go map 源码阅读

go map 源码阅读#

内存布局：golang中的map是怎么组成的
初始化
设置map值
获取map值
迭代map
- 为什么迭代顺序每次都不一样
扩容
- 什么时候才会扩容
- 为什么是渐进式扩容

源码地址#

src/runtime/map.go

内存布局#


// A header for a Go map. map的布局
type hmap struct {
	// Note: the format of the hmap is also encoded in cmd/compile/internal/gc/reflect.go.
	// Make sure this stays in sync with the compiler's definition.
	count     int // # live cells == size of map.  Must be first (used by len() builtin) 长度
	flags     uint8 // 读 写 迭代 扩容 标识位
	B         uint8  // log_2 of # of buckets (can hold up to loadFactor * 2^B items) 哈希表持有的 buckets 数量,是2的对数
	noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details 溢出桶数量近似值
	hash0     uint32 // hash seed hash 种子

	buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0. 桶的起始地址
	oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
	nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated) 扩容进度

	extra *mapextra // optional fields
}

// mapextra holds fields that are not present on all maps.
type mapextra struct {
	// If both key and elem do not contain pointers and are inline, then we mark bucket
	// type as containing no pointers. This avoids scanning such maps.
	// However, bmap.overflow is a pointer. In order to keep overflow buckets
	// alive, we store pointers to all overflow buckets in hmap.extra.overflow and hmap.extra.oldoverflow.
	// overflow and oldoverflow are only used if key and elem do not contain pointers.
	// overflow contains overflow buckets for hmap.buckets.
	// oldoverflow contains overflow buckets for hmap.oldbuckets.
	// The indirection allows to store a pointer to the slice in hiter.
	overflow    *[]*bmap
	oldoverflow *[]*bmap

	// nextOverflow holds a pointer to a free overflow bucket. 下一个空闲的溢出桶
	nextOverflow *bmap
}

// 一个bmap中有
// A bucket for a Go map.
type bmap struct {
	// tophash generally contains the top byte of the hash value
	// for each key in this bucket. If tophash[0] < minTopHash,
	// tophash[0] is a bucket evacuation state instead.
	tophash [bucketCnt]uint8
	// Followed by bucketCnt keys and then bucketCnt elems.
	// NOTE: packing all the keys together and then all the elems together makes the 将所有的keys 和 elems 放在一起会比 key/elem/key/elem 这种方式复杂，但是可以减少内存对齐的内存消耗
	// code a bit more complicated than alternating key/elem/key/elem/... but it allows
	// us to eliminate padding which would be needed for, e.g., map[int64]int8.
	// Followed by an overflow pointer.
}

map 结构是 hmap 的结构体实现，buckets 存放了 bmap 结构体
bmap 就是所谓的桶
一个 bmap 中只能存储 8 个 key/elem
bmap 的 key/elem 没有在结构体中定义下来
bmap 中有一个 overflow 指针，指向了下一个 bmap 结构

初始化#

make(map[string]string) // makemap_small

make(map[string]string, 1024) // makemap


// makemap_small implements Go map creation for make(map[k]v) and
// make(map[k]v, hint) when hint is known to be at most bucketCnt
// at compile time and the map needs to be allocated on the heap.
func makemap_small() *hmap {
	h := new(hmap)
	h.hash0 = fastrand() // 获取一个随机的hash种子
	return h
}

// makemap implements Go map creation for make(map[k]v, hint).
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If h.buckets != nil, bucket pointed to can be used as the first bucket.
func makemap(t *maptype, hint int, h *hmap) *hmap {
	mem, overflow := math.MulUintptr(uintptr(hint), t.bucket.size)
	if overflow || mem > maxAlloc {
		hint = 0
	}

	// initialize Hmap
	if h == nil {
		h = new(hmap)
	}
	h.hash0 = fastrand()

	// Find the size parameter B which will hold the requested # of elements.
	// For hint < 0 overLoadFactor returns false since hint < bucketCnt.
	B := uint8(0)
	for overLoadFactor(hint, B) {
		B++
	}
	h.B = B

