encoding/gob: use simple append-only buffer for encoding

Bytes buffers have more API and are a little slower. Since appending
is a key part of the path in encode, using a faster implementation
speeds things up measurably.
The couple of positive swings are likely garbage-collection related
since memory allocation looks different in the benchmark now.
I am not concerned by them.

benchmark                            old ns/op     new ns/op     delta
BenchmarkEndToEndPipe                6620          6388          -3.50%
BenchmarkEndToEndByteBuffer          3548          3600          +1.47%
BenchmarkEndToEndSliceByteBuffer     336678        367980        +9.30%
BenchmarkEncodeComplex128Slice       78199         71297         -8.83%
BenchmarkEncodeFloat64Slice          37731         32258         -14.51%
BenchmarkEncodeInt32Slice            26780         22977         -14.20%
BenchmarkEncodeStringSlice           35882         26492         -26.17%
BenchmarkDecodeComplex128Slice       194819        185126        -4.98%
BenchmarkDecodeFloat64Slice          120538        120102        -0.36%
BenchmarkDecodeInt32Slice            106442        107275        +0.78%
BenchmarkDecodeStringSlice           272902        269866        -1.11%

LGTM=ruiu
R=golang-codereviews, ruiu
CC=golang-codereviews
https://codereview.appspot.com/160990043

4 files changed, 59 insertions, 35 deletions

diff --git a/src/encoding/gob/codec_test.go b/src/encoding/gob/codec_test.go
index 4f17a2893..b3749e352 100644
--- a/src/encoding/gob/codec_test.go
+++ b/src/encoding/gob/codec_test.go
@@ -53,7 +53,7 @@ func testError(t *testing.T) {
 // Test basic encode/decode routines for unsigned integers
 func TestUintCodec(t *testing.T) {
 	defer testError(t)
-	b := new(bytes.Buffer)
+	b := new(encBuffer)
 	encState := newEncoderState(b)
 	for _, tt := range encodeT {
 		b.Reset()
@@ -62,10 +62,10 @@ func TestUintCodec(t *testing.T) {
 			t.Errorf("encodeUint: %#x encode: expected % x got % x", tt.x, tt.b, b.Bytes())
 		}
 	}
-	decState := newDecodeState(b)
 	for u := uint64(0); ; u = (u + 1) * 7 {
 		b.Reset()
 		encState.encodeUint(u)
+		decState := newDecodeState(bytes.NewBuffer(b.Bytes()))
 		v := decState.decodeUint()
 		if u != v {
 			t.Errorf("Encode/Decode: sent %#x received %#x", u, v)
@@ -78,10 +78,10 @@ func TestUintCodec(t *testing.T) {
 
 func verifyInt(i int64, t *testing.T) {
 	defer testError(t)
-	var b = new(bytes.Buffer)
+	var b = new(encBuffer)
 	encState := newEncoderState(b)
 	encState.encodeInt(i)
-	decState := newDecodeState(b)
+	decState := newDecodeState(bytes.NewBuffer(b.Bytes()))
 	decState.buf = make([]byte, 8)
 	j := decState.decodeInt()
 	if i != j {
@@ -125,7 +125,7 @@ func newDecodeState(buf *bytes.Buffer) *decoderState {
 	return d
 }
 
-func newEncoderState(b *bytes.Buffer) *encoderState {
+func newEncoderState(b *encBuffer) *encoderState {
 	b.Reset()
 	state := &encoderState{enc: nil, b: b}
 	state.fieldnum = -1
@@ -135,7 +135,7 @@ func newEncoderState(b *bytes.Buffer) *encoderState {
 // Test instruction execution for encoding.
 // Do not run the machine yet; instead do individual instructions crafted by hand.
 func TestScalarEncInstructions(t *testing.T) {
-	var b = new(bytes.Buffer)
+	var b = new(encBuffer)
 
 	// bool
 	{
diff --git a/src/encoding/gob/decoder.go b/src/encoding/gob/decoder.go
index dcad7a0e4..fe1494100 100644
--- a/src/encoding/gob/decoder.go
+++ b/src/encoding/gob/decoder.go
@@ -212,7 +212,7 @@ func (dec *Decoder) Decode(e interface{}) error {
 // Otherwise, it stores the value into v.  In that case, v must represent
 // a non-nil pointer to data or be an assignable reflect.Value (v.CanSet())
 // If the input is at EOF, DecodeValue returns io.EOF and
-// does not modify e.
+// does not modify v.
 func (dec *Decoder) DecodeValue(v reflect.Value) error {
 	if v.IsValid() {
 		if v.Kind() == reflect.Ptr && !v.IsNil() {
diff --git a/src/encoding/gob/encode.go b/src/encoding/gob/encode.go
index 3da848c85..f66279f14 100644
--- a/src/encoding/gob/encode.go
+++ b/src/encoding/gob/encode.go
@@ -7,7 +7,6 @@
 package gob
 
