src/internal/runtime/maps/map.go

// Copyright 2024 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 maps implements Go's builtin map type.
package maps

import (
	"internal/abi"
	"internal/goarch"
	"internal/runtime/math"
	"internal/runtime/sys"
	"unsafe"
)

// This package contains the implementation of Go's builtin map type.
//
// The map design is based on Abseil's "Swiss Table" map design
// (https://abseil.io/about/design/swisstables), with additional modifications
// to cover Go's additional requirements, discussed below.
//
// Terminology:
// - Slot: A storage location of a single key/element pair.
// - Group: A group of abi.SwissMapGroupSlots (8) slots, plus a control word.
// - Control word: An 8-byte word which denotes whether each slot is empty,
//   deleted, or used. If a slot is used, its control byte also contains the
//   lower 7 bits of the hash (H2).
// - H1: Upper 57 bits of a hash.
// - H2: Lower 7 bits of a hash.
// - Table: A complete "Swiss Table" hash table. A table consists of one or
//   more groups for storage plus metadata to handle operation and determining
//   when to grow.
// - Map: The top-level Map type consists of zero or more tables for storage.
//   The upper bits of the hash select which table a key belongs to.
// - Directory: Array of the tables used by the map.
//
// At its core, the table design is similar to a traditional open-addressed
// hash table. Storage consists of an array of groups, which effectively means
// an array of key/elem slots with some control words interspersed. Lookup uses
// the hash to determine an initial group to check. If, due to collisions, this
// group contains no match, the probe sequence selects the next group to check
// (see below for more detail about the probe sequence).
//
// The key difference occurs within a group. In a standard open-addressed
// linear probed hash table, we would check each slot one at a time to find a
// match. A swiss table utilizes the extra control word to check all 8 slots in
// parallel.
//
// Each byte in the control word corresponds to one of the slots in the group.
// In each byte, 1 bit is used to indicate whether the slot is in use, or if it
// is empty/deleted. The other 7 bits contain the lower 7 bits of the hash for
// the key in that slot. See [ctrl] for the exact encoding.
//
// During lookup, we can use some clever bitwise manipulation to compare all 8
// 7-bit hashes against the input hash in parallel (see [ctrlGroup.matchH2]).
// That is, we effectively perform 8 steps of probing in a single operation.
// With SIMD instructions, this could be extended to 16 slots with a 16-byte
// control word.
//
// Since we only use 7 bits of the 64 bit hash, there is a 1 in 128 (~0.7%)
// probability of false positive on each slot, but that's fine: we always need
// double check each match with a standard key comparison regardless.
//
// Probing
//
// Probing is done using the upper 57 bits (H1) of the hash as an index into
// the groups array. Probing walks through the groups using quadratic probing
// until it finds a group with a match or a group with an empty slot. See
// [probeSeq] for specifics about the probe sequence. Note the probe
// invariants: the number of groups must be a power of two, and the end of a
// probe sequence must be a group with an empty slot (the table can never be
// 100% full).
//
// Deletion
//
// Probing stops when it finds a group with an empty slot. This affects
// deletion: when deleting from a completely full group, we must not mark the
// slot as empty, as there could be more slots used later in a probe sequence
// and this deletion would cause probing to stop too early. Instead, we mark
// such slots as "deleted" with a tombstone. If the group still has an empty
// slot, we don't need a tombstone and directly mark the slot empty. Insert
// prioritizes reuse of tombstones over filling an empty slots. Otherwise,
// tombstones are only completely cleared during grow, as an in-place cleanup
// complicates iteration.
//
// Growth
//
// The probe sequence depends on the number of groups. Thus, when growing the
// group count all slots must be reordered to match the new probe sequence. In
// other words, an entire table must be grown at once.
//
// In order to support incremental growth, the map splits its contents across
// multiple tables. Each table is still a full hash table, but an individual
// table may only service a subset of the hash space. Growth occurs on
// individual tables, so while an entire table must grow at once, each of these
// grows is only a small portion of a map. The maximum size of a single grow is
// limited by limiting the maximum size of a table before it is split into
// multiple tables.
//
// A map starts with a single table. Up to [maxTableCapacity], growth simply
// replaces this table with a replacement with double capacity. Beyond this
// limit, growth splits the table into two.
//
// The map uses "extendible hashing" to select which table to use. In
// extendible hashing, we use the upper bits of the hash as an index into an
// array of tables (called the "directory"). The number of bits uses increases
// as the number of tables increases. For example, when there is only 1 table,
// we use 0 bits (no selection necessary). When there are 2 tables, we use 1
// bit to select either the 0th or 1st table. [Map.globalDepth] is the number
// of bits currently used for table selection, and by extension (1 <<
// globalDepth), the size of the directory.
//
// Note that each table has its own load factor and grows independently. If the
// 1st bucket grows, it will split. We'll need 2 bits to select tables, though
// we'll have 3 tables total rather than 4. We support this by allowing
// multiple indicies to point to the same table. This example:
//
//	directory (globalDepth=2)
//	+----+
//	| 00 | --\
//	+----+    +--> table (localDepth=1)
//	| 01 | --/
//	+----+
//	| 10 | ------> table (localDepth=2)
//	+----+
//	| 11 | ------> table (localDepth=2)
//	+----+
//
// Tables track the depth they were created at (localDepth). It is necessary to
// grow the directory when splitting a table where globalDepth == localDepth.
//
// Iteration
//
// Iteration is the most complex part of the map due to Go's generous iteration
// semantics. A summary of semantics from the spec:
// 1. Adding and/or deleting entries during iteration MUST NOT cause iteration
//    to return the same entry more than once.
// 2. Entries added during iteration MAY be returned by iteration.
// 3. Entries modified during iteration MUST return their latest value.
// 4. Entries deleted during iteration MUST NOT be returned by iteration.
// 5. Iteration order is unspecified. In the implementation, it is explicitly
//    randomized.
//
// If the map never grows, these semantics are straightforward: just iterate
// over every table in the directory and every group and slot in each table.
// These semantics all land as expected.
//
// If the map grows during iteration, things complicate significantly. First
// and foremost, we need to track which entries we already returned to satisfy
// (1). There are three types of grow:
// a. A table replaced by a single larger table.
// b. A table split into two replacement tables.
// c. Growing the directory (occurs as part of (b) if necessary).
//
// For all of these cases, the replacement table(s) will have a different probe
// sequence, so simply tracking the current group and slot indices is not
// sufficient.
//
// For (a) and (b), note that grows of tables other than the one we are
// currently iterating over are irrelevant.
//
// We handle (a) and (b) by having the iterator keep a reference to the table
// it is currently iterating over, even after the table is replaced. We keep
// iterating over the original table to maintain the iteration order and avoid
// violating (1). Any new entries added only to the replacement table(s) will
// be skipped (allowed by (2)). To avoid violating (3) or (4), while we use the
// original table to select the keys, we must look them up again in the new
// table(s) to determine if they have been modified or deleted. There is yet
// another layer of complexity if the key does not compare equal itself. See
// [Iter.Next] for the gory details.
//
// Note that for (b) once we finish iterating over the old table we'll need to
// skip the next entry in the directory, as that contains the second split of
// the old table. We can use the old table's localDepth to determine the next
// logical index to use.
//
// For (b), we must adjust the current directory index when the directory
// grows. This is more straightforward, as the directory orders remains the
// same after grow, so we just double the index if the directory size doubles.