	// allocate initial hash table
	// if B == 0, the buckets field is allocated lazily later (in mapassign) 如果 B 等于 0， 会在设置值得时候分配桶
	// If hint is large zeroing this memory could take a while.
	if h.B != 0 {// 如果没有超过 8 个初始化元素，就不会转换成链表结构
		var nextOverflow *bmap
		h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
		if nextOverflow != nil {
			h.extra = new(mapextra)
			h.extra.nextOverflow = nextOverflow
		}
	}

	return h
}

初始化 hash 种子
初始化时指定大小
- 获取一个合适的桶个数并初始化

设置map值#

map["xxx"] = "yyy" // mapassign

// Like mapaccess, but allocates a slot for the key if it is not present in the map.
func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
	if h == nil {
		panic(plainError("assignment to entry in nil map"))
	}
	if raceenabled {
		callerpc := getcallerpc()
		pc := funcPC(mapassign)
		racewritepc(unsafe.Pointer(h), callerpc, pc)
		raceReadObjectPC(t.key, key, callerpc, pc)
	}
	if msanenabled {
		msanread(key, t.key.size)
	}
	if h.flags&hashWriting != 0 { // 并发写判断
		throw("concurrent map writes")
	}
	hash := t.hasher(key, uintptr(h.hash0)) // 计算当前key的hash值

	// Set hashWriting after calling t.hasher, since t.hasher may panic,
	// in which case we have not actually done a write.
	h.flags ^= hashWriting // 设置标识位为-> 写

	if h.buckets == nil {
		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1) 初始化一个桶
	}

again:
	bucket := hash & bucketMask(h.B) // & 运算 找到对应的桶
	if h.growing() { // 当前map正在扩容
		growWork(t, h, bucket) // 渐进式迁移
	}
	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
	top := tophash(hash)

	var inserti *uint8
	var insertk unsafe.Pointer
	var elem unsafe.Pointer
bucketloop:
	for {
		for i := uintptr(0); i < bucketCnt; i++ {
			if b.tophash[i] != top {// 找到一个可以插入的空位置
				if isEmpty(b.tophash[i]) && inserti == nil { // tophash 为空，并且为第一个待插入点
					inserti = &b.tophash[i]
					insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
					elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
				}
				if b.tophash[i] == emptyRest {
					break bucketloop
				}
				continue
			}
			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
			if t.indirectkey() {
				k = *((*unsafe.Pointer)(k))
			}
			if !t.key.equal(key, k) { // 找到对应的key值, 判断key是否已经存在了,存在就直接更新
				continue
			}
			// already have a mapping for key. Update it.
			if t.needkeyupdate() {
				typedmemmove(t.key, k, key)
			}
			elem = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize))
			goto done
		}
		ovf := b.overflow(t) // 设置当前桶为溢出桶 并继续循环查找
		if ovf == nil {
			break
		}
		b = ovf
	}

	// Did not find mapping for key. Allocate new cell & add entry.

	// If we hit the max load factor or we have too many overflow buckets,
	// and we're not already in the middle of growing, start growing.
	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
		hashGrow(t, h)
		goto again // Growing the table invalidates everything, so try again
	}

	if inserti == nil { // 所有的桶里面已经存储了数据，就新添加一个溢出桶到最后的桶中
		// all current buckets are full, allocate a new one.
		newb := h.newoverflow(t, b)
		inserti = &newb.tophash[0]
		insertk = add(unsafe.Pointer(newb), dataOffset)
		elem = add(insertk, bucketCnt*uintptr(t.keysize))
	}

