Revert "internal/sync: optimize CompareAndSwap and Swap"

This reverts CL 606462.

Reason for revert: Breaks atomicity between operations. See #70970.

Change-Id: I1a899f2784da5a0f9da3193e3267275c23aea661
Reviewed-on: https://go-review.googlesource.com/c/go/+/638615
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/internal/sync/hashtriemap.go

@@ -219,22 +219,12 @@ func (ht *HashTrieMap[K, V]) Swap(key K, new V) (previous V, loaded bool) {
 
 			slot = &i.children[(hash>>hashShift)&nChildrenMask]
 			n = slot.Load()
-			if n == nil {
+			if n == nil || n.isEntry {
 				// We found a nil slot which is a candidate for insertion,
 				// or an existing entry that we'll replace.
 				haveInsertPoint = true
 				break
 			}
-			if n.isEntry {
-				// Swap if the keys compare.
-				old, swapped := n.entry().swap(key, new)
-				if swapped {
-					return old, true
-				}
-				// If we fail, that means we should try to insert.
-				haveInsertPoint = true
-				break
-			}
 			i = n.indirect()
 		}
 		if !haveInsertPoint {
@@ -261,10 +251,11 @@ func (ht *HashTrieMap[K, V]) Swap(key K, new V) (previous V, loaded bool) {
 	var zero V
 	var oldEntry *entry[K, V]
 	if n != nil {
-		// Between before and now, something got inserted. Swap if the keys compare.
+		// Swap if the keys compare.
 		oldEntry = n.entry()
-		old, swapped := oldEntry.swap(key, new)
+		newEntry, old, swapped := oldEntry.swap(key, new)
 		if swapped {
+			slot.Store(&newEntry.node)
 			return old, true
 		}
 	}
@@ -292,30 +283,25 @@ func (ht *HashTrieMap[K, V]) CompareAndSwap(key K, old, new V) (swapped bool) {
 		panic("called CompareAndSwap when value is not of comparable type")
 	}
 	hash := ht.keyHash(abi.NoEscape(unsafe.Pointer(&key)), ht.seed)
-	for {
-		// Find the key or return if it's not there.
-		i := ht.root.Load()
-		hashShift := 8 * goarch.PtrSize
-		found := false
-		for hashShift != 0 {
-			hashShift -= nChildrenLog2
 
-			slot := &i.children[(hash>>hashShift)&nChildrenMask]
-			n := slot.Load()
+	// Find a node with the key and compare with it. n != nil if we found the node.
+	i, _, slot, n := ht.find(key, hash, ht.valEqual, old)
+	if i != nil {
+		defer i.mu.Unlock()
+	}
 	if n == nil {
-				// Nothing to compare with. Give up.
 		return false
 	}
-			if n.isEntry {
-				// We found an entry. Try to compare and swap directly.
-				return n.entry().compareAndSwap(key, old, new, ht.valEqual)
-			}
-			i = n.indirect()
-		}
-		if !found {
-			panic("internal/concurrent.HashMapTrie: ran out of hash bits while iterating")
-		}
+
+	// Try to swap the entry.
+	e, swapped := n.entry().compareAndSwap(key, old, new, ht.valEqual)
+	if !swapped {
+		// Nothing was actually swapped, which means the node is no longer there.
+		return false
 	}
+	// Store the entry back because it changed.
+	slot.Store(&e.node)
+	return true
 }
 
 // LoadAndDelete deletes the value for a key, returning the previous value if any.
@@ -523,7 +509,7 @@ func (ht *HashTrieMap[K, V]) iter(i *indirect[K, V], yield func(key K, value V)
 		}
 		e := n.entry()
 		for e != nil {
-			if !yield(e.key, *e.value.Load()) {
+			if !yield(e.key, e.value) {
 				return false
 			}
 			e = e.overflow.Load()
@@ -579,22 +565,21 @@ type entry[K comparable, V any] struct {
 	node[K, V]
 	overflow atomic.Pointer[entry[K, V]] // Overflow for hash collisions.
 	key      K
-	value    atomic.Pointer[V]
+	value    V
 }
 
 func newEntryNode[K comparable, V any](key K, value V) *entry[K, V] {
-	e := &entry[K, V]{
+	return &entry[K, V]{
 		node:  node[K, V]{isEntry: true},
 		key:   key,
+		value: value,
 	}
-	e.value.Store(&value)
-	return e
 }
 
 func (e *entry[K, V]) lookup(key K) (V, bool) {
 	for e != nil {
 		if e.key == key {
-			return *e.value.Load(), true
+			return e.value, true
 		}
 		e = e.overflow.Load()
 	}
@@ -603,87 +588,69 @@ func (e *entry[K, V]) lookup(key K) (V, bool) {
 
 func (e *entry[K, V]) lookupWithValue(key K, value V, valEqual equalFunc) (V, bool) {
 	for e != nil {
-		oldp := e.value.Load()
-		if e.key == key && (valEqual == nil || valEqual(unsafe.Pointer(oldp), abi.NoEscape(unsafe.Pointer(&value)))) {
-			return *oldp, true
+		if e.key == key && (valEqual == nil || valEqual(unsafe.Pointer(&e.value), abi.NoEscape(unsafe.Pointer(&value)))) {
+			return e.value, true
 		}
 		e = e.overflow.Load()
 	}
 	return *new(V), false
 }
 