// Extracts the H1 portion of a hash: the 57 upper bits.
// TODO(prattmic): what about 32-bit systems?
func h1(h uintptr) uintptr {
	return h >> 7
}

// Extracts the H2 portion of a hash: the 7 bits not used for h1.
//
// These are used as an occupied control byte.
func h2(h uintptr) uintptr {
	return h & 0x7f
}

type Map struct {
	// The number of filled slots (i.e. the number of elements in all
	// tables). Excludes deleted slots.
	// Must be first (known by the compiler, for len() builtin).
	used uint64

	// seed is the hash seed, computed as a unique random number per map.
	seed uintptr

	// The directory of tables.
	//
	// Normally dirPtr points to an array of table pointers
	//
	// dirPtr *[dirLen]*table
	//
	// The length (dirLen) of this array is `1 << globalDepth`. Multiple
	// entries may point to the same table. See top-level comment for more
	// details.
	//
	// Small map optimization: if the map always contained
	// abi.SwissMapGroupSlots or fewer entries, it fits entirely in a
	// single group. In that case dirPtr points directly to a single group.
	//
	// dirPtr *group
	//
	// In this case, dirLen is 0. used counts the number of used slots in
	// the group. Note that small maps never have deleted slots (as there
	// is no probe sequence to maintain).
	dirPtr unsafe.Pointer
	dirLen int

