Source file src/runtime/map.go

Documentation: runtime

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  // This file contains the implementation of Go's map type.
     8  //
     9  // A map is just a hash table. The data is arranged
    10  // into an array of buckets. Each bucket contains up to
    11  // 8 key/value pairs. The low-order bits of the hash are
    12  // used to select a bucket. Each bucket contains a few
    13  // high-order bits of each hash to distinguish the entries
    14  // within a single bucket.
    15  //
    16  // If more than 8 keys hash to a bucket, we chain on
    17  // extra buckets.
    18  //
    19  // When the hashtable grows, we allocate a new array
    20  // of buckets twice as big. Buckets are incrementally
    21  // copied from the old bucket array to the new bucket array.
    22  //
    23  // Map iterators walk through the array of buckets and
    24  // return the keys in walk order (bucket #, then overflow
    25  // chain order, then bucket index).  To maintain iteration
    26  // semantics, we never move keys within their bucket (if
    27  // we did, keys might be returned 0 or 2 times).  When
    28  // growing the table, iterators remain iterating through the
    29  // old table and must check the new table if the bucket
    30  // they are iterating through has been moved ("evacuated")
    31  // to the new table.
    32  
    33  // Picking loadFactor: too large and we have lots of overflow
    34  // buckets, too small and we waste a lot of space. I wrote
    35  // a simple program to check some stats for different loads:
    36  // (64-bit, 8 byte keys and values)
    37  //  loadFactor    %overflow  bytes/entry     hitprobe    missprobe
    38  //        4.00         2.13        20.77         3.00         4.00
    39  //        4.50         4.05        17.30         3.25         4.50
    40  //        5.00         6.85        14.77         3.50         5.00
    41  //        5.50        10.55        12.94         3.75         5.50
    42  //        6.00        15.27        11.67         4.00         6.00
    43  //        6.50        20.90        10.79         4.25         6.50
    44  //        7.00        27.14        10.15         4.50         7.00
    45  //        7.50        34.03         9.73         4.75         7.50
    46  //        8.00        41.10         9.40         5.00         8.00
    47  //
    48  // %overflow   = percentage of buckets which have an overflow bucket
    49  // bytes/entry = overhead bytes used per key/value pair
    50  // hitprobe    = # of entries to check when looking up a present key
    51  // missprobe   = # of entries to check when looking up an absent key
    52  //
    53  // Keep in mind this data is for maximally loaded tables, i.e. just
    54  // before the table grows. Typical tables will be somewhat less loaded.
    55  
    56  import (
    57  	"runtime/internal/atomic"
    58  	"runtime/internal/math"
    59  	"runtime/internal/sys"
    60  	"unsafe"
    61  )
    62  
    63  const (
    64  	// Maximum number of key/value pairs a bucket can hold.
    65  	bucketCntBits = 3
    66  	bucketCnt     = 1 << bucketCntBits
    67  
    68  	// Maximum average load of a bucket that triggers growth is 6.5.
    69  	// Represent as loadFactorNum/loadFactDen, to allow integer math.
    70  	loadFactorNum = 13
    71  	loadFactorDen = 2
    72  
    73  	// Maximum key or value size to keep inline (instead of mallocing per element).
    74  	// Must fit in a uint8.
    75  	// Fast versions cannot handle big values - the cutoff size for
    76  	// fast versions in cmd/compile/internal/gc/walk.go must be at most this value.
    77  	maxKeySize   = 128
    78  	maxValueSize = 128
    79  
    80  	// data offset should be the size of the bmap struct, but needs to be
    81  	// aligned correctly. For amd64p32 this means 64-bit alignment
    82  	// even though pointers are 32 bit.
    83  	dataOffset = unsafe.Offsetof(struct {
    84  		b bmap
    85  		v int64
    86  	}{}.v)
    87  
    88  	// Possible tophash values. We reserve a few possibilities for special marks.
    89  	// Each bucket (including its overflow buckets, if any) will have either all or none of its
    90  	// entries in the evacuated* states (except during the evacuate() method, which only happens
    91  	// during map writes and thus no one else can observe the map during that time).
    92  	emptyRest      = 0 // this cell is empty, and there are no more non-empty cells at higher indexes or overflows.
    93  	emptyOne       = 1 // this cell is empty
    94  	evacuatedX     = 2 // key/value is valid.  Entry has been evacuated to first half of larger table.
    95  	evacuatedY     = 3 // same as above, but evacuated to second half of larger table.
    96  	evacuatedEmpty = 4 // cell is empty, bucket is evacuated.
    97  	minTopHash     = 5 // minimum tophash for a normal filled cell.
    98  
    99  	// flags
   100  	iterator     = 1 // there may be an iterator using buckets
   101  	oldIterator  = 2 // there may be an iterator using oldbuckets
   102  	hashWriting  = 4 // a goroutine is writing to the map
   103  	sameSizeGrow = 8 // the current map growth is to a new map of the same size
   104  
   105  	// sentinel bucket ID for iterator checks
   106  	noCheck = 1<<(8*sys.PtrSize) - 1
   107  )
   108  
   109  // isEmpty reports whether the given tophash array entry represents an empty bucket entry.
   110  func isEmpty(x uint8) bool {
   111  	return x <= emptyOne
   112  }
   113  
   114  // A header for a Go map.
   115  type hmap struct {
   116  	// Note: the format of the hmap is also encoded in cmd/compile/internal/gc/reflect.go.
   117  	// Make sure this stays in sync with the compiler's definition.
   118  	count     int // # live cells == size of map.  Must be first (used by len() builtin)
   119  	flags     uint8
   120  	B         uint8  // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
   121  	noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
   122  	hash0     uint32 // hash seed
   123  
   124  	buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
   125  	oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
   126  	nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)
   127  
   128  	extra *mapextra // optional fields
   129  }
   130  
   131  // mapextra holds fields that are not present on all maps.
   132  type mapextra struct {
   133  	// If both key and value do not contain pointers and are inline, then we mark bucket
   134  	// type as containing no pointers. This avoids scanning such maps.
   135  	// However, bmap.overflow is a pointer. In order to keep overflow buckets
   136  	// alive, we store pointers to all overflow buckets in hmap.extra.overflow and hmap.extra.oldoverflow.
   137  	// overflow and oldoverflow are only used if key and value do not contain pointers.
   138  	// overflow contains overflow buckets for hmap.buckets.
   139  	// oldoverflow contains overflow buckets for hmap.oldbuckets.
   140  	// The indirection allows to store a pointer to the slice in hiter.
   141  	overflow    *[]*bmap
   142  	oldoverflow *[]*bmap
   143  
   144  	// nextOverflow holds a pointer to a free overflow bucket.
   145  	nextOverflow *bmap
   146  }
   147  
   148  // A bucket for a Go map.
   149  type bmap struct {
   150  	// tophash generally contains the top byte of the hash value
   151  	// for each key in this bucket. If tophash[0] < minTopHash,
   152  	// tophash[0] is a bucket evacuation state instead.
