runtime: limit the number of map overflow buckets

Consider repeatedly adding many items to a map
and then deleting them all, as in #16070. The map
itself doesn't need to grow above the high water
mark of number of items. However, due to random
collisions, the map can accumulate overflow
buckets.

Prior to this CL, those overflow buckets were
never removed, which led to a slow memory leak.

The problem with removing overflow buckets is
iterators. The obvious approach is to repack
keys and values and eliminate unused overflow
buckets. However, keys, values, and overflow
buckets cannot be manipulated without disrupting
iterators.

This CL takes a different approach, which is to
reuse the existing map growth mechanism,
which is well established, well tested, and
safe in the presence of iterators.
When a map has accumulated enough overflow buckets
we trigger map growth, but grow into a map of the
same size as before. The old overflow buckets will
be left behind for garbage collection.

For the code in #16070, instead of climbing
(very slowly) forever, memory usage now cycles
between 264mb and 483mb every 15 minutes or so.

To avoid increasing the size of maps,
the overflow bucket counter is only 16 bits.
For large maps, the counter is incremented
stochastically.

Fixes #16070

Change-Id: If551d77613ec6836907efca58bda3deee304297e
Reviewed-on: https://go-review.googlesource.com/25049
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -182,20 +182,22 @@ func hmap(t *Type) *Type {
 	}
 
 	bucket := mapbucket(t)
-	var field [8]*Field
-	field[0] = makefield("count", Types[TINT])
-	field[1] = makefield("flags", Types[TUINT8])
-	field[2] = makefield("B", Types[TUINT8])
-	field[3] = makefield("hash0", Types[TUINT32])
-	field[4] = makefield("buckets", Ptrto(bucket))
-	field[5] = makefield("oldbuckets", Ptrto(bucket))
-	field[6] = makefield("nevacuate", Types[TUINTPTR])
-	field[7] = makefield("overflow", Types[TUNSAFEPTR])
+	fields := []*Field{
+		makefield("count", Types[TINT]),
+		makefield("flags", Types[TUINT8]),
+		makefield("B", Types[TUINT8]),
+		makefield("noverflow", Types[TUINT16]),
+		makefield("hash0", Types[TUINT32]),
+		makefield("buckets", Ptrto(bucket)),
+		makefield("oldbuckets", Ptrto(bucket)),
+		makefield("nevacuate", Types[TUINTPTR]),
+		makefield("overflow", Types[TUNSAFEPTR]),
+	}
 
 	h := typ(TSTRUCT)
 	h.Noalg = true
 	h.Local = t.Local
-	h.SetFields(field[:])
+	h.SetFields(fields)
 	dowidth(h)
 	t.MapType().Hmap = h
 	h.StructType().Map = t

src/runtime/hashmap.go

@@ -93,9 +93,10 @@ const (
 	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
+	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
@@ -105,10 +106,11 @@ const (
 type hmap struct {
 	// Note: the format of the Hmap is encoded in ../../cmd/internal/gc/reflect.go and
 	// ../reflect/type.go. Don't change this structure without also changing that code!
-	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)
-	hash0 uint32 // hash seed
+	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
@@ -165,7 +167,34 @@ func (b *bmap) overflow(t *maptype) *bmap {
 	return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize))
 }
 
+// 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) setoverflow(t *maptype, b, ovf *bmap) {
+	h.incrnoverflow()
 	if t.bucket.kind&kindNoPointers != 0 {
 		h.createOverflow()
 		*h.overflow[0] = append(*h.overflow[0], ovf)
@@ -238,7 +267,7 @@ func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
 
 	// find size parameter which will hold the requested # of elements
 	B := uint8(0)
-	for ; hint > bucketCnt && float32(hint) > loadFactor*float32(uintptr(1)<<B); B++ {
+	for ; overLoadFactor(hint, B); B++ {
 	}
 