	// store new key/elem at insert position
	if t.indirectkey() {
		kmem := newobject(t.key)
		*(*unsafe.Pointer)(insertk) = kmem
		insertk = kmem
	}
	if t.indirectelem() {
		vmem := newobject(t.elem)
		*(*unsafe.Pointer)(elem) = vmem
	}
	typedmemmove(t.key, insertk, key)
	*inserti = top
	h.count++ // map 长度加一

done:
	if h.flags&hashWriting == 0 {
		throw("concurrent map writes")
	}
	h.flags &^= hashWriting
	if t.indirectelem() {
		elem = *((*unsafe.Pointer)(elem))
	}
	return elem
}

计算获取的key 的hash值
通过最后B位确定 key 对应的hash桶
计算tophash， tophash 是为了方便快速定位前8位的值是否存在
遍历 8 个槽
- 如果tophash一致，比较key 值是否一致，如果一致就返回找到的地址
- 能确定第一个待插入点位置就插入第一个空闲位置
- 所有位置都满了，就创建新的溢出桶并插入新桶中

获取map值#

v := map[key] // mapaccess1 v     := *mapaccess1(maptype, hash, &key)
v,ok:=map[key] // mapaccess2 v, ok := mapaccess2(maptype, hash, &key)


// mapaccess1 returns a pointer to h[key].  Never returns nil, instead
// it will return a reference to the zero object for the elem type if
// the key is not in the map.
// NOTE: The returned pointer may keep the whole map live, so don't
// hold onto it for very long.
func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
	if raceenabled && h != nil {
		callerpc := getcallerpc()
		pc := funcPC(mapaccess1)
		racereadpc(unsafe.Pointer(h), callerpc, pc)
		raceReadObjectPC(t.key, key, callerpc, pc)
	}
	if msanenabled && h != nil {
		msanread(key, t.key.size)
	}
	if h == nil || h.count == 0 {
		if t.hashMightPanic() {
			t.hasher(key, 0) // see issue 23734
		}
		return unsafe.Pointer(&zeroVal[0]) // 返回对应对象的 0 值
	}
	if h.flags&hashWriting != 0 {
		throw("concurrent map read and map write")
	}
	hash := t.hasher(key, uintptr(h.hash0))
	m := bucketMask(h.B)
	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize))) // 起始的桶地址
	if c := h.oldbuckets; c != nil {
		if !h.sameSizeGrow() {
			// There used to be half as many buckets; mask down one more power of two.
			m >>= 1 // 两倍扩容时找到就的桶入口地址
		}
		oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
		if !evacuated(oldb) { // 旧桶扩容未完成
			b = oldb // 在旧桶上面查找
		}
	}
	top := tophash(hash) // 计算key对应的 tophash 值
bucketloop:
	for ; b != nil; b = b.overflow(t) {
		for i := uintptr(0); i < bucketCnt; i++ {
			if b.tophash[i] != top { // 找到相同的top hash位置 确定 i 的值
				if b.tophash[i] == emptyRest {
					break bucketloop
				}
				continue
			}
			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize)) // 通过key*i + offset 得到key 的偏移位置地址
			if t.indirectkey() {
				k = *((*unsafe.Pointer)(k))
			}
			if t.key.equal(key, k) { // 访问的key和map中保存的k 相同
				e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.elemsize)) // 找到 e 起始位置地址
				if t.indirectelem() {
					e = *((*unsafe.Pointer)(e))
				}
				return e
			}
		}
	}
	return unsafe.Pointer(&zeroVal[0])
}

计算获取的key 的hash值
通过最后B位确定 key 对应的hash桶
计算tophash， tophash 是为了方便快速定位前8位的值是否存在
遍历 8 个槽，如果tophash一致，比较key 值是否一致

ps: 判断key 是否存在的方法和取值方法实现是一致的

迭代map#

// it = new(hiter)
// mapiterinit(maptype,hash,it)
for k,v:=rang map {}  // mapiterinit(it)