   153  	tophash [bucketCnt]uint8
   154  	// Followed by bucketCnt keys and then bucketCnt values.
   155  	// NOTE: packing all the keys together and then all the values together makes the
   156  	// code a bit more complicated than alternating key/value/key/value/... but it allows
   157  	// us to eliminate padding which would be needed for, e.g., map[int64]int8.
   158  	// Followed by an overflow pointer.
   159  }
   160  
   161  // A hash iteration structure.
   162  // If you modify hiter, also change cmd/compile/internal/gc/reflect.go to indicate
   163  // the layout of this structure.
   164  type hiter struct {
   165  	key         unsafe.Pointer // Must be in first position.  Write nil to indicate iteration end (see cmd/internal/gc/range.go).
   166  	value       unsafe.Pointer // Must be in second position (see cmd/internal/gc/range.go).
   167  	t           *maptype
   168  	h           *hmap
   169  	buckets     unsafe.Pointer // bucket ptr at hash_iter initialization time
   170  	bptr        *bmap          // current bucket
   171  	overflow    *[]*bmap       // keeps overflow buckets of hmap.buckets alive
   172  	oldoverflow *[]*bmap       // keeps overflow buckets of hmap.oldbuckets alive
   173  	startBucket uintptr        // bucket iteration started at
   174  	offset      uint8          // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
   175  	wrapped     bool           // already wrapped around from end of bucket array to beginning
   176  	B           uint8
   177  	i           uint8
   178  	bucket      uintptr
   179  	checkBucket uintptr
   180  }
   181  
   182  // bucketShift returns 1<<b, optimized for code generation.
   183  func bucketShift(b uint8) uintptr {
   184  	if sys.GoarchAmd64|sys.GoarchAmd64p32|sys.Goarch386 != 0 {
   185  		b &= sys.PtrSize*8 - 1 // help x86 archs remove shift overflow checks
   186  	}
   187  	return uintptr(1) << b
   188  }
   189  
   190  // bucketMask returns 1<<b - 1, optimized for code generation.
   191  func bucketMask(b uint8) uintptr {
   192  	return bucketShift(b) - 1
   193  }
   194  
   195  // tophash calculates the tophash value for hash.
   196  func tophash(hash uintptr) uint8 {
   197  	top := uint8(hash >> (sys.PtrSize*8 - 8))
   198  	if top < minTopHash {
   199  		top += minTopHash
   200  	}
   201  	return top
   202  }
   203  
   204  func evacuated(b *bmap) bool {
   205  	h := b.tophash[0]
   206  	return h > emptyOne && h < minTopHash
   207  }
   208  
   209  func (b *bmap) overflow(t *maptype) *bmap {
   210  	return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize))
   211  }
   212  
   213  func (b *bmap) setoverflow(t *maptype, ovf *bmap) {
   214  	*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize)) = ovf
   215  }
   216  
   217  func (b *bmap) keys() unsafe.Pointer {
   218  	return add(unsafe.Pointer(b), dataOffset)
   219  }
   220  
   221  // incrnoverflow increments h.noverflow.
   222  // noverflow counts the number of overflow buckets.
   223  // This is used to trigger same-size map growth.
   224  // See also tooManyOverflowBuckets.
   225  // To keep hmap small, noverflow is a uint16.
   226  // When there are few buckets, noverflow is an exact count.
   227  // When there are many buckets, noverflow is an approximate count.
   228  func (h *hmap) incrnoverflow() {
   229  	// We trigger same-size map growth if there are
   230  	// as many overflow buckets as buckets.
   231  	// We need to be able to count to 1<<h.B.
   232  	if h.B < 16 {
   233  		h.noverflow++
   234  		return
   235  	}
   236  	// Increment with probability 1/(1<<(h.B-15)).
   237  	// When we reach 1<<15 - 1, we will have approximately
   238  	// as many overflow buckets as buckets.
   239  	mask := uint32(1)<<(h.B-15) - 1
   240  	// Example: if h.B == 18, then mask == 7,
   241  	// and fastrand & 7 == 0 with probability 1/8.
   242  	if fastrand()&mask == 0 {
   243  		h.noverflow++
   244  	}
   245  }
   246  
   247  func (h *hmap) newoverflow(t *maptype, b *bmap) *bmap {
   248  	var ovf *bmap
   249  	if h.extra != nil && h.extra.nextOverflow != nil {
   250  		// We have preallocated overflow buckets available.
   251  		// See makeBucketArray for more details.
   252  		ovf = h.extra.nextOverflow
   253  		if ovf.overflow(t) == nil {
   254  			// We're not at the end of the preallocated overflow buckets. Bump the pointer.
   255  			h.extra.nextOverflow = (*bmap)(add(unsafe.Pointer(ovf), uintptr(t.bucketsize)))
   256  		} else {
   257  			// This is the last preallocated overflow bucket.
   258  			// Reset the overflow pointer on this bucket,
   259  			// which was set to a non-nil sentinel value.
   260  			ovf.setoverflow(t, nil)
   261  			h.extra.nextOverflow = nil
   262  		}
   263  	} else {
   264  		ovf = (*bmap)(newobject(t.bucket))
   265  	}
   266  	h.incrnoverflow()
   267  	if t.bucket.kind&kindNoPointers != 0 {
   268  		h.createOverflow()
   269  		*h.extra.overflow = append(*h.extra.overflow, ovf)
   270  	}
   271  	b.setoverflow(t, ovf)
   272  	return ovf
   273  }
   274  
   275  func (h *hmap) createOverflow() {
   276  	if h.extra == nil {
   277  		h.extra = new(mapextra)
   278  	}
   279  	if h.extra.overflow == nil {
   280  		h.extra.overflow = new([]*bmap)
   281  	}
   282  }
   283  
   284  func makemap64(t *maptype, hint int64, h *hmap) *hmap {
   285  	if int64(int(hint)) != hint {
   286  		hint = 0
   287  	}
   288  	return makemap(t, int(hint), h)
   289  }
   290  
   291  // makehmap_small implements Go map creation for make(map[k]v) and
   292  // make(map[k]v, hint) when hint is known to be at most bucketCnt
   293  // at compile time and the map needs to be allocated on the heap.
   294  func makemap_small() *hmap {
   295  	h := new(hmap)
   296  	h.hash0 = fastrand()
   297  	return h
   298  }
   299  
   300  // makemap implements Go map creation for make(map[k]v, hint).
   301  // If the compiler has determined that the map or the first bucket
   302  // can be created on the stack, h and/or bucket may be non-nil.
   303  // If h != nil, the map can be created directly in h.
   304  // If h.buckets != nil, bucket pointed to can be used as the first bucket.
