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://golang.org/cl/160990043

1 files changed, 3 insertions, 4 deletions

diff --git a/src/encoding/gob/encoder.go b/src/encoding/gob/encoder.go
index 4b5dc16c79..a340e47b5e 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.