Source file src/runtime/map.go

Documentation: runtime

  // Copyright 2014 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 runtime
  
  // This file contains the implementation of Go's map type.
  //
  // A map is just a hash table. The data is arranged
  // into an array of buckets. Each bucket contains up to
  // 8 key/value pairs. The low-order bits of the hash are
  // used to select a bucket. Each bucket contains a few
  // high-order bits of each hash to distinguish the entries
  // within a single bucket.
  //
  // If more than 8 keys hash to a bucket, we chain on
  // extra buckets.
  //
  // When the hashtable grows, we allocate a new array
  // of buckets twice as big. Buckets are incrementally
  // copied from the old bucket array to the new bucket array.
  //
  // Map iterators walk through the array of buckets and
  // return the keys in walk order (bucket #, then overflow
  // chain order, then bucket index).  To maintain iteration
  // semantics, we never move keys within their bucket (if
  // we did, keys might be returned 0 or 2 times).  When
  // growing the table, iterators remain iterating through the
  // old table and must check the new table if the bucket
  // they are iterating through has been moved ("evacuated")
  // to the new table.
  
  // Picking loadFactor: too large and we have lots of overflow
  // buckets, too small and we waste a lot of space. I wrote
  // a simple program to check some stats for different loads:
  // (64-bit, 8 byte keys and values)
  //  loadFactor    %overflow  bytes/entry     hitprobe    missprobe
  //        4.00         2.13        20.77         3.00         4.00
  //        4.50         4.05        17.30         3.25         4.50
  //        5.00         6.85        14.77         3.50         5.00
  //        5.50        10.55        12.94         3.75         5.50
  //        6.00        15.27        11.67         4.00         6.00
  //        6.50        20.90        10.79         4.25         6.50
  //        7.00        27.14        10.15         4.50         7.00
  //        7.50        34.03         9.73         4.75         7.50
  //        8.00        41.10         9.40         5.00         8.00
  //
  // %overflow   = percentage of buckets which have an overflow bucket
  // bytes/entry = overhead bytes used per key/value pair
  // hitprobe    = # of entries to check when looking up a present key
  // missprobe   = # of entries to check when looking up an absent key
  //
  // Keep in mind this data is for maximally loaded tables, i.e. just
  // before the table grows. Typical tables will be somewhat less loaded.
  
  import (
  	"runtime/internal/atomic"
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  const (
  	// Maximum number of key/value pairs a bucket can hold.
  	bucketCntBits = 3
  	bucketCnt     = 1 << bucketCntBits
  
  	// Maximum average load of a bucket that triggers growth is 6.5.
  	// Represent as loadFactorNum/loadFactDen, to allow integer math.
  	loadFactorNum = 13
  	loadFactorDen = 2
  
  	// Maximum key or value size to keep inline (instead of mallocing per element).
  	// Must fit in a uint8.
  	// Fast versions cannot handle big values - the cutoff size for
  	// fast versions in cmd/compile/internal/gc/walk.go must be at most this value.
  	maxKeySize   = 128
  	maxValueSize = 128
  
  	// data offset should be the size of the bmap struct, but needs to be
  	// aligned correctly. For amd64p32 this means 64-bit alignment
  	// even though pointers are 32 bit.
  	dataOffset = unsafe.Offsetof(struct {
  		b bmap
  		v int64
  	}{}.v)
  
  	// Possible tophash values. We reserve a few possibilities for special marks.
  	// Each bucket (including its overflow buckets, if any) will have either all or none of its
  	// entries in the evacuated* states (except during the evacuate() method, which only happens
  	// during map writes and thus no one else can observe the map during that time).
  	empty          = 0 // cell is empty
  	evacuatedEmpty = 1 // cell is empty, bucket is evacuated.
  	evacuatedX     = 2 // key/value is valid.  Entry has been evacuated to first half of larger table.
  	evacuatedY     = 3 // same as above, but evacuated to second half of larger table.
  	minTopHash     = 4 // minimum tophash for a normal filled cell.
  