   305  func makemap(t *maptype, hint int, h *hmap) *hmap {
   306  	mem, overflow := math.MulUintptr(uintptr(hint), t.bucket.size)
   307  	if overflow || mem > maxAlloc {
   308  		hint = 0
   309  	}
   310  
   311  	// initialize Hmap
   312  	if h == nil {
   313  		h = new(hmap)
   314  	}
   315  	h.hash0 = fastrand()
   316  
   317  	// Find the size parameter B which will hold the requested # of elements.
   318  	// For hint < 0 overLoadFactor returns false since hint < bucketCnt.
   319  	B := uint8(0)
   320  	for overLoadFactor(hint, B) {
   321  		B++
   322  	}
   323  	h.B = B
   324  
   325  	// allocate initial hash table
   326  	// if B == 0, the buckets field is allocated lazily later (in mapassign)
   327  	// If hint is large zeroing this memory could take a while.
   328  	if h.B != 0 {
   329  		var nextOverflow *bmap
   330  		h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
   331  		if nextOverflow != nil {
   332  			h.extra = new(mapextra)
   333  			h.extra.nextOverflow = nextOverflow
   334  		}
   335  	}
   336  
   337  	return h
   338  }
   339  
   340  // makeBucketArray initializes a backing array for map buckets.
   341  // 1<<b is the minimum number of buckets to allocate.
   342  // dirtyalloc should either be nil or a bucket array previously
   343  // allocated by makeBucketArray with the same t and b parameters.
   344  // If dirtyalloc is nil a new backing array will be alloced and
   345  // otherwise dirtyalloc will be cleared and reused as backing array.
   346  func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
   347  	base := bucketShift(b)
   348  	nbuckets := base
   349  	// For small b, overflow buckets are unlikely.
   350  	// Avoid the overhead of the calculation.
   351  	if b >= 4 {
   352  		// Add on the estimated number of overflow buckets
   353  		// required to insert the median number of elements
   354  		// used with this value of b.
   355  		nbuckets += bucketShift(b - 4)
   356  		sz := t.bucket.size * nbuckets
   357  		up := roundupsize(sz)
   358  		if up != sz {
   359  			nbuckets = up / t.bucket.size
   360  		}
   361  	}
   362  
   363  	if dirtyalloc == nil {
   364  		buckets = newarray(t.bucket, int(nbuckets))
   365  	} else {
   366  		// dirtyalloc was previously generated by
   367  		// the above newarray(t.bucket, int(nbuckets))
   368  		// but may not be empty.
   369  		buckets = dirtyalloc
   370  		size := t.bucket.size * nbuckets
   371  		if t.bucket.kind&kindNoPointers == 0 {
   372  			memclrHasPointers(buckets, size)
   373  		} else {
   374  			memclrNoHeapPointers(buckets, size)
   375  		}
   376  	}
   377  
   378  	if base != nbuckets {
   379  		// We preallocated some overflow buckets.
   380  		// To keep the overhead of tracking these overflow buckets to a minimum,
   381  		// we use the convention that if a preallocated overflow bucket's overflow
   382  		// pointer is nil, then there are more available by bumping the pointer.
   383  		// We need a safe non-nil pointer for the last overflow bucket; just use buckets.
   384  		nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
   385  		last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
   386  		last.setoverflow(t, (*bmap)(buckets))
   387  	}
   388  	return buckets, nextOverflow
   389  }
   390  
   391  // mapaccess1 returns a pointer to h[key].  Never returns nil, instead
   392  // it will return a reference to the zero object for the value type if
   393  // the key is not in the map.
   394  // NOTE: The returned pointer may keep the whole map live, so don't
   395  // hold onto it for very long.
   396  func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
   397  	if raceenabled && h != nil {
   398  		callerpc := getcallerpc()
   399  		pc := funcPC(mapaccess1)
   400  		racereadpc(unsafe.Pointer(h), callerpc, pc)
   401  		raceReadObjectPC(t.key, key, callerpc, pc)
   402  	}
   403  	if msanenabled && h != nil {
   404  		msanread(key, t.key.size)
   405  	}
   406  	if h == nil || h.count == 0 {
   407  		if t.hashMightPanic() {
   408  			t.key.alg.hash(key, 0) // see issue 23734
   409  		}
   410  		return unsafe.Pointer(&zeroVal[0])
   411  	}
   412  	if h.flags&hashWriting != 0 {
   413  		throw("concurrent map read and map write")
   414  	}
   415  	alg := t.key.alg
   416  	hash := alg.hash(key, uintptr(h.hash0))
   417  	m := bucketMask(h.B)
   418  	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
   419  	if c := h.oldbuckets; c != nil {
   420  		if !h.sameSizeGrow() {
   421  			// There used to be half as many buckets; mask down one more power of two.
   422  			m >>= 1
   423  		}
   424  		oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
   425  		if !evacuated(oldb) {
   426  			b = oldb
   427  		}
   428  	}
   429  	top := tophash(hash)
   430  bucketloop:
   431  	for ; b != nil; b = b.overflow(t) {
   432  		for i := uintptr(0); i < bucketCnt; i++ {
   433  			if b.tophash[i] != top {
   434  				if b.tophash[i] == emptyRest {
   435  					break bucketloop
   436  				}
   437  				continue
   438  			}
   439  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   440  			if t.indirectkey() {
   441  				k = *((*unsafe.Pointer)(k))
   442  			}
   443  			if alg.equal(key, k) {
   444  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   445  				if t.indirectvalue() {
   446  					v = *((*unsafe.Pointer)(v))
   447  				}
   448  				return v
   449  			}
   450  		}
   451  	}
   452  	return unsafe.Pointer(&zeroVal[0])
   453  }
   454  
   455  func mapaccess2(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, bool) {
   456  	if raceenabled && h != nil {
   457  		callerpc := getcallerpc()
   458  		pc := funcPC(mapaccess2)
   459  		racereadpc(unsafe.Pointer(h), callerpc, pc)
   460  		raceReadObjectPC(t.key, key, callerpc, pc)
   461  	}
   462  	if msanenabled && h != nil {
   463  		msanread(key, t.key.size)
   464  	}
   465  	if h == nil || h.count == 0 {
   466  		if t.hashMightPanic() {
   467  			t.key.alg.hash(key, 0) // see issue 23734
   468  		}
   469  		return unsafe.Pointer(&zeroVal[0]), false
   470  	}
   471  	if h.flags&hashWriting != 0 {
   472  		throw("concurrent map read and map write")
   473  	}
   474  	alg := t.key.alg
   475  	hash := alg.hash(key, uintptr(h.hash0))
   476  	m := bucketMask(h.B)
   477  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
   478  	if c := h.oldbuckets; c != nil {
   479  		if !h.sameSizeGrow() {
   480  			// There used to be half as many buckets; mask down one more power of two.
