diff --git a/src/cmd/compile/internal/gc/builtin.go b/src/cmd/compile/internal/gc/builtin.go
index 3ca1adc1f7..6b416c8a5c 100644
--- a/src/cmd/compile/internal/gc/builtin.go
+++ b/src/cmd/compile/internal/gc/builtin.go
@@ -100,59 +100,60 @@ var runtimeDecls = [...]struct {
 	{"mapdelete_fast64", funcTag, 73},
 	{"mapdelete_faststr", funcTag, 73},
 	{"mapiternext", funcTag, 74},
-	{"makechan64", funcTag, 76},
-	{"makechan", funcTag, 77},
-	{"chanrecv1", funcTag, 79},
-	{"chanrecv2", funcTag, 80},
-	{"chansend1", funcTag, 82},
+	{"mapclear", funcTag, 75},
+	{"makechan64", funcTag, 77},
+	{"makechan", funcTag, 78},
+	{"chanrecv1", funcTag, 80},
+	{"chanrecv2", funcTag, 81},
+	{"chansend1", funcTag, 83},
 	{"closechan", funcTag, 23},
-	{"writeBarrier", varTag, 84},
-	{"typedmemmove", funcTag, 85},
-	{"typedmemclr", funcTag, 86},
-	{"typedslicecopy", funcTag, 87},
-	{"selectnbsend", funcTag, 88},
-	{"selectnbrecv", funcTag, 89},
-	{"selectnbrecv2", funcTag, 91},
+	{"writeBarrier", varTag, 85},
+	{"typedmemmove", funcTag, 86},
+	{"typedmemclr", funcTag, 87},
+	{"typedslicecopy", funcTag, 88},
+	{"selectnbsend", funcTag, 89},
+	{"selectnbrecv", funcTag, 90},
+	{"selectnbrecv2", funcTag, 92},
 	{"selectsetpc", funcTag, 56},
-	{"selectgo", funcTag, 92},
+	{"selectgo", funcTag, 93},
 	{"block", funcTag, 5},
-	{"makeslice", funcTag, 94},
-	{"makeslice64", funcTag, 95},
-	{"growslice", funcTag, 96},
-	{"memmove", funcTag, 97},
-	{"memclrNoHeapPointers", funcTag, 98},
-	{"memclrHasPointers", funcTag, 98},
-	{"memequal", funcTag, 99},
-	{"memequal8", funcTag, 100},
-	{"memequal16", funcTag, 100},
-	{"memequal32", funcTag, 100},
-	{"memequal64", funcTag, 100},
-	{"memequal128", funcTag, 100},
-	{"int64div", funcTag, 101},
-	{"uint64div", funcTag, 102},
-	{"int64mod", funcTag, 101},
-	{"uint64mod", funcTag, 102},
-	{"float64toint64", funcTag, 103},
-	{"float64touint64", funcTag, 104},
-	{"float64touint32", funcTag, 105},
-	{"int64tofloat64", funcTag, 106},
-	{"uint64tofloat64", funcTag, 107},
-	{"uint32tofloat64", funcTag, 108},
-	{"complex128div", funcTag, 109},
-	{"racefuncenter", funcTag, 110},
+	{"makeslice", funcTag, 95},
+	{"makeslice64", funcTag, 96},
+	{"growslice", funcTag, 97},
+	{"memmove", funcTag, 98},
+	{"memclrNoHeapPointers", funcTag, 99},
+	{"memclrHasPointers", funcTag, 99},
+	{"memequal", funcTag, 100},
+	{"memequal8", funcTag, 101},
+	{"memequal16", funcTag, 101},
+	{"memequal32", funcTag, 101},
+	{"memequal64", funcTag, 101},
+	{"memequal128", funcTag, 101},
+	{"int64div", funcTag, 102},
+	{"uint64div", funcTag, 103},
+	{"int64mod", funcTag, 102},
+	{"uint64mod", funcTag, 103},
+	{"float64toint64", funcTag, 104},
+	{"float64touint64", funcTag, 105},
+	{"float64touint32", funcTag, 106},
+	{"int64tofloat64", funcTag, 107},
+	{"uint64tofloat64", funcTag, 108},
+	{"uint32tofloat64", funcTag, 109},
+	{"complex128div", funcTag, 110},
+	{"racefuncenter", funcTag, 111},
 	{"racefuncexit", funcTag, 5},
-	{"raceread", funcTag, 110},
-	{"racewrite", funcTag, 110},
-	{"racereadrange", funcTag, 111},
-	{"racewriterange", funcTag, 111},
-	{"msanread", funcTag, 111},
-	{"msanwrite", funcTag, 111},
+	{"raceread", funcTag, 111},
+	{"racewrite", funcTag, 111},
+	{"racereadrange", funcTag, 112},
+	{"racewriterange", funcTag, 112},
+	{"msanread", funcTag, 112},
+	{"msanwrite", funcTag, 112},
 	{"support_popcnt", varTag, 11},
 	{"support_sse41", varTag, 11},
 }
 
 func runtimeTypes() []*types.Type {
-	var typs [112]*types.Type
+	var typs [113]*types.Type
 	typs[0] = types.Bytetype
 	typs[1] = types.NewPtr(typs[0])
 	typs[2] = types.Types[TANY]
@@ -228,42 +229,43 @@ func runtimeTypes() []*types.Type {
 	typs[72] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[62]), anonfield(typs[3])}, nil)
 	typs[73] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[62]), anonfield(typs[2])}, nil)
 	typs[74] = functype(nil, []*Node{anonfield(typs[3])}, nil)
-	typs[75] = types.NewChan(typs[2], types.Cboth)
-	typs[76] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15])}, []*Node{anonfield(typs[75])})