  	// flags
  	iterator     = 1 // there may be an iterator using buckets
  	oldIterator  = 2 // there may be an iterator using oldbuckets
  	hashWriting  = 4 // a goroutine is writing to the map
  	sameSizeGrow = 8 // the current map growth is to a new map of the same size
  
  	// sentinel bucket ID for iterator checks
  	noCheck = 1<<(8*sys.PtrSize) - 1
  )
  
  // A header for a Go map.
  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)
  	flags     uint8
  	B         uint8  // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
  	noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
  	hash0     uint32 // hash seed
  
  	buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
  	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)
  
  	extra *mapextra // optional fields
  }
  
  // mapextra holds fields that are not present on all maps.
  type mapextra struct {
  	// If both key and value 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 value 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.
  	nextOverflow *bmap
  }
  
  // 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 values.
  	// NOTE: packing all the keys together and then all the values together makes the
  	// code a bit more complicated than alternating key/value/key/value/... but it allows
  	// us to eliminate padding which would be needed for, e.g., map[int64]int8.
  	// Followed by an overflow pointer.
  }
  
  // A hash iteration structure.
  // If you modify hiter, also change cmd/compile/internal/gc/reflect.go to indicate
  // the layout of this structure.
  type hiter struct {
  	key         unsafe.Pointer // Must be in first position.  Write nil to indicate iteration end (see cmd/internal/gc/range.go).
  	value       unsafe.Pointer // Must be in second position (see cmd/internal/gc/range.go).
  	t           *maptype
  	h           *hmap
  	buckets     unsafe.Pointer // bucket ptr at hash_iter initialization time
  	bptr        *bmap          // current bucket
  	overflow    *[]*bmap       // keeps overflow buckets of hmap.buckets alive
  	oldoverflow *[]*bmap       // keeps overflow buckets of hmap.oldbuckets alive
  	startBucket uintptr        // bucket iteration started at
  	offset      uint8          // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
  	wrapped     bool           // already wrapped around from end of bucket array to beginning
  	B           uint8
  	i           uint8
  	bucket      uintptr
  	checkBucket uintptr
  }
  
  // bucketShift returns 1<<b, optimized for code generation.
  func bucketShift(b uint8) uintptr {
  	if sys.GoarchAmd64|sys.GoarchAmd64p32|sys.Goarch386 != 0 {
  		b &= sys.PtrSize*8 - 1 // help x86 archs remove shift overflow checks
  	}
  	return uintptr(1) << b
  }
  
  // bucketMask returns 1<<b - 1, optimized for code generation.
  func bucketMask(b uint8) uintptr {
  	return bucketShift(b) - 1
  }
  
  // tophash calculates the tophash value for hash.
  func tophash(hash uintptr) uint8 {
  	top := uint8(hash >> (sys.PtrSize*8 - 8))
  	if top < minTopHash {
  		top += minTopHash
  	}
  	return top
  }
  
  func evacuated(b *bmap) bool {
  	h := b.tophash[0]
  	return h > empty && h < minTopHash
  }
  
  func (b *bmap) overflow(t *maptype) *bmap {
  	return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize))
  }
  
  func (b *bmap) setoverflow(t *maptype, ovf *bmap) {
  	*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize)) = ovf
  }
  
  func (b *bmap) keys() unsafe.Pointer {
  	return add(unsafe.Pointer(b), dataOffset)
  }
  
  // incrnoverflow increments h.noverflow.
  // noverflow counts the number of overflow buckets.
  // This is used to trigger same-size map growth.
  // See also tooManyOverflowBuckets.
  // To keep hmap small, noverflow is a uint16.
  // When there are few buckets, noverflow is an exact count.
  // When there are many buckets, noverflow is an approximate count.
  func (h *hmap) incrnoverflow() {
  	// We trigger same-size map growth if there are
  	// as many overflow buckets as buckets.
  	// We need to be able to count to 1<<h.B.
  	if h.B < 16 {
  		h.noverflow++
  		return
  	}
  	// Increment with probability 1/(1<<(h.B-15)).
  	// When we reach 1<<15 - 1, we will have approximately
  	// as many overflow buckets as buckets.
  	mask := uint32(1)<<(h.B-15) - 1
  	// Example: if h.B == 18, then mask == 7,
  	// and fastrand & 7 == 0 with probability 1/8.
  	if fastrand()&mask == 0 {
  		h.noverflow++
  	}
  }
  