   481  			m >>= 1
   482  		}
   483  		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&m)*uintptr(t.bucketsize)))
   484  		if !evacuated(oldb) {
   485  			b = oldb
   486  		}
   487  	}
   488  	top := tophash(hash)
   489  bucketloop:
   490  	for ; b != nil; b = b.overflow(t) {
   491  		for i := uintptr(0); i < bucketCnt; i++ {
   492  			if b.tophash[i] != top {
   493  				if b.tophash[i] == emptyRest {
   494  					break bucketloop
   495  				}
   496  				continue
   497  			}
   498  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   499  			if t.indirectkey() {
   500  				k = *((*unsafe.Pointer)(k))
   501  			}
   502  			if alg.equal(key, k) {
   503  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   504  				if t.indirectvalue() {
   505  					v = *((*unsafe.Pointer)(v))
   506  				}
   507  				return v, true
   508  			}
   509  		}
   510  	}
   511  	return unsafe.Pointer(&zeroVal[0]), false
   512  }
   513  
   514  // returns both key and value. Used by map iterator
   515  func mapaccessK(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer) {
   516  	if h == nil || h.count == 0 {
   517  		return nil, nil
   518  	}
   519  	alg := t.key.alg
   520  	hash := alg.hash(key, uintptr(h.hash0))
   521  	m := bucketMask(h.B)
   522  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
   523  	if c := h.oldbuckets; c != nil {
   524  		if !h.sameSizeGrow() {
   525  			// There used to be half as many buckets; mask down one more power of two.
   526  			m >>= 1
   527  		}
   528  		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&m)*uintptr(t.bucketsize)))
   529  		if !evacuated(oldb) {
   530  			b = oldb
   531  		}
   532  	}
   533  	top := tophash(hash)
   534  bucketloop:
   535  	for ; b != nil; b = b.overflow(t) {
   536  		for i := uintptr(0); i < bucketCnt; i++ {
   537  			if b.tophash[i] != top {
   538  				if b.tophash[i] == emptyRest {
   539  					break bucketloop
   540  				}
   541  				continue
   542  			}
   543  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   544  			if t.indirectkey() {
   545  				k = *((*unsafe.Pointer)(k))
   546  			}
   547  			if alg.equal(key, k) {
   548  				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   549  				if t.indirectvalue() {
   550  					v = *((*unsafe.Pointer)(v))
   551  				}
   552  				return k, v
   553  			}
   554  		}
   555  	}
   556  	return nil, nil
   557  }
   558  
   559  func mapaccess1_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) unsafe.Pointer {
   560  	v := mapaccess1(t, h, key)
   561  	if v == unsafe.Pointer(&zeroVal[0]) {
   562  		return zero
   563  	}
   564  	return v
   565  }
   566  
   567  func mapaccess2_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) (unsafe.Pointer, bool) {
   568  	v := mapaccess1(t, h, key)
   569  	if v == unsafe.Pointer(&zeroVal[0]) {
   570  		return zero, false
   571  	}
   572  	return v, true
   573  }
   574  
   575  // Like mapaccess, but allocates a slot for the key if it is not present in the map.
   576  func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
   577  	if h == nil {
   578  		panic(plainError("assignment to entry in nil map"))
   579  	}
   580  	if raceenabled {
   581  		callerpc := getcallerpc()
   582  		pc := funcPC(mapassign)
   583  		racewritepc(unsafe.Pointer(h), callerpc, pc)
   584  		raceReadObjectPC(t.key, key, callerpc, pc)
   585  	}
   586  	if msanenabled {
   587  		msanread(key, t.key.size)
   588  	}
   589  	if h.flags&hashWriting != 0 {
   590  		throw("concurrent map writes")
   591  	}
   592  	alg := t.key.alg
   593  	hash := alg.hash(key, uintptr(h.hash0))
   594  
   595  	// Set hashWriting after calling alg.hash, since alg.hash may panic,
   596  	// in which case we have not actually done a write.
   597  	h.flags ^= hashWriting
   598  
   599  	if h.buckets == nil {
   600  		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
   601  	}
   602  
   603  again:
   604  	bucket := hash & bucketMask(h.B)
   605  	if h.growing() {
   606  		growWork(t, h, bucket)
   607  	}
   608  	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
   609  	top := tophash(hash)
   610  
   611  	var inserti *uint8
   612  	var insertk unsafe.Pointer
   613  	var val unsafe.Pointer
   614  bucketloop:
   615  	for {
   616  		for i := uintptr(0); i < bucketCnt; i++ {
   617  			if b.tophash[i] != top {
   618  				if isEmpty(b.tophash[i]) && inserti == nil {
   619  					inserti = &b.tophash[i]
   620  					insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   621  					val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   622  				}
   623  				if b.tophash[i] == emptyRest {
   624  					break bucketloop
   625  				}
   626  				continue
   627  			}
   628  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   629  			if t.indirectkey() {
   630  				k = *((*unsafe.Pointer)(k))
   631  			}
   632  			if !alg.equal(key, k) {
   633  				continue
   634  			}
   635  			// already have a mapping for key. Update it.
   636  			if t.needkeyupdate() {
   637  				typedmemmove(t.key, k, key)
   638  			}
   639  			val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   640  			goto done
   641  		}
   642  		ovf := b.overflow(t)
   643  		if ovf == nil {
   644  			break
   645  		}
   646  		b = ovf
   647  	}
   648  
   649  	// Did not find mapping for key. Allocate new cell & add entry.
   650  
   651  	// If we hit the max load factor or we have too many overflow buckets,
   652  	// and we're not already in the middle of growing, start growing.
   653  	if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
   654  		hashGrow(t, h)
   655  		goto again // Growing the table invalidates everything, so try again
   656  	}
   657  
   658  	if inserti == nil {
   659  		// all current buckets are full, allocate a new one.
   660  		newb := h.newoverflow(t, b)
   661  		inserti = &newb.tophash[0]
   662  		insertk = add(unsafe.Pointer(newb), dataOffset)
   663  		val = add(insertk, bucketCnt*uintptr(t.keysize))
   664  	}
   665  
   666  	// store new key/value at insert position
   667  	if t.indirectkey() {
   668  		kmem := newobject(t.key)
   669  		*(*unsafe.Pointer)(insertk) = kmem
   670  		insertk = kmem
   671  	}
   672  	if t.indirectvalue() {
   673  		vmem := newobject(t.elem)
   674  		*(*unsafe.Pointer)(val) = vmem
   675  	}
   676  	typedmemmove(t.key, insertk, key)
   677  	*inserti = top
   678  	h.count++
   679  
   680  done:
   681  	if h.flags&hashWriting == 0 {
   682  		throw("concurrent map writes")
   683  	}
   684  	h.flags &^= hashWriting
   685  	if t.indirectvalue() {
   686  		val = *((*unsafe.Pointer)(val))
   687  	}
   688  	return val
   689  }
   690  
   691  func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
   692  	if raceenabled && h != nil {
   693  		callerpc := getcallerpc()
   694  		pc := funcPC(mapdelete)
   695  		racewritepc(unsafe.Pointer(h), callerpc, pc)
   696  		raceReadObjectPC(t.key, key, callerpc, pc)
   697  	}
   698  	if msanenabled && h != nil {
   699  		msanread(key, t.key.size)
   700  	}
   701  	if h == nil || h.count == 0 {
   702  		if t.hashMightPanic() {
   703  			t.key.alg.hash(key, 0) // see issue 23734
   704  		}
   705  		return
   706  	}
   707  	if h.flags&hashWriting != 0 {
   708  		throw("concurrent map writes")
   709  	}
   710  
   711  	alg := t.key.alg
   712  	hash := alg.hash(key, uintptr(h.hash0))
   713  
   714  	// Set hashWriting after calling alg.hash, since alg.hash may panic,
   715  	// in which case we have not actually done a write (delete).