 	// allocate initial hash table
@@ -260,6 +289,7 @@ func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
 	h.buckets = buckets
 	h.oldbuckets = nil
 	h.nevacuate = 0
+	h.noverflow = 0
 
 	return h
 }
@@ -290,7 +320,11 @@ func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 	m := uintptr(1)<<h.B - 1
 	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 	if c := h.oldbuckets; c != nil {
-		oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+		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
 		}
@@ -344,7 +378,11 @@ func mapaccess2(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, bool)
 	m := uintptr(1)<<h.B - 1
 	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
 	if c := h.oldbuckets; c != nil {
-		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&(m>>1))*uintptr(t.bucketsize)))
+		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
 		}
@@ -387,7 +425,11 @@ func mapaccessK(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, unsafe
 	m := uintptr(1)<<h.B - 1
 	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + (hash&m)*uintptr(t.bucketsize)))
 	if c := h.oldbuckets; c != nil {
-		oldb := (*bmap)(unsafe.Pointer(uintptr(c) + (hash&(m>>1))*uintptr(t.bucketsize)))
+		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
 		}
@@ -465,7 +507,7 @@ func mapassign1(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
 
 again:
 	bucket := hash & (uintptr(1)<<h.B - 1)
-	if h.oldbuckets != nil {
+	if h.growing() {
 		growWork(t, h, bucket)
 	}
 	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
@@ -514,8 +556,11 @@ again:
 		b = ovf
 	}
 
-	// did not find mapping for key. Allocate new cell & add entry.
-	if float32(h.count) >= loadFactor*float32((uintptr(1)<<h.B)) && h.count >= bucketCnt {
+	// 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(int64(h.count), h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
 		hashGrow(t, h)
 		goto again // Growing the table invalidates everything, so try again
 	}
@@ -573,7 +618,7 @@ func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
 	bucket := hash & (uintptr(1)<<h.B - 1)
-	if h.oldbuckets != nil {
+	if h.growing() {
 		growWork(t, h, bucket)
 	}
 	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
@@ -700,12 +745,12 @@ next:
 			it.value = nil
 			return
 		}
-		if h.oldbuckets != nil && it.B == h.B {
+		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 & (uintptr(1)<<(it.B-1) - 1)
+			oldbucket := bucket & it.h.oldbucketmask()
 			b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
 			if !evacuated(b) {
 				checkBucket = bucket
@@ -729,9 +774,9 @@ next:
 		k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
 		v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
 		if b.tophash[offi] != empty && b.tophash[offi] != evacuatedEmpty {
-			if checkBucket != noCheck {
-				// Special case: iterator was started during a grow and the
-				// grow is not done yet. We're working on a bucket whose
+			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
@@ -817,21 +862,27 @@ next:
 }
 
 func hashGrow(t *maptype, h *hmap) {
-	if h.oldbuckets != nil {
-		throw("evacuation not done in time")
+	// 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(int64(h.count), h.B) {
+		bigger = 0
+		h.flags |= sameSizeGrow
 	}
 	oldbuckets := h.buckets
-	newbuckets := newarray(t.bucket, 1<<(h.B+1))
+	newbuckets := newarray(t.bucket, 1<<(h.B+bigger))
 	flags := h.flags &^ (iterator | oldIterator)
 	if h.flags&iterator != 0 {
 		flags |= oldIterator
 	}
 	// commit the grow (atomic wrt gc)
-	h.B++
+	h.B += bigger
 	h.flags = flags
 	h.oldbuckets = oldbuckets
 	h.buckets = newbuckets
 	h.nevacuate = 0
+	h.noverflow = 0
 
 	if h.overflow != nil {
 		// Promote current overflow buckets to the old generation.
@@ -846,35 +897,87 @@ func hashGrow(t *maptype, h *hmap) {
 	// by growWork() and evacuate().
 }
 
-func growWork(t *maptype, h *hmap, bucket uintptr) {
-	noldbuckets := uintptr(1) << (h.B - 1)
+// overLoadFactor reports whether count items placed in 1<<B buckets is over loadFactor.
+func overLoadFactor(count int64, B uint8) bool {
+	// TODO: rewrite to use integer math and comparison?
+	return count >= bucketCnt && float32(count) >= loadFactor*float32((uintptr(1)<<B))
+}
+
+// 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 < 16 {
+		return noverflow >= uint16(1)<<B
+	}
+	return noverflow >= 1<<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 uintptr(1) << 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&(noldbuckets-1))
+	evacuate(t, h, bucket&h.oldbucketmask())
 
 	// evacuate one more oldbucket to make progress on growing
-	if h.oldbuckets != nil {
+	if h.growing() {
 		evacuate(t, h, h.nevacuate)
 	}
 }
 
 func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
-	newbit := uintptr(1) << (h.B - 1)
+	newbit := h.noldbuckets()
 	alg := t.key.alg
 	if !evacuated(b) {
 		// TODO: reuse overflow buckets instead of using new ones, if there
 		// is no iterator using the old buckets.  (If !oldIterator.)
 