  func (h *hmap) newoverflow(t *maptype, b *bmap) *bmap {
  	var ovf *bmap
  	if h.extra != nil && h.extra.nextOverflow != nil {
  		// We have preallocated overflow buckets available.
  		// See makeBucketArray for more details.
  		ovf = h.extra.nextOverflow
  		if ovf.overflow(t) == nil {
  			// We're not at the end of the preallocated overflow buckets. Bump the pointer.
  			h.extra.nextOverflow = (*bmap)(add(unsafe.Pointer(ovf), uintptr(t.bucketsize)))
  		} else {
  			// This is the last preallocated overflow bucket.
  			// Reset the overflow pointer on this bucket,
  			// which was set to a non-nil sentinel value.
  			ovf.setoverflow(t, nil)
  			h.extra.nextOverflow = nil
  		}
  	} else {
  		ovf = (*bmap)(newobject(t.bucket))
  	}
  	h.incrnoverflow()
  	if t.bucket.kind&kindNoPointers != 0 {
  		h.createOverflow()
  		*h.extra.overflow = append(*h.extra.overflow, ovf)
  	}
  	b.setoverflow(t, ovf)
  	return ovf
  }
  
  func (h *hmap) createOverflow() {
  	if h.extra == nil {
  		h.extra = new(mapextra)
  	}
  	if h.extra.overflow == nil {
  		h.extra.overflow = new([]*bmap)
  	}
  }
  
  func makemap64(t *maptype, hint int64, h *hmap) *hmap {
  	if int64(int(hint)) != hint {
  		hint = 0
  	}
  	return makemap(t, int(hint), h)
  }
  
  // makehmap_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()
  	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 {
  	if hint < 0 || hint > int(maxSliceCap(t.bucket.size)) {
  		hint = 0
  	}
  
  	// initialize Hmap
  	if h == nil {
  		h = new(hmap)
  	}
  	h.hash0 = fastrand()
  
  	// find size parameter which will hold the requested # of elements
  	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)
  	// If hint is large zeroing this memory could take a while.
  	if h.B != 0 {
  		var nextOverflow *bmap
  		h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
  		if nextOverflow != nil {
  			h.extra = new(mapextra)
  			h.extra.nextOverflow = nextOverflow
  		}
  	}
  
  	return h
  }
  
  // makeBucketArray initializes a backing array for map buckets.
  // 1<<b is the minimum number of buckets to allocate.
  // dirtyalloc should either be nil or a bucket array previously
  // allocated by makeBucketArray with the same t and b parameters.
  // If dirtyalloc is nil a new backing array will be alloced and
  // otherwise dirtyalloc will be cleared and reused as backing array.
  func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
  	base := bucketShift(b)
  	nbuckets := base
  	// For small b, overflow buckets are unlikely.
  	// Avoid the overhead of the calculation.
  	if b >= 4 {
  		// Add on the estimated number of overflow buckets
  		// required to insert the median number of elements
  		// used with this value of b.
  		nbuckets += bucketShift(b - 4)
  		sz := t.bucket.size * nbuckets
  		up := roundupsize(sz)
  		if up != sz {
  			nbuckets = up / t.bucket.size
  		}
  	}
  
  	if dirtyalloc == nil {
  		buckets = newarray(t.bucket, int(nbuckets))
  	} else {
  		// dirtyalloc was previously generated by
  		// the above newarray(t.bucket, int(nbuckets))
  		// but may not be empty.
  		buckets = dirtyalloc
  		size := t.bucket.size * nbuckets
  		if t.bucket.kind&kindNoPointers == 0 {
  			memclrHasPointers(buckets, size)
  		} else {
  			memclrNoHeapPointers(buckets, size)
  		}
  	}
  
  	if base != nbuckets {
  		// We preallocated some overflow buckets.
  		// To keep the overhead of tracking these overflow buckets to a minimum,
  		// we use the convention that if a preallocated overflow bucket's overflow
  		// pointer is nil, then there are more available by bumping the pointer.
  		// We need a safe non-nil pointer for the last overflow bucket; just use buckets.
  		nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
  		last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
  		last.setoverflow(t, (*bmap)(buckets))
  	}
  	return buckets, nextOverflow
  }
  