	// The number of bits to use in table directory lookups.
	globalDepth uint8

	// The number of bits to shift out of the hash for directory lookups.
	// On 64-bit systems, this is 64 - globalDepth.
	globalShift uint8

	// writing is a flag that is toggled (XOR 1) while the map is being
	// written. Normally it is set to 1 when writing, but if there are
	// multiple concurrent writers, then toggling increases the probability
	// that both sides will detect the race.
	writing uint8

	// clearSeq is a sequence counter of calls to Clear. It is used to
	// detect map clears during iteration.
	clearSeq uint64
}

func depthToShift(depth uint8) uint8 {
	if goarch.PtrSize == 4 {
		return 32 - depth
	}
	return 64 - depth
}

// If m is non-nil, it should be used rather than allocating.
//
// maxAlloc should be runtime.maxAlloc.
//
// TODO(prattmic): Put maxAlloc somewhere accessible.
func NewMap(mt *abi.SwissMapType, hint uintptr, m *Map, maxAlloc uintptr) *Map {
	if m == nil {
		m = new(Map)
	}

	m.seed = uintptr(rand())

	if hint <= abi.SwissMapGroupSlots {
		// A small map can fill all 8 slots, so no need to increase
		// target capacity.
		//
		// In fact, since an 8 slot group is what the first assignment
		// to an empty map would allocate anyway, it doesn't matter if
		// we allocate here or on the first assignment.
		//
		// Thus we just return without allocating. (We'll save the
		// allocation completely if no assignment comes.)

		// Note that the compiler may have initialized m.dirPtr with a
		// pointer to a stack-allocated group, in which case we already
		// have a group. The control word is already initialized.

		return m
	}

	// Full size map.

	// Set initial capacity to hold hint entries without growing in the
	// average case.
	targetCapacity := (hint * abi.SwissMapGroupSlots) / maxAvgGroupLoad
	if targetCapacity < hint { // overflow
		return m // return an empty map.
	}

	dirSize := (uint64(targetCapacity) + maxTableCapacity - 1) / maxTableCapacity
	dirSize, overflow := alignUpPow2(dirSize)
	if overflow || dirSize > uint64(math.MaxUintptr) {
		return m // return an empty map.
	}

	// Reject hints that are obviously too large.
	groups, overflow := math.MulUintptr(uintptr(dirSize), maxTableCapacity)
	if overflow {
		return m // return an empty map.
	} else {
		mem, overflow := math.MulUintptr(groups, mt.GroupSize)
		if overflow || mem > maxAlloc {
			return m // return an empty map.
		}
	}

	m.globalDepth = uint8(sys.TrailingZeros64(dirSize))
	m.globalShift = depthToShift(m.globalDepth)

	directory := make([]*table, dirSize)

	for i := range directory {
		// TODO: Think more about initial table capacity.
		directory[i] = newTable(mt, uint64(targetCapacity)/dirSize, i, m.globalDepth)
	}

	m.dirPtr = unsafe.Pointer(&directory[0])
	m.dirLen = len(directory)

	return m
}

func NewEmptyMap() *Map {
	m := new(Map)
	m.seed = uintptr(rand())
	// See comment in NewMap. No need to eager allocate a group.
	return m
}

func (m *Map) directoryIndex(hash uintptr) uintptr {
	if m.dirLen == 1 {
		return 0
	}
	return hash >> (m.globalShift & 63)
}

func (m *Map) directoryAt(i uintptr) *table {
	return *(**table)(unsafe.Pointer(uintptr(m.dirPtr) + goarch.PtrSize*i))
}

func (m *Map) directorySet(i uintptr, nt *table) {
	*(**table)(unsafe.Pointer(uintptr(m.dirPtr) + goarch.PtrSize*i)) = nt
}

func (m *Map) replaceTable(nt *table) {
	// The number of entries that reference the same table doubles for each
	// time the globalDepth grows without the table splitting.
	entries := 1 << (m.globalDepth - nt.localDepth)
	for i := 0; i < entries; i++ {
		//m.directory[nt.index+i] = nt
		m.directorySet(uintptr(nt.index+i), nt)
	}
}