// mapiterinit initializes the hiter struct used for ranging over maps.
// The hiter struct pointed to by 'it' is allocated on the stack
// by the compilers order pass or on the heap by reflect_mapiterinit.
// Both need to have zeroed hiter since the struct contains pointers.
func mapiterinit(t *maptype, h *hmap, it *hiter) { // 初始化迭代器
	if raceenabled && h != nil {
		callerpc := getcallerpc()
		racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapiterinit))
	}

	if h == nil || h.count == 0 {
		return
	}

	if unsafe.Sizeof(hiter{})/sys.PtrSize != 12 {
		throw("hash_iter size incorrect") // see cmd/compile/internal/gc/reflect.go
	}
	it.t = t
	it.h = h

	// grab snapshot of bucket state
	it.B = h.B
	it.buckets = h.buckets
	if t.bucket.ptrdata == 0 {
		// Allocate the current slice and remember pointers to both current and old.
		// This preserves all relevant overflow buckets alive even if
		// the table grows and/or overflow buckets are added to the table
		// while we are iterating.
		h.createOverflow()
		it.overflow = h.extra.overflow
		it.oldoverflow = h.extra.oldoverflow
	}

	// decide where to start
	r := uintptr(fastrand()) // 随机迭代顺序
	if h.B > 31-bucketCntBits {
		r += uintptr(fastrand()) << 31
	}
	it.startBucket = r & bucketMask(h.B) // 找到迭代的起始桶
	it.offset = uint8(r >> h.B & (bucketCnt - 1)) // 当前桶内偏移量

	// iterator state
	it.bucket = it.startBucket

	// Remember we have an iterator.
	// Can run concurrently with another mapiterinit().
	if old := h.flags; old&(iterator|oldIterator) != iterator|oldIterator {
		atomic.Or8(&h.flags, iterator|oldIterator)
	}

	mapiternext(it)
}


func mapiternext(it *hiter) {// 这里为什么要分开
	h := it.h
	if raceenabled {
		callerpc := getcallerpc()
		racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapiternext))
	}
	if h.flags&hashWriting != 0 {
		throw("concurrent map iteration and map write")
	}
	t := it.t
	bucket := it.bucket
	b := it.bptr
	i := it.i
	checkBucket := it.checkBucket