  // mapaccess1 returns a pointer to h[key].  Never returns nil, instead
  // it will return a reference to the zero object for the value 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 {
  		return unsafe.Pointer(&zeroVal[0])
  	}
  	if h.flags&hashWriting != 0 {
  		throw("concurrent map read and map write")
  	}
  	alg := t.key.alg
  	hash := alg.hash(key, uintptr(h.hash0))
  	m := bucketMask(h.B)
  	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
  	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
  		}
  		oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
  		if !evacuated(oldb) {
  			b = oldb
  		}
  	}
  	top := tophash(hash)
  	for ; b != nil; b = b.overflow(t) {
  		for i := uintptr(0); i < bucketCnt; i++ {
  			if b.tophash[i] != top {
  				continue
  			}
  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  			if t.indirectkey {
  				k = *((*unsafe.Pointer)(k))
  			}
  			if alg.equal(key, k) {
  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  				if t.indirectvalue {
  					v = *((*unsafe.Pointer)(v))
  				}
  				return v
  			}
  		}
  	}
  	return unsafe.Pointer(&zeroVal[0])
  }
  
  func mapaccess2(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, bool) {
  	if raceenabled && h != nil {
  		callerpc := getcallerpc()
  		pc := funcPC(mapaccess2)
  		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 {
  		return unsafe.Pointer(&zeroVal[0]), false
  	}
  	if h.flags&hashWriting != 0 {
  		throw("concurrent map read and map write")
  	}
  	alg := t.key.alg
  	hash := alg.hash(key, uintptr(h.hash0))
  	m := bucketMask(h.B)
  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
  	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
  		}
  		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&m)*uintptr(t.bucketsize)))
  		if !evacuated(oldb) {
  			b = oldb
  		}
  	}
  	top := tophash(hash)
  	for ; b != nil; b = b.overflow(t) {
  		for i := uintptr(0); i < bucketCnt; i++ {
  			if b.tophash[i] != top {
  				continue
  			}
  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  			if t.indirectkey {
  				k = *((*unsafe.Pointer)(k))
  			}
  			if alg.equal(key, k) {
  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  				if t.indirectvalue {
  					v = *((*unsafe.Pointer)(v))
  				}
  				return v, true
  			}
  		}
  	}
  	return unsafe.Pointer(&zeroVal[0]), false
  }
  
  // returns both key and value. Used by map iterator
  func mapaccessK(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer) {
  	if h == nil || h.count == 0 {
  		return nil, nil
  	}
  	alg := t.key.alg
  	hash := alg.hash(key, uintptr(h.hash0))
  	m := bucketMask(h.B)
  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
  	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
  		}
  		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&m)*uintptr(t.bucketsize)))
  		if !evacuated(oldb) {
  			b = oldb
  		}
  	}
  	top := tophash(hash)
  	for ; b != nil; b = b.overflow(t) {
  		for i := uintptr(0); i < bucketCnt; i++ {
  			if b.tophash[i] != top {
  				continue
  			}
  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  			if t.indirectkey {
  				k = *((*unsafe.Pointer)(k))
  			}
  			if alg.equal(key, k) {
  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  				if t.indirectvalue {
  					v = *((*unsafe.Pointer)(v))
  				}
  				return k, v
  			}
  		}
  	}
  	return nil, nil
  }
  
  func mapaccess1_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) unsafe.Pointer {
  	v := mapaccess1(t, h, key)
  	if v == unsafe.Pointer(&zeroVal[0]) {
  		return zero
  	}
  	return v
  }
  
  func mapaccess2_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) (unsafe.Pointer, bool) {
  	v := mapaccess1(t, h, key)
  	if v == unsafe.Pointer(&zeroVal[0]) {
  		return zero, false
  	}
  	return v, true
  }
  