-	typs[77] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[32])}, []*Node{anonfield(typs[75])})
-	typs[78] = types.NewChan(typs[2], types.Crecv)
-	typs[79] = functype(nil, []*Node{anonfield(typs[78]), anonfield(typs[3])}, nil)
-	typs[80] = functype(nil, []*Node{anonfield(typs[78]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
-	typs[81] = types.NewChan(typs[2], types.Csend)
-	typs[82] = functype(nil, []*Node{anonfield(typs[81]), anonfield(typs[3])}, nil)
-	typs[83] = types.NewArray(typs[0], 3)
-	typs[84] = tostruct([]*Node{namedfield("enabled", typs[11]), namedfield("pad", typs[83]), namedfield("needed", typs[11]), namedfield("cgo", typs[11]), namedfield("alignme", typs[17])})
-	typs[85] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3]), anonfield(typs[3])}, nil)
-	typs[86] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3])}, nil)
-	typs[87] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2]), anonfield(typs[2])}, []*Node{anonfield(typs[32])})
-	typs[88] = functype(nil, []*Node{anonfield(typs[81]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
-	typs[89] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[78])}, []*Node{anonfield(typs[11])})
-	typs[90] = types.NewPtr(typs[11])
-	typs[91] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[90]), anonfield(typs[78])}, []*Node{anonfield(typs[11])})
-	typs[92] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[32])}, []*Node{anonfield(typs[32]), anonfield(typs[11])})
-	typs[93] = types.NewSlice(typs[2])
-	typs[94] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[32]), anonfield(typs[32])}, []*Node{anonfield(typs[93])})
-	typs[95] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[93])})
-	typs[96] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[93]), anonfield(typs[32])}, []*Node{anonfield(typs[93])})
-	typs[97] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[47])}, nil)
-	typs[98] = functype(nil, []*Node{anonfield(typs[58]), anonfield(typs[47])}, nil)
-	typs[99] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[47])}, []*Node{anonfield(typs[11])})
-	typs[100] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
-	typs[101] = functype(nil, []*Node{anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[15])})
-	typs[102] = functype(nil, []*Node{anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[17])})
-	typs[103] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[15])})
-	typs[104] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[17])})
-	typs[105] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[60])})
-	typs[106] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[13])})
-	typs[107] = functype(nil, []*Node{anonfield(typs[17])}, []*Node{anonfield(typs[13])})
-	typs[108] = functype(nil, []*Node{anonfield(typs[60])}, []*Node{anonfield(typs[13])})
-	typs[109] = functype(nil, []*Node{anonfield(typs[19]), anonfield(typs[19])}, []*Node{anonfield(typs[19])})
-	typs[110] = functype(nil, []*Node{anonfield(typs[47])}, nil)
-	typs[111] = functype(nil, []*Node{anonfield(typs[47]), anonfield(typs[47])}, nil)
+	typs[75] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[62])}, nil)
+	typs[76] = types.NewChan(typs[2], types.Cboth)
+	typs[77] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15])}, []*Node{anonfield(typs[76])})
+	typs[78] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[32])}, []*Node{anonfield(typs[76])})
+	typs[79] = types.NewChan(typs[2], types.Crecv)
+	typs[80] = functype(nil, []*Node{anonfield(typs[79]), anonfield(typs[3])}, nil)
+	typs[81] = functype(nil, []*Node{anonfield(typs[79]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
+	typs[82] = types.NewChan(typs[2], types.Csend)
+	typs[83] = functype(nil, []*Node{anonfield(typs[82]), anonfield(typs[3])}, nil)
+	typs[84] = types.NewArray(typs[0], 3)
+	typs[85] = tostruct([]*Node{namedfield("enabled", typs[11]), namedfield("pad", typs[84]), namedfield("needed", typs[11]), namedfield("cgo", typs[11]), namedfield("alignme", typs[17])})