-		x := (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
-		y := (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
-		xi := 0
-		yi := 0
-		xk := add(unsafe.Pointer(x), dataOffset)
-		yk := add(unsafe.Pointer(y), dataOffset)
-		xv := add(xk, bucketCnt*uintptr(t.keysize))
-		yv := add(yk, bucketCnt*uintptr(t.keysize))
+		var (
+			x, y   *bmap          // current low/high buckets in new map
+			xi, yi int            // key/val indices into x and y
+			xk, yk unsafe.Pointer // pointers to current x and y key storage
+			xv, yv unsafe.Pointer // pointers to current x and y value storage
+		)
+		x = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
+		xi = 0
+		xk = add(unsafe.Pointer(x), dataOffset)
+		xv = add(xk, bucketCnt*uintptr(t.keysize))
+		if !h.sameSizeGrow() {
+			// Only calculate y pointers if we're growing bigger.
+			// Otherwise GC can see bad pointers.
+			y = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
+			yi = 0
+			yk = add(unsafe.Pointer(y), dataOffset)
+			yv = add(yk, bucketCnt*uintptr(t.keysize))
+		}
 		for ; b != nil; b = b.overflow(t) {
 			k := add(unsafe.Pointer(b), dataOffset)
 			v := add(k, bucketCnt*uintptr(t.keysize))
@@ -891,34 +994,38 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 				if t.indirectkey {
 					k2 = *((*unsafe.Pointer)(k2))
 				}
-				// Compute hash to make our evacuation decision (whether we need
-				// to send this key/value to bucket x or bucket y).
-				hash := alg.hash(k2, uintptr(h.hash0))
-				if h.flags&iterator != 0 {
-					if !t.reflexivekey && !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.
-						if (top & 1) != 0 {
-							hash |= newbit
-						} else {
-							hash &^= newbit
-						}
-						top = uint8(hash >> (sys.PtrSize*8 - 8))
-						if top < minTopHash {
-							top += minTopHash
+				useX := true
+				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 := alg.hash(k2, uintptr(h.hash0))
+					if h.flags&iterator != 0 {
+						if !t.reflexivekey && !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.
+							if top&1 != 0 {
+								hash |= newbit
+							} else {
+								hash &^= newbit
+							}
+							top = uint8(hash >> (sys.PtrSize*8 - 8))
+							if top < minTopHash {
+								top += minTopHash
+							}
 						}
 					}
+					useX = hash&newbit == 0
 				}
-				if (hash & newbit) == 0 {
+				if useX {
 					b.tophash[i] = evacuatedX
 					if xi == bucketCnt {
 						newx := (*bmap)(newobject(t.bucket))
@@ -988,6 +1095,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 			if h.overflow != nil {
 				h.overflow[1] = nil
 			}
+			h.flags &^= sameSizeGrow
 		}
 	}
 }

src/runtime/hashmap_fast.go

@@ -29,7 +29,11 @@ func mapaccess1_fast32(t *maptype, h *hmap, key uint32) unsafe.Pointer {
 		m := uintptr(1)<<h.B - 1
 		b = (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 		if c := h.oldbuckets; c != nil {
-			oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+			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
 			}
@@ -74,7 +78,11 @@ func mapaccess2_fast32(t *maptype, h *hmap, key uint32) (unsafe.Pointer, bool) {
 		m := uintptr(1)<<h.B - 1
 		b = (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 		if c := h.oldbuckets; c != nil {
-			oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+			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
 			}
@@ -119,7 +127,11 @@ func mapaccess1_fast64(t *maptype, h *hmap, key uint64) unsafe.Pointer {
 		m := uintptr(1)<<h.B - 1
 		b = (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 		if c := h.oldbuckets; c != nil {
-			oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+			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
 			}
@@ -164,7 +176,11 @@ func mapaccess2_fast64(t *maptype, h *hmap, key uint64) (unsafe.Pointer, bool) {
 		m := uintptr(1)<<h.B - 1
 		b = (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 		if c := h.oldbuckets; c != nil {
-			oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+			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
 			}
@@ -264,7 +280,11 @@ dohash:
 	m := uintptr(1)<<h.B - 1
 	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 	if c := h.oldbuckets; c != nil {
-		oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+		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
 		}
@@ -367,7 +387,11 @@ dohash:
 	m := uintptr(1)<<h.B - 1
 	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
 	if c := h.oldbuckets; c != nil {
-		oldb := (*bmap)(add(c, (hash&(m>>1))*uintptr(t.bucketsize)))
+		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
 		}