   716  	h.flags ^= hashWriting
   717  
   718  	bucket := hash & bucketMask(h.B)
   719  	if h.growing() {
   720  		growWork(t, h, bucket)
   721  	}
   722  	b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
   723  	bOrig := b
   724  	top := tophash(hash)
   725  search:
   726  	for ; b != nil; b = b.overflow(t) {
   727  		for i := uintptr(0); i < bucketCnt; i++ {
   728  			if b.tophash[i] != top {
   729  				if b.tophash[i] == emptyRest {
   730  					break search
   731  				}
   732  				continue
   733  			}
   734  			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
   735  			k2 := k
   736  			if t.indirectkey() {
   737  				k2 = *((*unsafe.Pointer)(k2))
   738  			}
   739  			if !alg.equal(key, k2) {
   740  				continue
   741  			}
   742  			// Only clear key if there are pointers in it.
   743  			if t.indirectkey() {
   744  				*(*unsafe.Pointer)(k) = nil
   745  			} else if t.key.kind&kindNoPointers == 0 {
   746  				memclrHasPointers(k, t.key.size)
   747  			}
   748  			v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
   749  			if t.indirectvalue() {
   750  				*(*unsafe.Pointer)(v) = nil
   751  			} else if t.elem.kind&kindNoPointers == 0 {
   752  				memclrHasPointers(v, t.elem.size)
   753  			} else {
   754  				memclrNoHeapPointers(v, t.elem.size)
   755  			}
   756  			b.tophash[i] = emptyOne
   757  			// If the bucket now ends in a bunch of emptyOne states,
   758  			// change those to emptyRest states.
   759  			// It would be nice to make this a separate function, but
   760  			// for loops are not currently inlineable.
   761  			if i == bucketCnt-1 {
   762  				if b.overflow(t) != nil && b.overflow(t).tophash[0] != emptyRest {
   763  					goto notLast
   764  				}
   765  			} else {
   766  				if b.tophash[i+1] != emptyRest {
   767  					goto notLast
   768  				}
   769  			}
   770  			for {
   771  				b.tophash[i] = emptyRest
   772  				if i == 0 {
   773  					if b == bOrig {
   774  						break // beginning of initial bucket, we're done.
   775  					}
   776  					// Find previous bucket, continue at its last entry.
   777  					c := b
   778  					for b = bOrig; b.overflow(t) != c; b = b.overflow(t) {
   779  					}
   780  					i = bucketCnt - 1
   781  				} else {
   782  					i--
   783  				}
   784  				if b.tophash[i] != emptyOne {
   785  					break
   786  				}
   787  			}
   788  		notLast:
   789  			h.count--
   790  			break search
   791  		}
   792  	}
   793  
   794  	if h.flags&hashWriting == 0 {
   795  		throw("concurrent map writes")
   796  	}
   797  	h.flags &^= hashWriting
   798  }
   799  
   800  // mapiterinit initializes the hiter struct used for ranging over maps.
   801  // The hiter struct pointed to by 'it' is allocated on the stack
   802  // by the compilers order pass or on the heap by reflect_mapiterinit.
   803  // Both need to have zeroed hiter since the struct contains pointers.
   804  func mapiterinit(t *maptype, h *hmap, it *hiter) {
   805  	if raceenabled && h != nil {
   806  		callerpc := getcallerpc()
   807  		racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapiterinit))
   808  	}
   809  
   810  	if h == nil || h.count == 0 {
   811  		return
   812  	}
   813  
   814  	if unsafe.Sizeof(hiter{})/sys.PtrSize != 12 {
   815  		throw("hash_iter size incorrect") // see cmd/compile/internal/gc/reflect.go
   816  	}
   817  	it.t = t
   818  	it.h = h
   819  
   820  	// grab snapshot of bucket state
   821  	it.B = h.B
   822  	it.buckets = h.buckets
   823  	if t.bucket.kind&kindNoPointers != 0 {
   824  		// Allocate the current slice and remember pointers to both current and old.
   825  		// This preserves all relevant overflow buckets alive even if
   826  		// the table grows and/or overflow buckets are added to the table
   827  		// while we are iterating.
   828  		h.createOverflow()
   829  		it.overflow = h.extra.overflow
   830  		it.oldoverflow = h.extra.oldoverflow
   831  	}
   832  
   833  	// decide where to start
   834  	r := uintptr(fastrand())
   835  	if h.B > 31-bucketCntBits {
   836  		r += uintptr(fastrand()) << 31
   837  	}
   838  	it.startBucket = r & bucketMask(h.B)
   839  	it.offset = uint8(r >> h.B & (bucketCnt - 1))
   840  
   841  	// iterator state
   842  	it.bucket = it.startBucket
   843  
   844  	// Remember we have an iterator.
   845  	// Can run concurrently with another mapiterinit().
   846  	if old := h.flags; old&(iterator|oldIterator) != iterator|oldIterator {
   847  		atomic.Or8(&h.flags, iterator|oldIterator)
   848  	}
   849  
   850  	mapiternext(it)
   851  }
   852  
   853  func mapiternext(it *hiter) {
   854  	h := it.h
   855  	if raceenabled {
   856  		callerpc := getcallerpc()
   857  		racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapiternext))
   858  	}
   859  	if h.flags&hashWriting != 0 {
   860  		throw("concurrent map iteration and map write")
   861  	}
   862  	t := it.t
   863  	bucket := it.bucket
   864  	b := it.bptr
   865  	i := it.i
   866  	checkBucket := it.checkBucket
   867  	alg := t.key.alg
   868  
   869  next:
   870  	if b == nil {
   871  		if bucket == it.startBucket && it.wrapped {
   872  			// end of iteration
   873  			it.key = nil
   874  			it.value = nil
   875  			return
   876  		}
   877  		if h.growing() && it.B == h.B {
   878  			// Iterator was started in the middle of a grow, and the grow isn't done yet.
   879  			// If the bucket we're looking at hasn't been filled in yet (i.e. the old
   880  			// bucket hasn't been evacuated) then we need to iterate through the old
   881  			// bucket and only return the ones that will be migrated to this bucket.