next:
	if b == nil {
		if bucket == it.startBucket && it.wrapped { // 是否迭代了一轮
			// end of iteration
			it.key = nil
			it.elem = nil
			return
		}
		if h.growing() && it.B == h.B {
			// Iterator was started in the middle of a grow, and the grow isn't done yet.
			// If the bucket we're looking at hasn't been filled in yet (i.e. the old
			// bucket hasn't been evacuated) then we need to iterate through the old
			// bucket and only return the ones that will be migrated to this bucket.
			oldbucket := bucket & it.h.oldbucketmask() // 找到oldbucket 位置
			b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
			if !evacuated(b) {// 扩容未完成
				checkBucket = bucket
			} else {
				b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
				checkBucket = noCheck
			}
		} else {
			b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))// 找到下一个桶
			checkBucket = noCheck
		}
		bucket++
		if bucket == bucketShift(it.B) { // 下一个桶是最后一个桶时 设置桶位置为0起点(相当于一个环,又回到起点继续遍历)
			bucket = 0
			it.wrapped = true
		}
		i = 0
	}
	for ; i < bucketCnt; i++ {// 遍历8个桶的数据
		offi := (i + it.offset) & (bucketCnt - 1)
		if isEmpty(b.tophash[offi]) || b.tophash[offi] == evacuatedEmpty {
			// TODO: emptyRest is hard to use here, as we start iterating
			// in the middle of a bucket. It's feasible, just tricky.
			continue
		}
		k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
		if t.indirectkey() {
			k = *((*unsafe.Pointer)(k))
		}
		e := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.elemsize))
		if checkBucket != noCheck && !h.sameSizeGrow() {
			// Special case: iterator was started during a grow to a larger size
			// and the grow is not done yet. We're working on a bucket whose
			// oldbucket has not been evacuated yet. Or at least, it wasn't
			// evacuated when we started the bucket. So we're iterating
			// through the oldbucket, skipping any keys that will go
			// to the other new bucket (each oldbucket expands to two
			// buckets during a grow).
			if t.reflexivekey() || t.key.equal(k, k) {
				// If the item in the oldbucket is not destined for
				// the current new bucket in the iteration, skip it.
				hash := t.hasher(k, uintptr(h.hash0))
				if hash&bucketMask(it.B) != checkBucket {
					continue
				}
			} else {
				// Hash isn't repeatable if k != k (NaNs).  We need a
				// repeatable and randomish choice of which direction
				// to send NaNs during evacuation. We'll use the low
				// bit of tophash to decide which way NaNs go.
				// NOTE: this case is why we need two evacuate tophash
				// values, evacuatedX and evacuatedY, that differ in
				// their low bit.
				if checkBucket>>(it.B-1) != uintptr(b.tophash[offi]&1) {
					continue
				}
			}
		}
		if (b.tophash[offi] != evacuatedX && b.tophash[offi] != evacuatedY) ||
			!(t.reflexivekey() || t.key.equal(k, k)) {
			// This is the golden data, we can return it.
			// OR
			// key!=key, so the entry can't be deleted or updated, so we can just return it.
			// That's lucky for us because when key!=key we can't look it up successfully.
			it.key = k
			if t.indirectelem() {
				e = *((*unsafe.Pointer)(e))
			}
			it.elem = e
		} else {
			// The hash table has grown since the iterator was started.
			// The golden data for this key is now somewhere else.
			// Check the current hash table for the data.
			// This code handles the case where the key
			// has been deleted, updated, or deleted and reinserted.
			// NOTE: we need to regrab the key as it has potentially been
			// updated to an equal() but not identical key (e.g. +0.0 vs -0.0).
			rk, re := mapaccessK(t, h, k)
			if rk == nil {
				continue // key has been deleted
			}
			it.key = rk
			it.elem = re
		}
		it.bucket = bucket // 记录当前桶位置
		if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
			it.bptr = b
		}
		it.i = i + 1
		it.checkBucket = checkBucket
		return // 循环获取到一个就退出
	}
	b = b.overflow(t) // 取完一个桶后继续遍历后面的溢出桶
	i = 0
	goto next
}

先初始化迭代器,随机找到迭代起点桶,将迭代器指向hmap结构
依次遍历所有桶和桶内溢出桶
遍历到桶末尾时从头重新遍历,直到和起始遍历点重合

扩容#

当前桶数量已经超过装载因子 6.5 ，触发翻倍扩容 (装载因子 := 元素数量 / 桶数量)
当溢出桶使用过多，并且空间不连续，触发等量扩容（内存整理）


func hashGrow(t *maptype, h *hmap) {// 扩容
	// If we've hit the load factor, get bigger.
	// Otherwise, there are too many overflow buckets,
	// so keep the same number of buckets and "grow" laterally.
	bigger := uint8(1)
	if !overLoadFactor(h.count+1, h.B) {
		bigger = 0
		h.flags |= sameSizeGrow
	}
	oldbuckets := h.buckets
	newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)

	flags := h.flags &^ (iterator | oldIterator)
	if h.flags&iterator != 0 {
		flags |= oldIterator
	}
	// commit the grow (atomic wrt gc)
	h.B += bigger
	h.flags = flags
	h.oldbuckets = oldbuckets
	h.buckets = newbuckets
	h.nevacuate = 0
	h.noverflow = 0

	if h.extra != nil && h.extra.overflow != nil {
		// Promote current overflow buckets to the old generation.
		if h.extra.oldoverflow != nil {
			throw("oldoverflow is not nil")
		}
		h.extra.oldoverflow = h.extra.overflow
		h.extra.overflow = nil
	}
	if nextOverflow != nil {
		if h.extra == nil {
			h.extra = new(mapextra)
		}
		h.extra.nextOverflow = nextOverflow
	}

	// the actual copying of the hash table data is done incrementally
	// by growWork() and evacuate().
}



func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))// 旧桶起始地址
	newbit := h.noldbuckets() // 扩大到的数量
	if !evacuated(b) {
		// TODO: reuse overflow buckets instead of using new ones, if there
		// is no iterator using the old buckets.  (If !oldIterator.)