func (m *Map) installTableSplit(old, left, right *table) {
	if old.localDepth == m.globalDepth {
		// No room for another level in the directory. Grow the
		// directory.
		newDir := make([]*table, m.dirLen*2)
		for i := range m.dirLen {
			t := m.directoryAt(uintptr(i))
			newDir[2*i] = t
			newDir[2*i+1] = t
			// t may already exist in multiple indicies. We should
			// only update t.index once. Since the index must
			// increase, seeing the original index means this must
			// be the first time we've encountered this table.
			if t.index == i {
				t.index = 2 * i
			}
		}
		m.globalDepth++
		m.globalShift--
		//m.directory = newDir
		m.dirPtr = unsafe.Pointer(&newDir[0])
		m.dirLen = len(newDir)
	}

	// N.B. left and right may still consume multiple indicies if the
	// directory has grown multiple times since old was last split.
	left.index = old.index
	m.replaceTable(left)

	entries := 1 << (m.globalDepth - left.localDepth)
	right.index = left.index + entries
	m.replaceTable(right)
}

func (m *Map) Used() uint64 {
	return m.used
}

// Get performs a lookup of the key that key points to. It returns a pointer to
// the element, or false if the key doesn't exist.
func (m *Map) Get(typ *abi.SwissMapType, key unsafe.Pointer) (unsafe.Pointer, bool) {
	return m.getWithoutKey(typ, key)
}

func (m *Map) getWithKey(typ *abi.SwissMapType, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer, bool) {
	if m.Used() == 0 {
		return nil, nil, false
	}

	if m.writing != 0 {
		fatal("concurrent map read and map write")
	}

	hash := typ.Hasher(key, m.seed)

	if m.dirLen == 0 {
		return m.getWithKeySmall(typ, hash, key)
	}

	idx := m.directoryIndex(hash)
	return m.directoryAt(idx).getWithKey(typ, hash, key)
}

func (m *Map) getWithoutKey(typ *abi.SwissMapType, key unsafe.Pointer) (unsafe.Pointer, bool) {
	if m.Used() == 0 {
		return nil, false
	}

	if m.writing != 0 {
		fatal("concurrent map read and map write")
	}

	hash := typ.Hasher(key, m.seed)

	if m.dirLen == 0 {
		_, elem, ok := m.getWithKeySmall(typ, hash, key)
		return elem, ok
	}

	idx := m.directoryIndex(hash)
	return m.directoryAt(idx).getWithoutKey(typ, hash, key)
}

func (m *Map) getWithKeySmall(typ *abi.SwissMapType, hash uintptr, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer, bool) {
	g := groupReference{
		data: m.dirPtr,
	}

	match := g.ctrls().matchH2(h2(hash))

	for match != 0 {
		i := match.first()

		slotKey := g.key(typ, i)
		if typ.IndirectKey() {
			slotKey = *((*unsafe.Pointer)(slotKey))
		}

		if typ.Key.Equal(key, slotKey) {
			slotElem := g.elem(typ, i)
			if typ.IndirectElem() {
				slotElem = *((*unsafe.Pointer)(slotElem))
			}
			return slotKey, slotElem, true
		}

		match = match.removeFirst()
	}

	// No match here means key is not in the map.
	// (A single group means no need to probe or check for empty).
	return nil, nil, false
}

func (m *Map) Put(typ *abi.SwissMapType, key, elem unsafe.Pointer) {
	slotElem := m.PutSlot(typ, key)
	typedmemmove(typ.Elem, slotElem, elem)
}

// PutSlot returns a pointer to the element slot where an inserted element
// should be written.
//
// PutSlot never returns nil.
func (m *Map) PutSlot(typ *abi.SwissMapType, key unsafe.Pointer) unsafe.Pointer {
	if m.writing != 0 {
		fatal("concurrent map writes")
	}

	hash := typ.Hasher(key, m.seed)