   882  			oldbucket := bucket & it.h.oldbucketmask()
   883  			b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
   884  			if !evacuated(b) {
   885  				checkBucket = bucket
   886  			} else {
   887  				b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
   888  				checkBucket = noCheck
   889  			}
   890  		} else {
   891  			b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
   892  			checkBucket = noCheck
   893  		}
   894  		bucket++
   895  		if bucket == bucketShift(it.B) {
   896  			bucket = 0
   897  			it.wrapped = true
   898  		}
   899  		i = 0
   900  	}
   901  	for ; i < bucketCnt; i++ {
   902  		offi := (i + it.offset) & (bucketCnt - 1)
   903  		if isEmpty(b.tophash[offi]) || b.tophash[offi] == evacuatedEmpty {
   904  			// TODO: emptyRest is hard to use here, as we start iterating
   905  			// in the middle of a bucket. It's feasible, just tricky.
   906  			continue
   907  		}
   908  		k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
   909  		if t.indirectkey() {
   910  			k = *((*unsafe.Pointer)(k))
   911  		}
   912  		v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
   913  		if checkBucket != noCheck && !h.sameSizeGrow() {
   914  			// Special case: iterator was started during a grow to a larger size
   915  			// and the grow is not done yet. We're working on a bucket whose
   916  			// oldbucket has not been evacuated yet. Or at least, it wasn't
   917  			// evacuated when we started the bucket. So we're iterating
   918  			// through the oldbucket, skipping any keys that will go
   919  			// to the other new bucket (each oldbucket expands to two
   920  			// buckets during a grow).
   921  			if t.reflexivekey() || alg.equal(k, k) {
   922  				// If the item in the oldbucket is not destined for
   923  				// the current new bucket in the iteration, skip it.
   924  				hash := alg.hash(k, uintptr(h.hash0))
   925  				if hash&bucketMask(it.B) != checkBucket {
   926  					continue
   927  				}
   928  			} else {
   929  				// Hash isn't repeatable if k != k (NaNs).  We need a
   930  				// repeatable and randomish choice of which direction
   931  				// to send NaNs during evacuation. We'll use the low
   932  				// bit of tophash to decide which way NaNs go.
   933  				// NOTE: this case is why we need two evacuate tophash
   934  				// values, evacuatedX and evacuatedY, that differ in
   935  				// their low bit.
   936  				if checkBucket>>(it.B-1) != uintptr(b.tophash[offi]&1) {
   937  					continue
   938  				}
   939  			}
   940  		}
   941  		if (b.tophash[offi] != evacuatedX && b.tophash[offi] != evacuatedY) ||
   942  			!(t.reflexivekey() || alg.equal(k, k)) {
   943  			// This is the golden data, we can return it.
   944  			// OR
   945  			// key!=key, so the entry can't be deleted or updated, so we can just return it.
   946  			// That's lucky for us because when key!=key we can't look it up successfully.
   947  			it.key = k
   948  			if t.indirectvalue() {
   949  				v = *((*unsafe.Pointer)(v))
   950  			}
   951  			it.value = v
   952  		} else {
   953  			// The hash table has grown since the iterator was started.
   954  			// The golden data for this key is now somewhere else.
   955  			// Check the current hash table for the data.
   956  			// This code handles the case where the key
   957  			// has been deleted, updated, or deleted and reinserted.
   958  			// NOTE: we need to regrab the key as it has potentially been
   959  			// updated to an equal() but not identical key (e.g. +0.0 vs -0.0).
   960  			rk, rv := mapaccessK(t, h, k)
   961  			if rk == nil {
   962  				continue // key has been deleted
   963  			}
   964  			it.key = rk
   965  			it.value = rv
   966  		}
   967  		it.bucket = bucket
   968  		if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
   969  			it.bptr = b
   970  		}
   971  		it.i = i + 1
   972  		it.checkBucket = checkBucket
   973  		return
   974  	}
   975  	b = b.overflow(t)
   976  	i = 0
   977  	goto next
   978  }
   979  
   980  // mapclear deletes all keys from a map.
   981  func mapclear(t *maptype, h *hmap) {
   982  	if raceenabled && h != nil {
   983  		callerpc := getcallerpc()
   984  		pc := funcPC(mapclear)
   985  		racewritepc(unsafe.Pointer(h), callerpc, pc)
   986  	}
   987  
   988  	if h == nil || h.count == 0 {
   989  		return
   990  	}
   991  
   992  	if h.flags&hashWriting != 0 {
   993  		throw("concurrent map writes")
   994  	}
   995  
   996  	h.flags ^= hashWriting
   997  
   998  	h.flags &^= sameSizeGrow
   999  	h.oldbuckets = nil
  1000  	h.nevacuate = 0
  1001  	h.noverflow = 0
  1002  	h.count = 0
  1003  
  1004  	// Keep the mapextra allocation but clear any extra information.
  1005  	if h.extra != nil {
  1006  		*h.extra = mapextra{}
  1007  	}
  1008  
  1009  	// makeBucketArray clears the memory pointed to by h.buckets
  1010  	// and recovers any overflow buckets by generating them
  1011  	// as if h.buckets was newly alloced.
  1012  	_, nextOverflow := makeBucketArray(t, h.B, h.buckets)
  1013  	if nextOverflow != nil {
  1014  		// If overflow buckets are created then h.extra
  1015  		// will have been allocated during initial bucket creation.
  1016  		h.extra.nextOverflow = nextOverflow
  1017  	}
  1018  
  1019  	if h.flags&hashWriting == 0 {
  1020  		throw("concurrent map writes")
  1021  	}
  1022  	h.flags &^= hashWriting
  1023  }
  1024  
  1025  func hashGrow(t *maptype, h *hmap) {
  1026  	// If we've hit the load factor, get bigger.
  1027  	// Otherwise, there are too many overflow buckets,
  1028  	// so keep the same number of buckets and "grow" laterally.
  1029  	bigger := uint8(1)
  1030  	if !overLoadFactor(h.count+1, h.B) {
  1031  		bigger = 0
  1032  		h.flags |= sameSizeGrow
  1033  	}
  1034  	oldbuckets := h.buckets
  1035  	newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)
  1036  
  1037  	flags := h.flags &^ (iterator | oldIterator)
  1038  	if h.flags&iterator != 0 {
  1039  		flags |= oldIterator
  1040  	}
  1041  	// commit the grow (atomic wrt gc)
  1042  	h.B += bigger
  1043  	h.flags = flags
  1044  	h.oldbuckets = oldbuckets
  1045  	h.buckets = newbuckets
  1046  	h.nevacuate = 0
  1047  	h.noverflow = 0
  1048  
  1049  	if h.extra != nil && h.extra.overflow != nil {
  1050  		// Promote current overflow buckets to the old generation.