  // 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 {
  		throw("concurrent map writes")
  	}
  	alg := t.key.alg
  	hash := alg.hash(key, uintptr(h.hash0))
  
  	// Set hashWriting after calling alg.hash, since alg.hash may panic,
  	// in which case we have not actually done a write.
  	h.flags |= hashWriting
  
  	if h.buckets == nil {
  		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
  	}
  
  again:
  	bucket := hash & bucketMask(h.B)
  	if h.growing() {
  		growWork(t, h, bucket)
  	}
  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
  	top := tophash(hash)
  
  	var inserti *uint8
  	var insertk unsafe.Pointer
  	var val unsafe.Pointer
  	for {
  		for i := uintptr(0); i < bucketCnt; i++ {
  			if b.tophash[i] != top {
  				if b.tophash[i] == empty && inserti == nil {
  					inserti = &b.tophash[i]
  					insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  					val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  				}
  				continue
  			}
  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  			if t.indirectkey {
  				k = *((*unsafe.Pointer)(k))
  			}
  			if !alg.equal(key, k) {
  				continue
  			}
  			// already have a mapping for key. Update it.
  			if t.needkeyupdate {
  				typedmemmove(t.key, k, key)
  			}
  			val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  			goto done
  		}
  		ovf := b.overflow(t)
  		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 {
  		// all current buckets are full, allocate a new one.
  		newb := h.newoverflow(t, b)
  		inserti = &newb.tophash[0]
  		insertk = add(unsafe.Pointer(newb), dataOffset)
  		val = add(insertk, bucketCnt*uintptr(t.keysize))
  	}
  
  	// store new key/value at insert position
  	if t.indirectkey {
  		kmem := newobject(t.key)
  		*(*unsafe.Pointer)(insertk) = kmem
  		insertk = kmem
  	}
  	if t.indirectvalue {
  		vmem := newobject(t.elem)
  		*(*unsafe.Pointer)(val) = vmem
  	}
  	typedmemmove(t.key, insertk, key)
  	*inserti = top
  	h.count++
  
  done:
  	if h.flags&hashWriting == 0 {
  		throw("concurrent map writes")
  	}
  	h.flags &^= hashWriting
  	if t.indirectvalue {
  		val = *((*unsafe.Pointer)(val))
  	}
  	return val
  }
  
  func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
  	if raceenabled && h != nil {
  		callerpc := getcallerpc()
  		pc := funcPC(mapdelete)
  		racewritepc(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 {
  		return
  	}
  	if h.flags&hashWriting != 0 {
  		throw("concurrent map writes")
  	}
  
  	alg := t.key.alg
  	hash := alg.hash(key, uintptr(h.hash0))
  
  	// Set hashWriting after calling alg.hash, since alg.hash may panic,
  	// in which case we have not actually done a write (delete).
  	h.flags |= hashWriting
  
  	bucket := hash & bucketMask(h.B)
  	if h.growing() {
  		growWork(t, h, bucket)
  	}
  	b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
  	top := tophash(hash)
  search:
  	for ; b != nil; b = b.overflow(t) {
  		for i := uintptr(0); i < bucketCnt; i++ {
  			if b.tophash[i] != top {
  				continue
  			}
  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
  			k2 := k
  			if t.indirectkey {
  				k2 = *((*unsafe.Pointer)(k2))
  			}
  			if !alg.equal(key, k2) {
  				continue
  			}
  			// Only clear key if there are pointers in it.
  			if t.indirectkey {
  				*(*unsafe.Pointer)(k) = nil
  			} else if t.key.kind&kindNoPointers == 0 {
  				memclrHasPointers(k, t.key.size)
  			}
  			v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
  			if t.indirectvalue {
  				*(*unsafe.Pointer)(v) = nil
  			} else if t.elem.kind&kindNoPointers == 0 {
  				memclrHasPointers(v, t.elem.size)
  			} else {
  				memclrNoHeapPointers(v, t.elem.size)
  			}
  			b.tophash[i] = empty
  			h.count--
  			break search
  		}
  	}
  
  	if h.flags&hashWriting == 0 {
  		throw("concurrent map writes")
  	}
  	h.flags &^= hashWriting
  }
  