	// Set writing after calling Hasher, since Hasher may panic, in which
	// case we have not actually done a write.
	m.writing ^= 1 // toggle, see comment on writing

	if m.dirPtr == nil {
		m.growToSmall(typ)
	}

	if m.dirLen == 0 {
		if m.used < abi.SwissMapGroupSlots {
			elem := m.putSlotSmall(typ, hash, key)

			if m.writing == 0 {
				fatal("concurrent map writes")
			}
			m.writing ^= 1

			return elem
		}

		// Can't fit another entry, grow to full size map.
		//
		// TODO(prattmic): If this is an update to an existing key then
		// we actually don't need to grow.
		m.growToTable(typ)
	}

	for {
		idx := m.directoryIndex(hash)
		elem, ok := m.directoryAt(idx).PutSlot(typ, m, hash, key)
		if !ok {
			continue
		}

		if m.writing == 0 {
			fatal("concurrent map writes")
		}
		m.writing ^= 1

		return elem
	}
}

func (m *Map) putSlotSmall(typ *abi.SwissMapType, hash uintptr, key unsafe.Pointer) unsafe.Pointer {
	g := groupReference{
		data: m.dirPtr,
	}

	match := g.ctrls().matchH2(h2(hash))

	// Look for an existing slot containing this key.
	for match != 0 {
		i := match.first()

		slotKey := g.key(typ, i)
		if typ.IndirectKey() {
			slotKey = *((*unsafe.Pointer)(slotKey))
		}
		if typ.Key.Equal(key, slotKey) {
			if typ.NeedKeyUpdate() {
				typedmemmove(typ.Key, slotKey, key)
			}

			slotElem := g.elem(typ, i)
			if typ.IndirectElem() {
				slotElem = *((*unsafe.Pointer)(slotElem))
			}

			return slotElem
		}
		match = match.removeFirst()
	}

	// There can't be deleted slots, small maps can't have them
	// (see deleteSmall). Use matchEmptyOrDeleted as it is a bit
	// more efficient than matchEmpty.
	match = g.ctrls().matchEmptyOrDeleted()
	if match == 0 {
		fatal("small map with no empty slot (concurrent map writes?)")
		return nil
	}

	i := match.first()

	slotKey := g.key(typ, i)
	if typ.IndirectKey() {
		kmem := newobject(typ.Key)
		*(*unsafe.Pointer)(slotKey) = kmem
		slotKey = kmem
	}
	typedmemmove(typ.Key, slotKey, key)

	slotElem := g.elem(typ, i)
	if typ.IndirectElem() {
		emem := newobject(typ.Elem)
		*(*unsafe.Pointer)(slotElem) = emem
		slotElem = emem
	}

	g.ctrls().set(i, ctrl(h2(hash)))
	m.used++

	return slotElem
}

func (m *Map) growToSmall(typ *abi.SwissMapType) {
	grp := newGroups(typ, 1)
	m.dirPtr = grp.data

	g := groupReference{
		data: m.dirPtr,
	}
	g.ctrls().setEmpty()
}

func (m *Map) growToTable(typ *abi.SwissMapType) {
	tab := newTable(typ, 2*abi.SwissMapGroupSlots, 0, 0)

	g := groupReference{
		data: m.dirPtr,
	}

	for i := uintptr(0); i < abi.SwissMapGroupSlots; i++ {
		if (g.ctrls().get(i) & ctrlEmpty) == ctrlEmpty {
			// Empty
			continue
		}

		key := g.key(typ, i)
		if typ.IndirectKey() {
			key = *((*unsafe.Pointer)(key))
		}

		elem := g.elem(typ, i)
		if typ.IndirectElem() {
			elem = *((*unsafe.Pointer)(elem))
		}

		hash := typ.Hasher(key, m.seed)

		tab.uncheckedPutSlot(typ, hash, key, elem)
	}

	directory := make([]*table, 1)

	directory[0] = tab

	m.dirPtr = unsafe.Pointer(&directory[0])
	m.dirLen = len(directory)

	m.globalDepth = 0
	m.globalShift = depthToShift(m.globalDepth)
}

func (m *Map) Delete(typ *abi.SwissMapType, key unsafe.Pointer) {
	if m == nil || m.Used() == 0 {
		if err := mapKeyError(typ, key); err != nil {
			panic(err) // see issue 23734
		}
		return
	}

	if m.writing != 0 {
		fatal("concurrent map writes")
	}

	hash := typ.Hasher(key, m.seed)