+	typs[86] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3]), anonfield(typs[3])}, nil)
+	typs[87] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3])}, nil)
+	typs[88] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[2]), anonfield(typs[2])}, []*Node{anonfield(typs[32])})
+	typs[89] = functype(nil, []*Node{anonfield(typs[82]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
+	typs[90] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[79])}, []*Node{anonfield(typs[11])})
+	typs[91] = types.NewPtr(typs[11])
+	typs[92] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[91]), anonfield(typs[79])}, []*Node{anonfield(typs[11])})
+	typs[93] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[32])}, []*Node{anonfield(typs[32]), anonfield(typs[11])})
+	typs[94] = types.NewSlice(typs[2])
+	typs[95] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[32]), anonfield(typs[32])}, []*Node{anonfield(typs[94])})
+	typs[96] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[94])})
+	typs[97] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[94]), anonfield(typs[32])}, []*Node{anonfield(typs[94])})
+	typs[98] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[47])}, nil)
+	typs[99] = functype(nil, []*Node{anonfield(typs[58]), anonfield(typs[47])}, nil)
+	typs[100] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3]), anonfield(typs[47])}, []*Node{anonfield(typs[11])})
+	typs[101] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[11])})
+	typs[102] = functype(nil, []*Node{anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[15])})
+	typs[103] = functype(nil, []*Node{anonfield(typs[17]), anonfield(typs[17])}, []*Node{anonfield(typs[17])})
+	typs[104] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[15])})
+	typs[105] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[17])})
+	typs[106] = functype(nil, []*Node{anonfield(typs[13])}, []*Node{anonfield(typs[60])})
+	typs[107] = functype(nil, []*Node{anonfield(typs[15])}, []*Node{anonfield(typs[13])})
+	typs[108] = functype(nil, []*Node{anonfield(typs[17])}, []*Node{anonfield(typs[13])})
+	typs[109] = functype(nil, []*Node{anonfield(typs[60])}, []*Node{anonfield(typs[13])})
+	typs[110] = functype(nil, []*Node{anonfield(typs[19]), anonfield(typs[19])}, []*Node{anonfield(typs[19])})
+	typs[111] = functype(nil, []*Node{anonfield(typs[47])}, nil)
+	typs[112] = functype(nil, []*Node{anonfield(typs[47]), anonfield(typs[47])}, nil)
 	return typs[:]
 }
diff --git a/src/cmd/compile/internal/gc/builtin/runtime.go b/src/cmd/compile/internal/gc/builtin/runtime.go
index 1d3f17c0d1..d459c07cbe 100644
--- a/src/cmd/compile/internal/gc/builtin/runtime.go
+++ b/src/cmd/compile/internal/gc/builtin/runtime.go
@@ -122,6 +122,7 @@ func mapdelete_fast32(mapType *byte, hmap map[any]any, key any)
 func mapdelete_fast64(mapType *byte, hmap map[any]any, key any)
 func mapdelete_faststr(mapType *byte, hmap map[any]any, key any)
 func mapiternext(hiter *any)
+func mapclear(mapType *byte, hmap map[any]any)
 
 // *byte is really *runtime.Type
 func makechan64(chanType *byte, size int64) (hchan chan any)
diff --git a/src/cmd/compile/internal/gc/order.go b/src/cmd/compile/internal/gc/order.go
index 45a3b5cc42..dce68a6c17 100644
--- a/src/cmd/compile/internal/gc/order.go
+++ b/src/cmd/compile/internal/gc/order.go
@@ -695,6 +695,8 @@ func (o *Order) stmt(n *Node) {
 
 		t := o.markTemp()
 		n.Right = o.expr(n.Right, nil)
+
+		orderBody := true
 		switch n.Type.Etype {
 		default:
 			Fatalf("orderstmt range %v", n.Type)
@@ -721,6 +723,14 @@ func (o *Order) stmt(n *Node) {
 			n.Right = o.copyExpr(r, r.Type, false)
 
 		case TMAP:
+			if isMapClear(n) {
+				// Preserve the body of the map clear pattern so it can
+				// be detected during walk. The loop body will not be used
+				// when optimizing away the range loop to a runtime call.
+				orderBody = false
+				break
+			}
+
 			// copy the map value in case it is a map literal.
 			// TODO(rsc): Make tmp = literal expressions reuse tmp.