 import (
-	"bytes"
 	"encoding"
 	"math"
 	"reflect"
@@ -23,14 +22,46 @@ type encHelper func(state *encoderState, v reflect.Value) bool
 // 0 terminates the structure.
 type encoderState struct {
 	enc      *Encoder
-	b        *bytes.Buffer
+	b        *encBuffer
 	sendZero bool                 // encoding an array element or map key/value pair; send zero values
 	fieldnum int                  // the last field number written.
 	buf      [1 + uint64Size]byte // buffer used by the encoder; here to avoid allocation.
 	next     *encoderState        // for free list
 }
 
-func (enc *Encoder) newEncoderState(b *bytes.Buffer) *encoderState {
+// encBuffer is an extremely simple, fast implementation of a write-only byte buffer.
+// It never returns a non-nil error, but Write returns an error value so it matches io.Writer.
+type encBuffer struct {
+	data    []byte
+	scratch [64]byte
+}
+
+func (e *encBuffer) WriteByte(c byte) {
+	e.data = append(e.data, c)
+}
+
+func (e *encBuffer) Write(p []byte) (int, error) {
+	e.data = append(e.data, p...)
+	return len(p), nil
+}
+
+func (e *encBuffer) WriteString(s string) {
+	e.data = append(e.data, s...)
+}
+
+func (e *encBuffer) Len() int {
+	return len(e.data)
+}
+
+func (e *encBuffer) Bytes() []byte {
+	return e.data
+}
+
+func (e *encBuffer) Reset() {
+	e.data = e.data[0:0]
+}
+
+func (enc *Encoder) newEncoderState(b *encBuffer) *encoderState {
 	e := enc.freeList
 	if e == nil {
 		e = new(encoderState)
@@ -41,6 +72,9 @@ func (enc *Encoder) newEncoderState(b *bytes.Buffer) *encoderState {
 	e.sendZero = false
 	e.fieldnum = 0
 	e.b = b
+	if len(b.data) == 0 {
+		b.data = b.scratch[0:0]
+	}
 	return e
 }
 
@@ -57,10 +91,7 @@ func (enc *Encoder) freeEncoderState(e *encoderState) {
 // encodeUint writes an encoded unsigned integer to state.b.
 func (state *encoderState) encodeUint(x uint64) {
 	if x <= 0x7F {
-		err := state.b.WriteByte(uint8(x))
-		if err != nil {
-			error_(err)
-		}
+		state.b.WriteByte(uint8(x))
 		return
 	}
 	i := uint64Size
@@ -70,10 +101,7 @@ func (state *encoderState) encodeUint(x uint64) {
 		i--
 	}
 	state.buf[i] = uint8(i - uint64Size) // = loop count, negated
-	_, err := state.b.Write(state.buf[i : uint64Size+1])
-	if err != nil {
-		error_(err)
-	}
+	state.b.Write(state.buf[i : uint64Size+1])
 }
 
 // encodeInt writes an encoded signed integer to state.w.
@@ -251,7 +279,7 @@ func valid(v reflect.Value) bool {
 }
 
 // encodeSingle encodes a single top-level non-struct value.
-func (enc *Encoder) encodeSingle(b *bytes.Buffer, engine *encEngine, value reflect.Value) {
+func (enc *Encoder) encodeSingle(b *encBuffer, engine *encEngine, value reflect.Value) {
 	state := enc.newEncoderState(b)
 	defer enc.freeEncoderState(state)
 	state.fieldnum = singletonField
@@ -268,7 +296,7 @@ func (enc *Encoder) encodeSingle(b *bytes.Buffer, engine *encEngine, value refle
 }
 
 // encodeStruct encodes a single struct value.
-func (enc *Encoder) encodeStruct(b *bytes.Buffer, engine *encEngine, value reflect.Value) {
+func (enc *Encoder) encodeStruct(b *encBuffer, engine *encEngine, value reflect.Value) {
 	if !valid(value) {
 		return
 	}
@@ -295,7 +323,7 @@ func (enc *Encoder) encodeStruct(b *bytes.Buffer, engine *encEngine, value refle
 }
 
 // encodeArray encodes an array.
-func (enc *Encoder) encodeArray(b *bytes.Buffer, value reflect.Value, op encOp, elemIndir int, length int, helper encHelper) {
+func (enc *Encoder) encodeArray(b *encBuffer, value reflect.Value, op encOp, elemIndir int, length int, helper encHelper) {
 	state := enc.newEncoderState(b)
 	defer enc.freeEncoderState(state)
 	state.fieldnum = -1
@@ -329,7 +357,7 @@ func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir in
 }
 