	// Set writing after calling Hasher, since Hasher may panic, in which
	// case we have not actually done a write.
	m.writing ^= 1 // toggle, see comment on writing

	if m.dirLen == 0 {
		m.deleteSmall(typ, hash, key)
	} else {
		idx := m.directoryIndex(hash)
		m.directoryAt(idx).Delete(typ, m, hash, key)
	}

	if m.used == 0 {
		// Reset the hash seed to make it more difficult for attackers
		// to repeatedly trigger hash collisions. See
		// https://go.dev/issue/25237.
		m.seed = uintptr(rand())
	}

	if m.writing == 0 {
		fatal("concurrent map writes")
	}
	m.writing ^= 1
}

func (m *Map) deleteSmall(typ *abi.SwissMapType, hash uintptr, key unsafe.Pointer) {
	g := groupReference{
		data: m.dirPtr,
	}

	match := g.ctrls().matchH2(h2(hash))

	for match != 0 {
		i := match.first()
		slotKey := g.key(typ, i)
		origSlotKey := slotKey
		if typ.IndirectKey() {
			slotKey = *((*unsafe.Pointer)(slotKey))
		}
		if typ.Key.Equal(key, slotKey) {
			m.used--

			if typ.IndirectKey() {
				// Clearing the pointer is sufficient.
				*(*unsafe.Pointer)(origSlotKey) = nil
			} else if typ.Key.Pointers() {
				// Only bother clearing if there are pointers.
				typedmemclr(typ.Key, slotKey)
			}

			slotElem := g.elem(typ, i)
			if typ.IndirectElem() {
				// Clearing the pointer is sufficient.
				*(*unsafe.Pointer)(slotElem) = nil
			} else {
				// Unlike keys, always clear the elem (even if
				// it contains no pointers), as compound
				// assignment operations depend on cleared
				// deleted values. See
				// https://go.dev/issue/25936.
				typedmemclr(typ.Elem, slotElem)
			}

			// We only have 1 group, so it is OK to immediately
			// reuse deleted slots.
			g.ctrls().set(i, ctrlEmpty)
			return
		}
		match = match.removeFirst()
	}
}

// Clear deletes all entries from the map resulting in an empty map.
func (m *Map) Clear(typ *abi.SwissMapType) {
	if m == nil || m.Used() == 0 {
		return
	}

	if m.writing != 0 {
		fatal("concurrent map writes")
	}
	m.writing ^= 1 // toggle, see comment on writing

	if m.dirLen == 0 {
		m.clearSmall(typ)
	} else {
		var lastTab *table
		for i := range m.dirLen {
			t := m.directoryAt(uintptr(i))
			if t == lastTab {
				continue
			}
			t.Clear(typ)
			lastTab = t
		}
		m.used = 0
		m.clearSeq++
		// TODO: shrink directory?
	}

	// Reset the hash seed to make it more difficult for attackers to
	// repeatedly trigger hash collisions. See https://go.dev/issue/25237.
	m.seed = uintptr(rand())

	if m.writing == 0 {
		fatal("concurrent map writes")
	}
	m.writing ^= 1
}

func (m *Map) clearSmall(typ *abi.SwissMapType) {
	g := groupReference{
		data: m.dirPtr,
	}

	typedmemclr(typ.Group, g.data)
	g.ctrls().setEmpty()

	m.used = 0
	m.clearSeq++
}

func (m *Map) Clone(typ *abi.SwissMapType) *Map {
	// Note: this should never be called with a nil map.
	if m.writing != 0 {
		fatal("concurrent map clone and map write")
	}

	// Shallow copy the Map structure.
	m2 := new(Map)
	*m2 = *m
	m = m2

	// We need to just deep copy the dirPtr field.
	if m.dirPtr == nil {
		// delayed group allocation, nothing to do.
	} else if m.dirLen == 0 {
		// Clone one group.
		oldGroup := groupReference{data: m.dirPtr}
		newGroup := groupReference{data: newGroups(typ, 1).data}
		cloneGroup(typ, newGroup, oldGroup)
		m.dirPtr = newGroup.data
	} else {
		// Clone each (different) table.
		oldDir := unsafe.Slice((**table)(m.dirPtr), m.dirLen)
		newDir := make([]*table, m.dirLen)
		for i, t := range oldDir {
			if i > 0 && t == oldDir[i-1] {
				newDir[i] = newDir[i-1]
				continue
			}
			newDir[i] = t.clone(typ)
		}
		m.dirPtr = unsafe.Pointer(&newDir[0])
	}

	return m
}