-// swap replaces a value in the overflow chain if keys compare equal.
-// Returns the old value, and whether or not anything was swapped.
+// swap replaces an entry in the overflow chain if keys compare equal. Returns the new entry chain,
+// the old value, and whether or not anything was swapped.
 //
 // swap must be called under the mutex of the indirect node which e is a child of.
-func (head *entry[K, V]) swap(key K, newv V) (V, bool) {
+func (head *entry[K, V]) swap(key K, new V) (*entry[K, V], V, bool) {
 	if head.key == key {
-		vp := new(V)
-		*vp = newv
-		oldp := head.value.Swap(vp)
-		return *oldp, true
+		// Return the new head of the list.
+		e := newEntryNode(key, new)
+		if chain := head.overflow.Load(); chain != nil {
+			e.overflow.Store(chain)
+		}
+		return e, head.value, true
 	}
 	i := &head.overflow
 	e := i.Load()
 	for e != nil {
 		if e.key == key {
-			vp := new(V)
-			*vp = newv
-			oldp := e.value.Swap(vp)
-			return *oldp, true
+			eNew := newEntryNode(key, new)
+			eNew.overflow.Store(e.overflow.Load())
+			i.Store(eNew)
+			return head, e.value, true
 		}
 		i = &e.overflow
 		e = e.overflow.Load()
 	}
 	var zero V
-	return zero, false
+	return head, zero, false
 }
 
-// compareAndSwap replaces a value for a matching key and existing value in the overflow chain.
-// Returns whether or not anything was swapped.
+// compareAndSwap replaces an entry in the overflow chain if both the key and value compare
+// equal. Returns the new entry chain and whether or not anything was swapped.
 //
 // compareAndSwap must be called under the mutex of the indirect node which e is a child of.
-func (head *entry[K, V]) compareAndSwap(key K, oldv, newv V, valEqual equalFunc) bool {
-	var vbox *V
-outerLoop:
-	for {
-		oldvp := head.value.Load()
-		if head.key == key && valEqual(unsafe.Pointer(oldvp), abi.NoEscape(unsafe.Pointer(&oldv))) {
+func (head *entry[K, V]) compareAndSwap(key K, old, new V, valEqual equalFunc) (*entry[K, V], bool) {
+	if head.key == key && valEqual(unsafe.Pointer(&head.value), abi.NoEscape(unsafe.Pointer(&old))) {
 		// Return the new head of the list.
-			if vbox == nil {
-				// Delay explicit creation of a new value to hold newv. If we just pass &newv
-				// to CompareAndSwap, then newv will unconditionally escape, even if the CAS fails.
-				vbox = new(V)
-				*vbox = newv
+		e := newEntryNode(key, new)
+		if chain := head.overflow.Load(); chain != nil {
+			e.overflow.Store(chain)
 		}
-			if head.value.CompareAndSwap(oldvp, vbox) {
-				return true
-			}
-			// We need to restart from the head of the overflow list in case, due to a removal, a node
-			// is moved up the list and we miss it.
-			continue outerLoop
+		return e, true
 	}
 	i := &head.overflow
 	e := i.Load()
 	for e != nil {
-			oldvp := e.value.Load()
-			if e.key == key && valEqual(unsafe.Pointer(oldvp), abi.NoEscape(unsafe.Pointer(&oldv))) {
-				if vbox == nil {
-					// Delay explicit creation of a new value to hold newv. If we just pass &newv
-					// to CompareAndSwap, then newv will unconditionally escape, even if the CAS fails.
-					vbox = new(V)
-					*vbox = newv
-				}
-				if e.value.CompareAndSwap(oldvp, vbox) {
-					return true
-				}
-				continue outerLoop
+		if e.key == key && valEqual(unsafe.Pointer(&e.value), abi.NoEscape(unsafe.Pointer(&old))) {
+			eNew := newEntryNode(key, new)
+			eNew.overflow.Store(e.overflow.Load())
+			i.Store(eNew)
+			return head, true
 		}
 		i = &e.overflow
 		e = e.overflow.Load()
 	}
-		return false
-	}
+	return head, false
 }
 
 // loadAndDelete deletes an entry in the overflow chain by key. Returns the value for the key, the new
@@ -693,14 +660,14 @@ outerLoop:
 func (head *entry[K, V]) loadAndDelete(key K) (V, *entry[K, V], bool) {
 	if head.key == key {
 		// Drop the head of the list.
-		return *head.value.Load(), head.overflow.Load(), true
+		return head.value, head.overflow.Load(), true
 	}
 	i := &head.overflow
 	e := i.Load()
 	for e != nil {
 		if e.key == key {
 			i.Store(e.overflow.Load())
-			return *e.value.Load(), head, true
+			return e.value, head, true
 		}
 		i = &e.overflow
 		e = e.overflow.Load()
@@ -713,14 +680,14 @@ func (head *entry[K, V]) loadAndDelete(key K) (V, *entry[K, V], bool) {
 //
 // compareAndDelete must be called under the mutex of the indirect node which e is a child of.
 func (head *entry[K, V]) compareAndDelete(key K, value V, valEqual equalFunc) (*entry[K, V], bool) {
-	if head.key == key && valEqual(unsafe.Pointer(head.value.Load()), abi.NoEscape(unsafe.Pointer(&value))) {
+	if head.key == key && valEqual(unsafe.Pointer(&head.value), abi.NoEscape(unsafe.Pointer(&value))) {
 		// Drop the head of the list.
 		return head.overflow.Load(), true
 	}
 	i := &head.overflow
 	e := i.Load()
 	for e != nil {
-		if e.key == key && valEqual(unsafe.Pointer(e.value.Load()), abi.NoEscape(unsafe.Pointer(&value))) {
+		if e.key == key && valEqual(unsafe.Pointer(&e.value), abi.NoEscape(unsafe.Pointer(&value))) {
 			i.Store(e.overflow.Load())
 			return head, true
 		}