
-		if !ok {
-			continue
-		}
-		if !f(k, v) {
-			break
-		}
-	}
-}
-
-func (m *Map) missLocked() {
-	m.misses++
-	if m.misses < len(m.dirty) {
-		return
-	}
-	m.read.Store(readOnly{m: m.dirty})
-	m.dirty = nil
-	m.misses = 0
-}
-
-func (m *Map) dirtyLocked() {
-	if m.dirty != nil {
-		return
-	}
-
-	read, _ := m.read.Load().(readOnly)
-	m.dirty = make(map[interface{}]*entry, len(read.m))
-	for k, e := range read.m {
-		if !e.tryExpungeLocked() {
-			m.dirty[k] = e
-		}
-	}
-}
-
-func (e *entry) tryExpungeLocked() (isExpunged bool) {
-	p := atomic.LoadPointer(&e.p)
-	for p == nil {
-		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
-			return true
-		}
-		p = atomic.LoadPointer(&e.p)
-	}
-	return p == expunged
-}
diff --git a/syncmap/pre_go19.go b/syncmap/pre_go19.go
new file mode 100644
index 0000000..01a7be7
--- /dev/null
+++ b/syncmap/pre_go19.go
@@ -0,0 +1,370 @@
+// Copyright 2016 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.
+
+// +build !go1.9
+
+package syncmap
+
+import (
+	"sync"
+	"sync/atomic"
+	"unsafe"
+)
+
+// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map struct {
+	mu sync.Mutex
+
+	// read contains the portion of the map's contents that are safe for
+	// concurrent access (with or without mu held).
+	//
+	// The read field itself is always safe to load, but must only be stored with
+	// mu held.
+	//
+	// Entries stored in read may be updated concurrently without mu, but updating
+	// a previously-expunged entry requires that the entry be copied to the dirty
+	// map and unexpunged with mu held.
+	read atomic.Value // readOnly
+
+	// dirty contains the portion of the map's contents that require mu to be
+	// held. To ensure that the dirty map can be promoted to the read map quickly,
+	// it also includes all of the non-expunged entries in the read map.
+	//
+	// Expunged entries are not stored in the dirty map. An expunged entry in the
+	// clean map must be unexpunged and added to the dirty map before a new value
+	// can be stored to it.
+	//
+	// If the dirty map is nil, the next write to the map will initialize it by
+	// making a shallow copy of the clean map, omitting stale entries.
+	dirty map[interface{}]*entry
+
+	// misses counts the number of loads since the read map was last updated that
+	// needed to lock mu to determine whether the key was present.
+	//
+	// Once enough misses have occurred to cover the cost of copying the dirty
+	// map, the dirty map will be promoted to the read map (in the unamended
+	// state) and the next store to the map will make a new dirty copy.
+	misses int
+}
+
+// readOnly is an immutable struct stored atomically in the Map.read field.
+type readOnly struct {
+	m       map[interface{}]*entry
+	amended bool // true if the dirty map contains some key not in m.
+}
+
+// expunged is an arbitrary pointer that marks entries which have been deleted
+// from the dirty map.
+var expunged = unsafe.Pointer(new(interface{}))
+
+// An entry is a slot in the map corresponding to a particular key.
+type entry struct {
+	// p points to the interface{} value stored for the entry.
+	//
+	// If p == nil, the entry has been deleted and m.dirty == nil.
+	//
+	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
+	// is missing from m.dirty.
+	//
+	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
+	// != nil, in m.dirty[key].
+	//
+	// An entry can be deleted by atomic replacement with nil: when m.dirty is
+	// next created, it will atomically replace nil with expunged and leave
+	// m.dirty[key] unset.
+	//
+	// An entry's associated value can be updated by atomic replacement, provided
+	// p != expunged. If p == expunged, an entry's associated value can be updated
+	// only after first setting m.dirty[key] = e so that lookups using the dirty
+	// map find the entry.
+	p unsafe.Pointer // *interface{}
+}
+
+func newEntry(i interface{}) *entry {
+	return &entry{p: unsafe.Pointer(&i)}
+}
+
+// Load returns the value stored in the map for a key, or nil if no
+// value is present.
+// The ok result indicates whether value was found in the map.
+func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		// Avoid reporting a spurious miss if m.dirty got promoted while we were
+		// blocked on m.mu. (If further loads of the same key will not miss, it's
+		// not worth copying the dirty map for this key.)
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			e, ok = m.dirty[key]
+			// Regardless of whether the entry was present, record a miss: this key
+			// will take the slow path until the dirty map is promoted to the read
+			// map.
+			m.missLocked()
+		}
+		m.mu.Unlock()
+	}
+	if !ok {
+		return nil, false
+	}
+	return e.load()
+}
+
+func (e *entry) load() (value interface{}, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == nil || p == expunged {
+		return nil, false
+	}
+	return *(*interface{})(p), true
+}
+
+// Store sets the value for a key.