  1051  		if h.extra.oldoverflow != nil {
  1052  			throw("oldoverflow is not nil")
  1053  		}
  1054  		h.extra.oldoverflow = h.extra.overflow
  1055  		h.extra.overflow = nil
  1056  	}
  1057  	if nextOverflow != nil {
  1058  		if h.extra == nil {
  1059  			h.extra = new(mapextra)
  1060  		}
  1061  		h.extra.nextOverflow = nextOverflow
  1062  	}
  1063  
  1064  	// the actual copying of the hash table data is done incrementally
  1065  	// by growWork() and evacuate().
  1066  }
  1067  
  1068  // overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor.
  1069  func overLoadFactor(count int, B uint8) bool {
  1070  	return count > bucketCnt && uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen)
  1071  }
  1072  
  1073  // tooManyOverflowBuckets reports whether noverflow buckets is too many for a map with 1<<B buckets.
  1074  // Note that most of these overflow buckets must be in sparse use;
  1075  // if use was dense, then we'd have already triggered regular map growth.
  1076  func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
  1077  	// If the threshold is too low, we do extraneous work.
  1078  	// If the threshold is too high, maps that grow and shrink can hold on to lots of unused memory.
  1079  	// "too many" means (approximately) as many overflow buckets as regular buckets.
  1080  	// See incrnoverflow for more details.
  1081  	if B > 15 {
  1082  		B = 15
  1083  	}
  1084  	// The compiler doesn't see here that B < 16; mask B to generate shorter shift code.
  1085  	return noverflow >= uint16(1)<<(B&15)
  1086  }
  1087  
  1088  // growing reports whether h is growing. The growth may be to the same size or bigger.
  1089  func (h *hmap) growing() bool {
  1090  	return h.oldbuckets != nil
  1091  }
  1092  
  1093  // sameSizeGrow reports whether the current growth is to a map of the same size.
  1094  func (h *hmap) sameSizeGrow() bool {
  1095  	return h.flags&sameSizeGrow != 0
  1096  }
  1097  
  1098  // noldbuckets calculates the number of buckets prior to the current map growth.
  1099  func (h *hmap) noldbuckets() uintptr {
  1100  	oldB := h.B
  1101  	if !h.sameSizeGrow() {
  1102  		oldB--
  1103  	}
  1104  	return bucketShift(oldB)
  1105  }
  1106  
  1107  // oldbucketmask provides a mask that can be applied to calculate n % noldbuckets().
  1108  func (h *hmap) oldbucketmask() uintptr {
  1109  	return h.noldbuckets() - 1
  1110  }
  1111  
  1112  func growWork(t *maptype, h *hmap, bucket uintptr) {
  1113  	// make sure we evacuate the oldbucket corresponding
  1114  	// to the bucket we're about to use
  1115  	evacuate(t, h, bucket&h.oldbucketmask())
  1116  
  1117  	// evacuate one more oldbucket to make progress on growing
  1118  	if h.growing() {
  1119  		evacuate(t, h, h.nevacuate)
  1120  	}
  1121  }
  1122  
  1123  func bucketEvacuated(t *maptype, h *hmap, bucket uintptr) bool {
  1124  	b := (*bmap)(add(h.oldbuckets, bucket*uintptr(t.bucketsize)))
  1125  	return evacuated(b)
  1126  }
  1127  
  1128  // evacDst is an evacuation destination.
  1129  type evacDst struct {
  1130  	b *bmap          // current destination bucket
  1131  	i int            // key/val index into b
  1132  	k unsafe.Pointer // pointer to current key storage
  1133  	v unsafe.Pointer // pointer to current value storage
  1134  }
  1135  
  1136  func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
  1137  	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
  1138  	newbit := h.noldbuckets()
  1139  	if !evacuated(b) {
  1140  		// TODO: reuse overflow buckets instead of using new ones, if there
  1141  		// is no iterator using the old buckets.  (If !oldIterator.)
  1142  
  1143  		// xy contains the x and y (low and high) evacuation destinations.
  1144  		var xy [2]evacDst
  1145  		x := &xy[0]
  1146  		x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
  1147  		x.k = add(unsafe.Pointer(x.b), dataOffset)
  1148  		x.v = add(x.k, bucketCnt*uintptr(t.keysize))
  1149  
  1150  		if !h.sameSizeGrow() {
  1151  			// Only calculate y pointers if we're growing bigger.
  1152  			// Otherwise GC can see bad pointers.
  1153  			y := &xy[1]
  1154  			y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
  1155  			y.k = add(unsafe.Pointer(y.b), dataOffset)
  1156  			y.v = add(y.k, bucketCnt*uintptr(t.keysize))
  1157  		}
  1158  
  1159  		for ; b != nil; b = b.overflow(t) {
  1160  			k := add(unsafe.Pointer(b), dataOffset)
  1161  			v := add(k, bucketCnt*uintptr(t.keysize))
  1162  			for i := 0; i < bucketCnt; i, k, v = i+1, add(k, uintptr(t.keysize)), add(v, uintptr(t.valuesize)) {
  1163  				top := b.tophash[i]
  1164  				if isEmpty(top) {
  1165  					b.tophash[i] = evacuatedEmpty
  1166  					continue
  1167  				}
  1168  				if top < minTopHash {
  1169  					throw("bad map state")
  1170  				}
  1171  				k2 := k
  1172  				if t.indirectkey() {
  1173  					k2 = *((*unsafe.Pointer)(k2))
  1174  				}
  1175  				var useY uint8
  1176  				if !h.sameSizeGrow() {
  1177  					// Compute hash to make our evacuation decision (whether we need
  1178  					// to send this key/value to bucket x or bucket y).
  1179  					hash := t.key.alg.hash(k2, uintptr(h.hash0))
  1180  					if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.alg.equal(k2, k2) {
  1181  						// If key != key (NaNs), then the hash could be (and probably
  1182  						// will be) entirely different from the old hash. Moreover,
  1183  						// it isn't reproducible. Reproducibility is required in the
  1184  						// presence of iterators, as our evacuation decision must
  1185  						// match whatever decision the iterator made.
  1186  						// Fortunately, we have the freedom to send these keys either
  1187  						// way. Also, tophash is meaningless for these kinds of keys.
  1188  						// We let the low bit of tophash drive the evacuation decision.
  1189  						// We recompute a new random tophash for the next level so
  1190  						// these keys will get evenly distributed across all buckets
  1191  						// after multiple grows.
  1192  						useY = top & 1
  1193  						top = tophash(hash)
  1194  					} else {
  1195  						if hash&newbit != 0 {
  1196  							useY = 1
  1197  						}
  1198  					}
  1199  				}
  1200  
  1201  				if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
  1202  					throw("bad evacuatedN")
  1203  				}
  1204  
  1205  				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
  1206  				dst := &xy[useY]                 // evacuation destination
  1207  
  1208  				if dst.i == bucketCnt {
  1209  					dst.b = h.newoverflow(t, dst.b)
  1210  					dst.i = 0
  1211  					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
  1212  					dst.v = add(dst.k, bucketCnt*uintptr(t.keysize))
  1213  				}
  1214  				dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
  1215  				if t.indirectkey() {
  1216  					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
  1217  				} else {
  1218  					typedmemmove(t.key, dst.k, k) // copy value
  1219  				}
  1220  				if t.indirectvalue() {
  1221  					*(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
  1222  				} else {
  1223  					typedmemmove(t.elem, dst.v, v)
  1224  				}
  1225  				dst.i++
  1226  				// These updates might push these pointers past the end of the
  1227  				// key or value arrays.  That's ok, as we have the overflow pointer
  1228  				// at the end of the bucket to protect against pointing past the
  1229  				// end of the bucket.
