reflect: add MakeMapWithSize for creating maps with size hint

Providing size hint when creating a map allows avoiding re-allocating
underlying data structure if we know how many elements are going to
be inserted. This can be used for example during decoding maps in
gob.

Fixes #19599

Change-Id: I108035fec29391215d2261a73eaed1310b46bab1
Reviewed-on: https://go-review.googlesource.com/38335Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -5850,6 +5850,24 @@ func TestMapAlloc(t *testing.T) {
 	if allocs > 0.5 {
 		t.Errorf("allocs per map assignment: want 0 got %f", allocs)
 	}
+
+	const size = 1000
+	tmp := 0
+	val := ValueOf(&tmp).Elem()
+	allocs = testing.AllocsPerRun(100, func() {
+		mv := MakeMapWithSize(TypeOf(map[int]int{}), size)
+		// Only adding half of the capacity to not trigger re-allocations due too many overloaded buckets.
+		for i := 0; i < size/2; i++ {
+			val.SetInt(int64(i))
+			mv.SetMapIndex(val, val)
+		}
+	})
+	if allocs > 10 {
+		t.Errorf("allocs per map assignment: want at most 10 got %f", allocs)
+	}
+	// Empirical testing shows that with capacity hint single run will trigger 3 allocations and without 91. I set
+	// the threshold to 10, to not make it overly brittle if something changes in the initial allocation of the
+	// map, but to still catch a regression where we keep re-allocating in the hashmap as new entries are added.
 }
 
 func TestChanAlloc(t *testing.T) {

src/reflect/value.go

@@ -2077,12 +2077,17 @@ func MakeChan(typ Type, buffer int) Value {
 	return Value{typ.common(), ch, flag(Chan)}
 }
 
-// MakeMap creates a new map of the specified type.
+// MakeMap creates a new map with the specified type.
 func MakeMap(typ Type) Value {
+	return MakeMapWithSize(typ, 0)
+}
+
+// MakeMapWithSize creates a new map with the specified type and initial capacity.
+func MakeMapWithSize(typ Type, cap int) Value {
 	if typ.Kind() != Map {
-		panic("reflect.MakeMap of non-map type")
+		panic("reflect.MakeMapWithSize of non-map type")
 	}
-	m := makemap(typ.(*rtype))
+	m := makemap(typ.(*rtype), cap)
 	return Value{typ.common(), m, flag(Map)}
 }
 
@@ -2477,7 +2482,7 @@ func chanrecv(ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, receive
 func chansend(ch unsafe.Pointer, val unsafe.Pointer, nb bool) bool
 
 func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)
-func makemap(t *rtype) (m unsafe.Pointer)
+func makemap(t *rtype, cap int) (m unsafe.Pointer)
 
 //go:noescape
 func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)

src/runtime/hashmap.go

@@ -1139,8 +1139,8 @@ func ismapkey(t *_type) bool {
 // Reflect stubs. Called from ../reflect/asm_*.s
 
 //go:linkname reflect_makemap reflect.makemap
-func reflect_makemap(t *maptype) *hmap {
-	return makemap(t, 0, nil, nil)
+func reflect_makemap(t *maptype, cap int) *hmap {
+	return makemap(t, int64(cap), nil, nil)
 }
 
 //go:linkname reflect_mapaccess reflect.mapaccess