 // encodeMap encodes a map as unsigned count followed by key:value pairs.
-func (enc *Encoder) encodeMap(b *bytes.Buffer, mv reflect.Value, keyOp, elemOp encOp, keyIndir, elemIndir int) {
+func (enc *Encoder) encodeMap(b *encBuffer, mv reflect.Value, keyOp, elemOp encOp, keyIndir, elemIndir int) {
 	state := enc.newEncoderState(b)
 	state.fieldnum = -1
 	state.sendZero = true
@@ -347,7 +375,7 @@ func (enc *Encoder) encodeMap(b *bytes.Buffer, mv reflect.Value, keyOp, elemOp e
 // by the type identifier (which might require defining that type right now), followed
 // by the concrete value.  A nil value gets sent as the empty string for the name,
 // followed by no value.
-func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
+func (enc *Encoder) encodeInterface(b *encBuffer, iv reflect.Value) {
 	// Gobs can encode nil interface values but not typed interface
 	// values holding nil pointers, since nil pointers point to no value.
 	elem := iv.Elem()
@@ -371,10 +399,7 @@ func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
 	}
 	// Send the name.
 	state.encodeUint(uint64(len(name)))
-	_, err := state.b.WriteString(name)
-	if err != nil {
-		error_(err)
-	}
+	state.b.WriteString(name)
 	// Define the type id if necessary.
 	enc.sendTypeDescriptor(enc.writer(), state, ut)
 	// Send the type id.
@@ -382,7 +407,7 @@ func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
 	// Encode the value into a new buffer.  Any nested type definitions
 	// should be written to b, before the encoded value.
 	enc.pushWriter(b)
-	data := new(bytes.Buffer)
+	data := new(encBuffer)
 	data.Write(spaceForLength)
 	enc.encode(data, elem, ut)
 	if enc.err != nil {
@@ -391,7 +416,7 @@ func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
 	enc.popWriter()
 	enc.writeMessage(b, data)
 	if enc.err != nil {
-		error_(err)
+		error_(enc.err)
 	}
 	enc.freeEncoderState(state)
 }
@@ -433,7 +458,7 @@ func isZero(val reflect.Value) bool {
 
 // encGobEncoder encodes a value that implements the GobEncoder interface.
 // The data is sent as a byte array.
-func (enc *Encoder) encodeGobEncoder(b *bytes.Buffer, ut *userTypeInfo, v reflect.Value) {
+func (enc *Encoder) encodeGobEncoder(b *encBuffer, ut *userTypeInfo, v reflect.Value) {
 	// TODO: should we catch panics from the called method?
 
 	var data []byte
@@ -653,7 +678,7 @@ func buildEncEngine(info *typeInfo, ut *userTypeInfo, building map[*typeInfo]boo
 	return enc
 }
 
-func (enc *Encoder) encode(b *bytes.Buffer, value reflect.Value, ut *userTypeInfo) {
+func (enc *Encoder) encode(b *encBuffer, value reflect.Value, ut *userTypeInfo) {
 	defer catchError(&enc.err)
 	engine := getEncEngine(ut, nil)
 	indir := ut.indir
diff --git a/src/encoding/gob/encoder.go b/src/encoding/gob/encoder.go
index 4b5dc16c7..a340e47b5 100644
--- a/src/encoding/gob/encoder.go
+++ b/src/encoding/gob/encoder.go
@@ -5,7 +5,6 @@
 package gob
 
 import (
-	"bytes"
 	"io"
 	"reflect"
 	"sync"
@@ -19,7 +18,7 @@ type Encoder struct {
 	sent       map[reflect.Type]typeId // which types we've already sent
 	countState *encoderState           // stage for writing counts
 	freeList   *encoderState           // list of free encoderStates; avoids reallocation
-	byteBuf    bytes.Buffer            // buffer for top-level encoderState
+	byteBuf    encBuffer               // buffer for top-level encoderState
 	err        error
 }
 
@@ -34,7 +33,7 @@ func NewEncoder(w io.Writer) *Encoder {
 	enc := new(Encoder)
 	enc.w = []io.Writer{w}
 	enc.sent = make(map[reflect.Type]typeId)
-	enc.countState = enc.newEncoderState(new(bytes.Buffer))
+	enc.countState = enc.newEncoderState(new(encBuffer))
 	return enc
 }
 
@@ -60,7 +59,7 @@ func (enc *Encoder) setError(err error) {
 }
 
 // writeMessage sends the data item preceded by a unsigned count of its length.
-func (enc *Encoder) writeMessage(w io.Writer, b *bytes.Buffer) {
+func (enc *Encoder) writeMessage(w io.Writer, b *encBuffer) {
 	// Space has been reserved for the length at the head of the message.
 	// This is a little dirty: we grab the slice from the bytes.Buffer and massage
 	// it by hand.