reflect: use zero buffer to back the Value returned by Zero

In the common case (<1KB types), no allocation is required
by reflect.Zero.

Also use memclr instead of memmove in Set when the source
is known to be zero.

Fixes #33136

Change-Id: Ic66871930fbb53328032e587153ebd12995ccf55
Reviewed-on: https://go-review.googlesource.com/c/go/+/192331
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

src/reflect/all_test.go

@@ -6006,6 +6006,14 @@ func TestReflectMethodTraceback(t *testing.T) {
 	}
 }
 
+func TestSmallZero(t *testing.T) {
+	type T [10]byte
+	typ := TypeOf(T{})
+	if allocs := testing.AllocsPerRun(100, func() { Zero(typ) }); allocs > 0 {
+		t.Errorf("Creating small zero values caused %f allocs, want 0", allocs)
+	}
+}
+
 func TestBigZero(t *testing.T) {
 	const size = 1 << 10
 	var v [size]byte
@@ -6017,6 +6025,27 @@ func TestBigZero(t *testing.T) {
 	}
 }
 
+func TestZeroSet(t *testing.T) {
+	type T [16]byte
+	type S struct {
+		a uint64
+		T T
+		b uint64
+	}
+	v := S{
+		a: 0xaaaaaaaaaaaaaaaa,
+		T: T{9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9},
+		b: 0xbbbbbbbbbbbbbbbb,
+	}
+	ValueOf(&v).Elem().Field(1).Set(Zero(TypeOf(T{})))
+	if v != (S{
+		a: 0xaaaaaaaaaaaaaaaa,
+		b: 0xbbbbbbbbbbbbbbbb,
+	}) {
+		t.Fatalf("Setting a field to a Zero value didn't work")
+	}
+}
+
 func TestFieldByIndexNil(t *testing.T) {
 	type P struct {
 		F int

src/reflect/value.go

@@ -1553,7 +1553,11 @@ func (v Value) Set(x Value) {
 	}
 	x = x.assignTo("reflect.Set", v.typ, target)
 	if x.flag&flagIndir != 0 {
-		typedmemmove(v.typ, v.ptr, x.ptr)
+		if x.ptr == unsafe.Pointer(&zeroVal[0]) {
+			typedmemclr(v.typ, v.ptr)
+		} else {
+			typedmemmove(v.typ, v.ptr, x.ptr)
+		}
 	} else {
 		*(*unsafe.Pointer)(v.ptr) = x.ptr
 	}
@@ -2360,11 +2364,23 @@ func Zero(typ Type) Value {
 	t := typ.(*rtype)
 	fl := flag(t.Kind())
 	if ifaceIndir(t) {
-		return Value{t, unsafe_New(t), fl | flagIndir}
+		var p unsafe.Pointer
+		if t.size <= maxZero {
+			p = unsafe.Pointer(&zeroVal[0])
+		} else {
+			p = unsafe_New(t)
+		}
+		return Value{t, p, fl | flagIndir}
 	}
 	return Value{t, nil, fl}
 }
 
+// must match declarations in runtime/map.go.
+const maxZero = 1024
+
+//go:linkname zeroVal runtime.zeroVal
+var zeroVal [maxZero]byte
+
 // New returns a Value representing a pointer to a new zero value
 // for the specified type. That is, the returned Value's Type is PtrTo(typ).
 func New(typ Type) Value {

src/runtime/map.go

@@ -1380,5 +1380,5 @@ func reflectlite_maplen(h *hmap) int {
 	return h.count
 }
 
-const maxZero = 1024 // must match value in cmd/compile/internal/gc/walk.go:zeroValSize
+const maxZero = 1024 // must match value in reflect/value.go:maxZero cmd/compile/internal/gc/walk.go:zeroValSize
 var zeroVal [maxZero]byte