 			// For maps tmp is just one word so it hardly matters.
@@ -732,7 +742,9 @@ func (o *Order) stmt(n *Node) {
 			prealloc[n] = o.newTemp(hiter(n.Type), true)
 		}
 		o.exprListInPlace(n.List)
-		orderBlock(&n.Nbody)
+		if orderBody {
+			orderBlock(&n.Nbody)
+		}
 		o.out = append(o.out, n)
 		o.cleanTemp(t)
 
diff --git a/src/cmd/compile/internal/gc/range.go b/src/cmd/compile/internal/gc/range.go
index 5c3c5ca088..af818f6f4c 100644
--- a/src/cmd/compile/internal/gc/range.go
+++ b/src/cmd/compile/internal/gc/range.go
@@ -154,6 +154,14 @@ func cheapComputableIndex(width int64) bool {
 // Node n may also be modified in place, and may also be
 // the returned node.
 func walkrange(n *Node) *Node {
+	if isMapClear(n) {
+		m := n.Right
+		lno := setlineno(m)
+		n = mapClear(m)
+		lineno = lno
+		return n
+	}
+
 	// variable name conventions:
 	//	ohv1, hv1, hv2: hidden (old) val 1, 2
 	//	ha, hit: hidden aggregate, iterator
@@ -449,6 +457,69 @@ func walkrange(n *Node) *Node {
 	return n
 }
 
+// isMapClear checks if n is of the form:
+//
+// for k := range m {
+//   delete(m, k)
+// }
+//
+// where == for keys of map m is reflexive.
+func isMapClear(n *Node) bool {
+	if Debug['N'] != 0 || instrumenting {
+		return false
+	}
+
+	if n.Op != ORANGE || n.Type.Etype != TMAP || n.List.Len() != 1 {
+		return false
+	}
+
+	k := n.List.First()
+	if k == nil || k.isBlank() {
+		return false
+	}
+
+	// Require k to be a new variable name.
+	if k.Name == nil || k.Name.Defn != n {
+		return false
+	}
+
+	if n.Nbody.Len() != 1 {
+		return false
+	}
+
+	stmt := n.Nbody.First() // only stmt in body
+	if stmt == nil || stmt.Op != ODELETE {
+		return false
+	}
+
+	m := n.Right
+	if !samesafeexpr(stmt.List.First(), m) || !samesafeexpr(stmt.List.Second(), k) {
+		return false
+	}
+
+	// Keys where equality is not reflexive can not be deleted from maps.
+	if !isreflexive(m.Type.Key()) {
+		return false
+	}
+
+	return true
+}
+
+// mapClear constructs a call to runtime.mapclear for the map m.
+func mapClear(m *Node) *Node {
+	t := m.Type
+
+	// instantiate mapclear(typ *type, hmap map[any]any)
+	fn := syslook("mapclear")
+	fn = substArgTypes(fn, t.Key(), t.Elem())
+	n := mkcall1(fn, nil, nil, typename(t), m)
+
+	n = typecheck(n, Etop)
+	n = walkstmt(n)
+
+	return n
+}
+
 // Lower n into runtime·memclr if possible, for
 // fast zeroing of slices and arrays (issue 5373).
 // Look for instances of
diff --git a/src/runtime/map.go b/src/runtime/map.go
index 1926123458..cc1358a977 100644
--- a/src/runtime/map.go
+++ b/src/runtime/map.go
@@ -318,7 +318,7 @@ func makemap(t *maptype, hint int, h *hmap) *hmap {
 	// 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)
+		h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
 		if nextOverflow != nil {
 			h.extra = new(mapextra)
 			h.extra.nextOverflow = nextOverflow
@@ -328,6 +328,57 @@ func makemap(t *maptype, hint int, h *hmap) *hmap {
 	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.
@@ -855,34 +906,49 @@ next:
 	goto next
 }
 
-func makeBucketArray(t *maptype, b uint8) (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
-		}
+// 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)
 	}
-	buckets = newarray(t.bucket, int(nbuckets))
-	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))
+
+	if h == nil || h.count == 0 {
+		return
 	}
-	return buckets, nextOverflow
+
+	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) {
@@ -895,7 +961,7 @@ func hashGrow(t *maptype, h *hmap) {
 		h.flags |= sameSizeGrow
 	}
 	oldbuckets := h.buckets
-	newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger)
+	newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)
 
 	flags := h.flags &^ (iterator | oldIterator)
 	if h.flags&iterator != 0 {
diff --git a/src/runtime/map_benchmark_test.go b/src/runtime/map_benchmark_test.go
index aec0c51f3f..025c0398d3 100644
--- a/src/runtime/map_benchmark_test.go
+++ b/src/runtime/map_benchmark_test.go
@@ -341,3 +341,32 @@ func BenchmarkComplexAlgMap(b *testing.B) {
 		_ = m[k]
 	}
 }
+
+func BenchmarkGoMapClear(b *testing.B) {
+	b.Run("Reflexive", func(b *testing.B) {
+		for size := 1; size < 100000; size *= 10 {
+			b.Run(strconv.Itoa(size), func(b *testing.B) {
+				m := make(map[int]int, size)
+				for i := 0; i < b.N; i++ {
+					m[0] = size // Add one element so len(m) != 0 avoiding fast paths.