  // 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) {
  	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.kind&kindNoPointers != 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())
  	if h.B > 31-bucketCntBits {
  		r += uintptr(fastrand()) << 31
  	}
  	it.startBucket = r & bucketMask(h.B)
  	it.offset = uint8(r >> h.B & (bucketCnt - 1))
  
  	// 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) {
  	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
  	alg := t.key.alg
  
  next:
  	if b == nil {
  		if bucket == it.startBucket && it.wrapped {
  			// end of iteration
  			it.key = nil
  			it.value = 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()
  			b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
  			if !evacuated(b) {
  				checkBucket = bucket
  			} else {
  				b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
  				checkBucket = noCheck
  			}
  		} else {
  			b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
  			checkBucket = noCheck
  		}
  		bucket++
  		if bucket == bucketShift(it.B) {
  			bucket = 0
  			it.wrapped = true
  		}
  		i = 0
  	}
  	for ; i < bucketCnt; i++ {
  		offi := (i + it.offset) & (bucketCnt - 1)
  		if b.tophash[offi] == empty || b.tophash[offi] == evacuatedEmpty {
  			continue
  		}
  		k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
  		if t.indirectkey {
  			k = *((*unsafe.Pointer)(k))
  		}
  		v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
  		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 || alg.equal(k, k) {
  				// If the item in the oldbucket is not destined for
  				// the current new bucket in the iteration, skip it.
  				hash := alg.hash(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 || alg.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.indirectvalue {
  				v = *((*unsafe.Pointer)(v))
  			}
  			it.value = v
  		} 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, rv := mapaccessK(t, h, k)
  			if rk == nil {
  				continue // key has been deleted
  			}
  			it.key = rk
  			it.value = rv
  		}
  		it.bucket = bucket
  		if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
  			it.bptr = b
  		}
  		it.i = i + 1
  		it.checkBucket = checkBucket
  		return
  	}
  	b = b.overflow(t)
  	i = 0
  	goto next
  }
  
  // mapclear deletes all keys from a map.
  func mapclear(t *maptype, h *hmap) {
  	if raceenabled && h != nil {
  		callerpc := getcallerpc()
  		pc := funcPC(mapclear)
  		racewritepc(unsafe.Pointer(h), callerpc, pc)
  	}
  
  	if h == nil || h.count == 0 {
  		return
  	}
  
  	if h.flags&hashWriting != 0 {
  		throw("concurrent map writes")
  	}
  
  	h.flags |= hashWriting
  
  	h.flags &^= sameSizeGrow
  	h.oldbuckets = nil
  	h.nevacuate = 0
  	h.noverflow = 0
  	h.count = 0
  
  	// Keep the mapextra allocation but clear any extra information.
  	if h.extra != nil {
  		*h.extra = mapextra{}
  	}
  
  	// makeBucketArray clears the memory pointed to by h.buckets
  	// and recovers any overflow buckets by generating them
  	// as if h.buckets was newly alloced.
  	_, nextOverflow := makeBucketArray(t, h.B, h.buckets)
  	if nextOverflow != nil {
  		// If overflow buckets are created then h.extra
  		// will have been allocated during initial bucket creation.
  		h.extra.nextOverflow = nextOverflow
  	}
  
  	if h.flags&hashWriting == 0 {
  		throw("concurrent map writes")
  	}
  	h.flags &^= hashWriting
  }
  
  func hashGrow(t *maptype, h *hmap) {
  	// 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().
  }
  
  // overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor.
  func overLoadFactor(count int, B uint8) bool {
  	return count > bucketCnt && uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen)
  }
  