  1230  				dst.k = add(dst.k, uintptr(t.keysize))
  1231  				dst.v = add(dst.v, uintptr(t.valuesize))
  1232  			}
  1233  		}
  1234  		// Unlink the overflow buckets & clear key/value to help GC.
  1235  		if h.flags&oldIterator == 0 && t.bucket.kind&kindNoPointers == 0 {
  1236  			b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
  1237  			// Preserve b.tophash because the evacuation
  1238  			// state is maintained there.
  1239  			ptr := add(b, dataOffset)
  1240  			n := uintptr(t.bucketsize) - dataOffset
  1241  			memclrHasPointers(ptr, n)
  1242  		}
  1243  	}
  1244  
  1245  	if oldbucket == h.nevacuate {
  1246  		advanceEvacuationMark(h, t, newbit)
  1247  	}
  1248  }
  1249  
  1250  func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
  1251  	h.nevacuate++
  1252  	// Experiments suggest that 1024 is overkill by at least an order of magnitude.
  1253  	// Put it in there as a safeguard anyway, to ensure O(1) behavior.
  1254  	stop := h.nevacuate + 1024
  1255  	if stop > newbit {
  1256  		stop = newbit
  1257  	}
  1258  	for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
  1259  		h.nevacuate++
  1260  	}
  1261  	if h.nevacuate == newbit { // newbit == # of oldbuckets
  1262  		// Growing is all done. Free old main bucket array.
  1263  		h.oldbuckets = nil
  1264  		// Can discard old overflow buckets as well.
  1265  		// If they are still referenced by an iterator,
  1266  		// then the iterator holds a pointers to the slice.
  1267  		if h.extra != nil {
  1268  			h.extra.oldoverflow = nil
  1269  		}
  1270  		h.flags &^= sameSizeGrow
  1271  	}
  1272  }
  1273  
  1274  func ismapkey(t *_type) bool {
  1275  	return t.alg.hash != nil
  1276  }
  1277  
  1278  // Reflect stubs. Called from ../reflect/asm_*.s
  1279  
  1280  //go:linkname reflect_makemap reflect.makemap
  1281  func reflect_makemap(t *maptype, cap int) *hmap {
  1282  	// Check invariants and reflects math.
  1283  	if !ismapkey(t.key) {
  1284  		throw("runtime.reflect_makemap: unsupported map key type")
  1285  	}
  1286  	if t.key.size > maxKeySize && (!t.indirectkey() || t.keysize != uint8(sys.PtrSize)) ||
  1287  		t.key.size <= maxKeySize && (t.indirectkey() || t.keysize != uint8(t.key.size)) {
  1288  		throw("key size wrong")
  1289  	}
  1290  	if t.elem.size > maxValueSize && (!t.indirectvalue() || t.valuesize != uint8(sys.PtrSize)) ||
  1291  		t.elem.size <= maxValueSize && (t.indirectvalue() || t.valuesize != uint8(t.elem.size)) {
  1292  		throw("value size wrong")
  1293  	}
  1294  	if t.key.align > bucketCnt {
  1295  		throw("key align too big")
  1296  	}
  1297  	if t.elem.align > bucketCnt {
  1298  		throw("value align too big")
  1299  	}
  1300  	if t.key.size%uintptr(t.key.align) != 0 {
  1301  		throw("key size not a multiple of key align")
  1302  	}
  1303  	if t.elem.size%uintptr(t.elem.align) != 0 {
  1304  		throw("value size not a multiple of value align")
  1305  	}
  1306  	if bucketCnt < 8 {
  1307  		throw("bucketsize too small for proper alignment")
  1308  	}
  1309  	if dataOffset%uintptr(t.key.align) != 0 {
  1310  		throw("need padding in bucket (key)")
  1311  	}
  1312  	if dataOffset%uintptr(t.elem.align) != 0 {
  1313  		throw("need padding in bucket (value)")
  1314  	}
  1315  
  1316  	return makemap(t, cap, nil)
  1317  }
  1318  
  1319  //go:linkname reflect_mapaccess reflect.mapaccess
  1320  func reflect_mapaccess(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
  1321  	val, ok := mapaccess2(t, h, key)
  1322  	if !ok {
  1323  		// reflect wants nil for a missing element
  1324  		val = nil
  1325  	}
  1326  	return val
  1327  }
  1328  
  1329  //go:linkname reflect_mapassign reflect.mapassign
  1330  func reflect_mapassign(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
  1331  	p := mapassign(t, h, key)
  1332  	typedmemmove(t.elem, p, val)
  1333  }
  1334  
  1335  //go:linkname reflect_mapdelete reflect.mapdelete
  1336  func reflect_mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
  1337  	mapdelete(t, h, key)
  1338  }
  1339  
  1340  //go:linkname reflect_mapiterinit reflect.mapiterinit
  1341  func reflect_mapiterinit(t *maptype, h *hmap) *hiter {
  1342  	it := new(hiter)
  1343  	mapiterinit(t, h, it)
  1344  	return it
  1345  }
  1346  
  1347  //go:linkname reflect_mapiternext reflect.mapiternext
  1348  func reflect_mapiternext(it *hiter) {
  1349  	mapiternext(it)
  1350  }
  1351  
  1352  //go:linkname reflect_mapiterkey reflect.mapiterkey
  1353  func reflect_mapiterkey(it *hiter) unsafe.Pointer {
  1354  	return it.key
  1355  }
  1356  
  1357  //go:linkname reflect_mapitervalue reflect.mapitervalue
  1358  func reflect_mapitervalue(it *hiter) unsafe.Pointer {
  1359  	return it.value
  1360  }
  1361  
  1362  //go:linkname reflect_maplen reflect.maplen
  1363  func reflect_maplen(h *hmap) int {
  1364  	if h == nil {
  1365  		return 0
  1366  	}
  1367  	if raceenabled {
  1368  		callerpc := getcallerpc()
  1369  		racereadpc(unsafe.Pointer(h), callerpc, funcPC(reflect_maplen))
  1370  	}
  1371  	return h.count
  1372  }
  1373  
  1374  //go:linkname reflect_ismapkey reflect.ismapkey
  1375  func reflect_ismapkey(t *_type) bool {
  1376  	return ismapkey(t)
  1377  }
  1378  
  1379  const maxZero = 1024 // must match value in cmd/compile/internal/gc/walk.go
  1380  var zeroVal [maxZero]byte
  1381  

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