+func (m *Map) Store(key, value interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok && e.tryStore(&value) {
+		return
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			// The entry was previously expunged, which implies that there is a
+			// non-nil dirty map and this entry is not in it.
+			m.dirty[key] = e
+		}
+		e.storeLocked(&value)
+	} else if e, ok := m.dirty[key]; ok {
+		e.storeLocked(&value)
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+	}
+	m.mu.Unlock()
+}
+
+// tryStore stores a value if the entry has not been expunged.
+//
+// If the entry is expunged, tryStore returns false and leaves the entry
+// unchanged.
+func (e *entry) tryStore(i *interface{}) bool {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return false
+	}
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return false
+		}
+	}
+}
+
+// unexpungeLocked ensures that the entry is not marked as expunged.
+//
+// If the entry was previously expunged, it must be added to the dirty map
+// before m.mu is unlocked.
+func (e *entry) unexpungeLocked() (wasExpunged bool) {
+	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
+}
+
+// storeLocked unconditionally stores a value to the entry.
+//
+// The entry must be known not to be expunged.
+func (e *entry) storeLocked(i *interface{}) {
+	atomic.StorePointer(&e.p, unsafe.Pointer(i))
+}
+
+// LoadOrStore returns the existing value for the key if present.
+// Otherwise, it stores and returns the given value.
+// The loaded result is true if the value was loaded, false if stored.
+func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	// Avoid locking if it's a clean hit.
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		actual, loaded, ok := e.tryLoadOrStore(value)
+		if ok {
+			return actual, loaded
+		}
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			m.dirty[key] = e
+		}
+		actual, loaded, _ = e.tryLoadOrStore(value)
+	} else if e, ok := m.dirty[key]; ok {
+		actual, loaded, _ = e.tryLoadOrStore(value)
+		m.missLocked()
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+		actual, loaded = value, false
+	}
+	m.mu.Unlock()
+
+	return actual, loaded
+}
+
+// tryLoadOrStore atomically loads or stores a value if the entry is not
+// expunged.
+//
+// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
+// returns with ok==false.
+func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return nil, false, false
+	}
+	if p != nil {
+		return *(*interface{})(p), true, true
+	}
+
+	// Copy the interface after the first load to make this method more amenable
+	// to escape analysis: if we hit the "load" path or the entry is expunged, we
+	// shouldn't bother heap-allocating.
+	ic := i
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
+			return i, false, true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return nil, false, false
+		}
+		if p != nil {
+			return *(*interface{})(p), true, true
+		}
+	}
+}
+
+// Delete deletes the value for a key.
+func (m *Map) Delete(key interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			delete(m.dirty, key)
+		}
+		m.mu.Unlock()
+	}
+	if ok {
+		e.delete()
+	}
+}
+
+func (e *entry) delete() (hadValue bool) {
+	for {
+		p := atomic.LoadPointer(&e.p)
+		if p == nil || p == expunged {
+			return false
+		}
+		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
+			return true
+		}
+	}
+}
+
+// Range calls f sequentially for each key and value present in the map.
+// If f returns false, range stops the iteration.
+//
+// Range does not necessarily correspond to any consistent snapshot of the Map's
+// contents: no key will be visited more than once, but if the value for any key
+// is stored or deleted concurrently, Range may reflect any mapping for that key
+// from any point during the Range call.
+//
+// Range may be O(N) with the number of elements in the map even if f returns
+// false after a constant number of calls.
+func (m *Map) Range(f func(key, value interface{}) bool) {
+	// We need to be able to iterate over all of the keys that were already
+	// present at the start of the call to Range.
+	// If read.amended is false, then read.m satisfies that property without
+	// requiring us to hold m.mu for a long time.
+	read, _ := m.read.Load().(readOnly)
+	if read.amended {
+		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
+		// (assuming the caller does not break out early), so a call to Range
+		// amortizes an entire copy of the map: we can promote the dirty copy
+		// immediately!
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		if read.amended {
+			read = readOnly{m: m.dirty}
+			m.read.Store(read)
+			m.dirty = nil
+			m.misses = 0
+		}
+		m.mu.Unlock()
+	}
+
+	for k, e := range read.m {
+		v, ok := e.load()
+		if !ok {
+			continue
+		}
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *Map) missLocked() {
+	m.misses++
+	if m.misses < len(m.dirty) {
+		return
+	}
+	m.read.Store(readOnly{m: m.dirty})
+	m.dirty = nil
+	m.misses = 0
+}
+
+func (m *Map) dirtyLocked() {
+	if m.dirty != nil {
+		return
+	}
+
+	read, _ := m.read.Load().(readOnly)
+	m.dirty = make(map[interface{}]*entry, len(read.m))
+	for k, e := range read.m {
+		if !e.tryExpungeLocked() {
+			m.dirty[k] = e
+		}
+	}
+}
+
+func (e *entry) tryExpungeLocked() (isExpunged bool) {
+	p := atomic.LoadPointer(&e.p)
+	for p == nil {
+		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+	}
+	return p == expunged
+}