+					for k := range m {
+						delete(m, k)
+					}
+				}
+			})
+		}
+	})
+	b.Run("NonReflexive", func(b *testing.B) {
+		for size := 1; size < 100000; size *= 10 {
+			b.Run(strconv.Itoa(size), func(b *testing.B) {
+				m := make(map[float64]int, size)
+				for i := 0; i < b.N; i++ {
+					m[1.0] = size // Add one element so len(m) != 0 avoiding fast paths.
+					for k := range m {
+						delete(m, k)
+					}
+				}
+			})
+		}
+	})
+}
diff --git a/test/codegen/maps.go b/test/codegen/maps.go
index 57e219ca06..d167715898 100644
--- a/test/codegen/maps.go
+++ b/test/codegen/maps.go
@@ -36,3 +36,60 @@ func AccessString2(m map[string]int) bool {
 	_, ok := m["abc"]
 	return ok
 }
+
+// ------------------- //
+//     Map Clear       //
+// ------------------- //
+
+// Optimization of map clear idiom (Issue #20138).
+
+func MapClearReflexive(m map[int]int) {
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearIndirect(m map[int]int) {
+	s := struct{ m map[int]int }{m: m}
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range s.m {
+		delete(s.m, k)
+	}
+}
+
+func MapClearPointer(m map[*byte]int) {
+	// amd64:`.*runtime\.mapclear`
+	// amd64:-`.*runtime\.mapiterinit`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearNotReflexive(m map[float64]int) {
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearInterface(m map[interface{}]int) {
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k := range m {
+		delete(m, k)
+	}
+}
+
+func MapClearSideEffect(m map[int]int) int {
+	k := 0
+	// amd64:`.*runtime\.mapiterinit`
+	// amd64:-`.*runtime\.mapclear`
+	for k = range m {
+		delete(m, k)
+	}
+	return k
+}
diff --git a/test/mapclear.go b/test/mapclear.go
new file mode 100644
index 0000000000..a29f30da74
--- /dev/null
+++ b/test/mapclear.go
@@ -0,0 +1,89 @@
+// run
+
+// Copyright 2018 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.
+
+// Ensure that range loops over maps with delete statements
+// have the requisite side-effects.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func checkcleared() {
+	m := make(map[byte]int)
+	m[1] = 1
+	m[2] = 2
+	for k := range m {
+		delete(m, k)
+	}
+	l := len(m)
+	if want := 0; l != want {
+		fmt.Printf("len after map clear = %d want %d\n", l, want)
+		os.Exit(1)
+	}
+
+	m[0] = 0 // To have non empty map and avoid internal map code fast paths.
+	n := 0
+	for range m {
+		n++
+	}
+	if want := 1; n != want {
+		fmt.Printf("number of keys found = %d want %d\n", n, want)
+		os.Exit(1)
+	}
+}
+
+func checkloopvars() {
+	k := 0
+	m := make(map[int]int)
+	m[42] = 0
+	for k = range m {
+		delete(m, k)
+	}
+	if want := 42; k != want {
+		fmt.Printf("var after range with side-effect = %d want %d\n", k, want)
+		os.Exit(1)
+	}
+}
+
+func checksideeffects() {
+	var x int
+	f := func() int {
+		x++
+		return 0
+	}
+	m := make(map[int]int)
+	m[0] = 0
+	m[1] = 1
+	for k := range m {
+		delete(m, k+f())
+	}
+	if want := 2; x != want {
+		fmt.Printf("var after range with side-effect = %d want %d\n", x, want)
+		os.Exit(1)
+	}
+
+	var n int
+	m = make(map[int]int)
+	m[0] = 0
+	m[1] = 1
+	for k := range m {
+		delete(m, k)
+		n++
+	}
+	if want := 2; n != want {
+		fmt.Printf("counter for range with side-effect = %d want %d\n", n, want)
+		os.Exit(1)
+	}
+}
+
+func main() {
+	checkcleared()
+	checkloopvars()
+	checksideeffects()
+}