		// xy contains the x and y (low and high) evacuation destinations.
		var xy [2]evacDst
		x := &xy[0]
		x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))// 桶的起始地址+偏移量 = 当前桶的起始地址
		x.k = add(unsafe.Pointer(x.b), dataOffset)
		x.e = add(x.k, bucketCnt*uintptr(t.keysize))

		if !h.sameSizeGrow() {
			// Only calculate y pointers if we're growing bigger.
			// Otherwise GC can see bad pointers.
			y := &xy[1]
			y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
			y.k = add(unsafe.Pointer(y.b), dataOffset)
			y.e = add(y.k, bucketCnt*uintptr(t.keysize))
		}

		for ; b != nil; b = b.overflow(t) {
			k := add(unsafe.Pointer(b), dataOffset) // 计算key起始地址
			e := add(k, bucketCnt*uintptr(t.keysize)) // 计算 val 起始地址
			for i := 0; i < bucketCnt; i, k, e = i+1, add(k, uintptr(t.keysize)), add(e, uintptr(t.elemsize)) {
				top := b.tophash[i]
				if isEmpty(top) {
					b.tophash[i] = evacuatedEmpty
					continue
				}
				if top < minTopHash {
					throw("bad map state")
				}
				k2 := k
				if t.indirectkey() {
					k2 = *((*unsafe.Pointer)(k2))
				}
				var useY uint8
				if !h.sameSizeGrow() {
					// Compute hash to make our evacuation decision (whether we need
					// to send this key/elem to bucket x or bucket y).
					hash := t.hasher(k2, uintptr(h.hash0))
					if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.equal(k2, k2) {
						// If key != key (NaNs), then the hash could be (and probably
						// will be) entirely different from the old hash. Moreover,
						// it isn't reproducible. Reproducibility is required in the
						// presence of iterators, as our evacuation decision must
						// match whatever decision the iterator made.
						// Fortunately, we have the freedom to send these keys either
						// way. Also, tophash is meaningless for these kinds of keys.
						// We let the low bit of tophash drive the evacuation decision.
						// We recompute a new random tophash for the next level so
						// these keys will get evenly distributed across all buckets
						// after multiple grows.
						useY = top & 1
						top = tophash(hash)
					} else {
						if hash&newbit != 0 {
							useY = 1
						}
					}
				}

				if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
					throw("bad evacuatedN")
				}

				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
				dst := &xy[useY]                 // evacuation destination

				if dst.i == bucketCnt { // 创建nextoverflow
					dst.b = h.newoverflow(t, dst.b)
					dst.i = 0
					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
					dst.e = add(dst.k, bucketCnt*uintptr(t.keysize))
				}
				dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
				if t.indirectkey() {
					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
				} else {
					typedmemmove(t.key, dst.k, k) // copy elem
				}
				if t.indirectelem() {
					*(*unsafe.Pointer)(dst.e) = *(*unsafe.Pointer)(e)
				} else {
					typedmemmove(t.elem, dst.e, e)
				}
				dst.i++
				// These updates might push these pointers past the end of the
				// key or elem arrays.  That's ok, as we have the overflow pointer
				// at the end of the bucket to protect against pointing past the
				// end of the bucket.
				dst.k = add(dst.k, uintptr(t.keysize))
				dst.e = add(dst.e, uintptr(t.elemsize))
			}
		}
		// Unlink the overflow buckets & clear key/elem to help GC.
		if h.flags&oldIterator == 0 && t.bucket.ptrdata != 0 {
			b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
			// Preserve b.tophash because the evacuation
			// state is maintained there.
			ptr := add(b, dataOffset)
			n := uintptr(t.bucketsize) - dataOffset
			memclrHasPointers(ptr, n)
		}
	}

	if oldbucket == h.nevacuate {
		advanceEvacuationMark(h, t, newbit)
	}
}