2018-23-01 Paul Thomas <pault@gcc.gnu.org>

[official-gcc.git] / libgo / go / sync / map.go

// 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.

package sync

import (
        "sync/atomic"
        "unsafe"
)

// Map is like a Go map[interface{}]interface{} but is safe for concurrent use
// by multiple goroutines without additional locking or coordination.
// Loads, stores, and deletes run in amortized constant time.
//
// The Map type is specialized. Most code should use a plain Go map instead,
// with separate locking or coordination, for better type safety and to make it
// easier to maintain other invariants along with the map content.
//
// The Map type is optimized for two common use cases: (1) when the entry for a given
// key is only ever written once but read many times, as in caches that only grow,
// or (2) when multiple goroutines read, write, and overwrite entries for disjoint
// sets of keys. In these two cases, use of a Map may significantly reduce lock
// contention compared to a Go map paired with a separate Mutex or RWMutex.
//
// The zero Map is empty and ready for use. A Map must not be copied after first use.
type Map struct {
        mu 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
}