  // tooManyOverflowBuckets reports whether noverflow buckets is too many for a map with 1<<B buckets.
  // Note that most of these overflow buckets must be in sparse use;
  // if use was dense, then we'd have already triggered regular map growth.
  func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
  	// If the threshold is too low, we do extraneous work.
  	// If the threshold is too high, maps that grow and shrink can hold on to lots of unused memory.
  	// "too many" means (approximately) as many overflow buckets as regular buckets.
  	// See incrnoverflow for more details.
  	if B > 15 {
  		B = 15
  	}
  	// The compiler doesn't see here that B < 16; mask B to generate shorter shift code.
  	return noverflow >= uint16(1)<<(B&15)
  }
  
  // growing reports whether h is growing. The growth may be to the same size or bigger.
  func (h *hmap) growing() bool {
  	return h.oldbuckets != nil
  }
  
  // sameSizeGrow reports whether the current growth is to a map of the same size.
  func (h *hmap) sameSizeGrow() bool {
  	return h.flags&sameSizeGrow != 0
  }
  
  // noldbuckets calculates the number of buckets prior to the current map growth.
  func (h *hmap) noldbuckets() uintptr {
  	oldB := h.B
  	if !h.sameSizeGrow() {
  		oldB--
  	}
  	return bucketShift(oldB)
  }
  
  // oldbucketmask provides a mask that can be applied to calculate n % noldbuckets().
  func (h *hmap) oldbucketmask() uintptr {
  	return h.noldbuckets() - 1
  }
  
  func growWork(t *maptype, h *hmap, bucket uintptr) {
  	// make sure we evacuate the oldbucket corresponding
  	// to the bucket we're about to use
  	evacuate(t, h, bucket&h.oldbucketmask())
  
  	// evacuate one more oldbucket to make progress on growing
  	if h.growing() {
  		evacuate(t, h, h.nevacuate)
  	}
  }
  
  func bucketEvacuated(t *maptype, h *hmap, bucket uintptr) bool {
  	b := (*bmap)(add(h.oldbuckets, bucket*uintptr(t.bucketsize)))
  	return evacuated(b)
  }
  
  // evacDst is an evacuation destination.
  type evacDst struct {
  	b *bmap          // current destination bucket
  	i int            // key/val index into b
  	k unsafe.Pointer // pointer to current key storage
  	v unsafe.Pointer // pointer to current value storage
  }
  
  func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
  	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
  	newbit := h.noldbuckets()
  	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)))
  		x.k = add(unsafe.Pointer(x.b), dataOffset)
  		x.v = 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.v = add(y.k, bucketCnt*uintptr(t.keysize))
  		}
  
  		for ; b != nil; b = b.overflow(t) {
  			k := add(unsafe.Pointer(b), dataOffset)
  			v := add(k, bucketCnt*uintptr(t.keysize))
  			for i := 0; i < bucketCnt; i, k, v = i+1, add(k, uintptr(t.keysize)), add(v, uintptr(t.valuesize)) {
  				top := b.tophash[i]
  				if top == empty {
  					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/value to bucket x or bucket y).
  					hash := t.key.alg.hash(k2, uintptr(h.hash0))
  					if h.flags&iterator != 0 && !t.reflexivekey && !t.key.alg.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 {
  					throw("bad evacuatedN")
  				}
  
  				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
  				dst := &xy[useY]                 // evacuation destination
  
  				if dst.i == bucketCnt {
  					dst.b = h.newoverflow(t, dst.b)
  					dst.i = 0
  					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
  					dst.v = 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 value
  				}
  				if t.indirectvalue {
  					*(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
  				} else {
  					typedmemmove(t.elem, dst.v, v)
  				}
  				dst.i++
  				// These updates might push these pointers past the end of the
  				// key or value 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.v = add(dst.v, uintptr(t.valuesize))
  			}
  		}
  		// Unlink the overflow buckets & clear key/value to help GC.
  		if h.flags&oldIterator == 0 && t.bucket.kind&kindNoPointers == 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)
  	}
  }
  
