encoding/json: allocate less in NewEncoder

The *Encoder is almost always garbage. It doesn't need an
encodeState inside of it (and its bytes.Buffer), since it's
only needed locally inside of Encode.

benchmark                 old ns/op    new ns/op    delta
BenchmarkEncoderEncode         2562         2553   -0.35%

benchmark                 old bytes    new bytes    delta
BenchmarkEncoderEncode          283          102  -63.96%

R=r
CC=gobot, golang-dev
https://golang.org/cl/9365044

src/pkg/encoding/json/encode.go

@@ -227,6 +227,26 @@ type encodeState struct {
 	scratch      [64]byte
 }
 
+// TODO(bradfitz): use a sync.Cache here
+var encodeStatePool = make(chan *encodeState, 8)
+
+func newEncodeState() *encodeState {
+	select {
+	case e := <-encodeStatePool:
+		e.Reset()
+		return e
+	default:
+		return new(encodeState)
+	}
+}
+
+func putEncodeState(e *encodeState) {
+	select {
+	case encodeStatePool <- e:
+	default:
+	}
+}
+
 func (e *encodeState) marshal(v interface{}) (err error) {
 	defer func() {
 		if r := recover(); r != nil {

src/pkg/encoding/json/stream.go

@@ -156,8 +156,8 @@ func (enc *Encoder) Encode(v interface{}) error {
 	if enc.err != nil {
 		return enc.err
 	}
-	enc.e.Reset()
-	err := enc.e.marshal(v)
+	e := newEncodeState()
+	err := e.marshal(v)
 	if err != nil {
 		return err
 	}
@@ -168,11 +168,12 @@ func (enc *Encoder) Encode(v interface{}) error {
 	// is required if the encoded value was a number,
 	// so that the reader knows there aren't more
 	// digits coming.
-	enc.e.WriteByte('\n')
+	e.WriteByte('\n')
 
-	if _, err = enc.w.Write(enc.e.Bytes()); err != nil {
+	if _, err = enc.w.Write(e.Bytes()); err != nil {
 		enc.err = err
 	}
+	putEncodeState(e)
 	return err
 }
 

src/pkg/encoding/json/stream_test.go

@@ -191,3 +191,16 @@ func TestBlocking(t *testing.T) {
 		w.Close()
 	}
 }
+
+func BenchmarkEncoderEncode(b *testing.B) {
+	b.ReportAllocs()
+	type T struct {
+		X, Y string
+	}
+	v := &T{"foo", "bar"}
+	for i := 0; i < b.N; i++ {
+		if err := NewEncoder(ioutil.Discard).Encode(v); err != nil {
+			b.Fatal(err)
+		}
+	}
+}