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>

1 files changed, 29 insertions, 0 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 5a12699472..abdfe41908 100644
--- a/src/reflect/all_test.go
+++ b/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