  func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
  	h.nevacuate++
  	// Experiments suggest that 1024 is overkill by at least an order of magnitude.
  	// Put it in there as a safeguard anyway, to ensure O(1) behavior.
  	stop := h.nevacuate + 1024
  	if stop > newbit {
  		stop = newbit
  	}
  	for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
  		h.nevacuate++
  	}
  	if h.nevacuate == newbit { // newbit == # of oldbuckets
  		// Growing is all done. Free old main bucket array.
  		h.oldbuckets = nil
  		// Can discard old overflow buckets as well.
  		// If they are still referenced by an iterator,
  		// then the iterator holds a pointers to the slice.
  		if h.extra != nil {
  			h.extra.oldoverflow = nil
  		}
  		h.flags &^= sameSizeGrow
  	}
  }
  
  func ismapkey(t *_type) bool {
  	return t.alg.hash != nil
  }
  
  // Reflect stubs. Called from ../reflect/asm_*.s
  
  //go:linkname reflect_makemap reflect.makemap
  func reflect_makemap(t *maptype, cap int) *hmap {
  	// Check invariants and reflects math.
  	if !ismapkey(t.key) {
  		throw("runtime.reflect_makemap: unsupported map key type")
  	}
  	if t.key.size > maxKeySize && (!t.indirectkey || t.keysize != uint8(sys.PtrSize)) ||
  		t.key.size <= maxKeySize && (t.indirectkey || t.keysize != uint8(t.key.size)) {
  		throw("key size wrong")
  	}
  	if t.elem.size > maxValueSize && (!t.indirectvalue || t.valuesize != uint8(sys.PtrSize)) ||
  		t.elem.size <= maxValueSize && (t.indirectvalue || t.valuesize != uint8(t.elem.size)) {
  		throw("value size wrong")
  	}
  	if t.key.align > bucketCnt {
  		throw("key align too big")
  	}
  	if t.elem.align > bucketCnt {
  		throw("value align too big")
  	}
  	if t.key.size%uintptr(t.key.align) != 0 {
  		throw("key size not a multiple of key align")
  	}
  	if t.elem.size%uintptr(t.elem.align) != 0 {
  		throw("value size not a multiple of value align")
  	}
  	if bucketCnt < 8 {
  		throw("bucketsize too small for proper alignment")
  	}
  	if dataOffset%uintptr(t.key.align) != 0 {
  		throw("need padding in bucket (key)")
  	}
  	if dataOffset%uintptr(t.elem.align) != 0 {
  		throw("need padding in bucket (value)")
  	}
  
  	return makemap(t, cap, nil)
  }
  
  //go:linkname reflect_mapaccess reflect.mapaccess
  func reflect_mapaccess(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
  	val, ok := mapaccess2(t, h, key)
  	if !ok {
  		// reflect wants nil for a missing element
  		val = nil
  	}
  	return val
  }
  
  //go:linkname reflect_mapassign reflect.mapassign
  func reflect_mapassign(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
  	p := mapassign(t, h, key)
  	typedmemmove(t.elem, p, val)
  }
  
  //go:linkname reflect_mapdelete reflect.mapdelete
  func reflect_mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
  	mapdelete(t, h, key)
  }
  
  //go:linkname reflect_mapiterinit reflect.mapiterinit
  func reflect_mapiterinit(t *maptype, h *hmap) *hiter {
  	it := new(hiter)
  	mapiterinit(t, h, it)
  	return it
  }
  
  //go:linkname reflect_mapiternext reflect.mapiternext
  func reflect_mapiternext(it *hiter) {
  	mapiternext(it)
  }
  
  //go:linkname reflect_mapiterkey reflect.mapiterkey
  func reflect_mapiterkey(it *hiter) unsafe.Pointer {
  	return it.key
  }
  
  //go:linkname reflect_maplen reflect.maplen
  func reflect_maplen(h *hmap) int {
  	if h == nil {
  		return 0
  	}
  	if raceenabled {
  		callerpc := getcallerpc()
  		racereadpc(unsafe.Pointer(h), callerpc, funcPC(reflect_maplen))
  	}
  	return h.count
  }
  
  //go:linkname reflect_ismapkey reflect.ismapkey
  func reflect_ismapkey(t *_type) bool {
  	return ismapkey(t)
  }
  
  const maxZero = 1024 // must match value in cmd/compile/internal/gc/walk.go
  var zeroVal [maxZero]byte
  

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