encoding/gob: reduce the amount of memory allocations.

Benchmark results:

benchmark                              old ns/op     new ns/op     delta
BenchmarkEndToEndPipe-4                7547          7294          -3.35%
BenchmarkEndToEndByteBuffer-4          5146          5092          -1.05%
BenchmarkEndToEndSliceByteBuffer-4     552779        439768        -20.44%
BenchmarkEncodeComplex128Slice-4       266370        266184        -0.07%
BenchmarkEncodeFloat64Slice-4          111891        110258        -1.46%
BenchmarkEncodeInt32Slice-4            74482         74080         -0.54%
BenchmarkEncodeStringSlice-4           84404         84279         -0.15%
BenchmarkEncodeInterfaceSlice-4        3942925       3045995       -22.75%
BenchmarkDecodeComplex128Slice-4       451837        415282        -8.09%
BenchmarkDecodeFloat64Slice-4          283584        262558        -7.41%
BenchmarkDecodeInt32Slice-4            246571        237383        -3.73%
BenchmarkDecodeStringSlice-4           734210        479625        -34.67%
BenchmarkDecodeInterfaceSlice-4        4778225       4160935       -12.92%

benchmark                              old allocs     new allocs     delta
BenchmarkEndToEndPipe-4                3              2              -33.33%
BenchmarkEndToEndByteBuffer-4          3              2              -33.33%
BenchmarkEndToEndSliceByteBuffer-4     1002           402            -59.88%
BenchmarkEncodeComplex128Slice-4       1              1              +0.00%
BenchmarkEncodeFloat64Slice-4          1              1              +0.00%
BenchmarkEncodeInt32Slice-4            1              1              +0.00%
BenchmarkEncodeStringSlice-4           1              1              +0.00%
BenchmarkEncodeInterfaceSlice-4        3001           1              -99.97%
BenchmarkDecodeComplex128Slice-4       188            185            -1.60%
BenchmarkDecodeFloat64Slice-4          188            185            -1.60%
BenchmarkDecodeInt32Slice-4            188            185            -1.60%
BenchmarkDecodeStringSlice-4           2188           1185           -45.84%
BenchmarkDecodeInterfaceSlice-4        6197           4194           -32.32%

benchmark                              old bytes     new bytes     delta
BenchmarkEndToEndPipe-4                64            48            -25.00%
BenchmarkEndToEndByteBuffer-4          64            48            -25.00%
BenchmarkEndToEndSliceByteBuffer-4     34551         10554         -69.45%
BenchmarkEncodeComplex128Slice-4       55            55            +0.00%
BenchmarkEncodeFloat64Slice-4          33            33            +0.00%
BenchmarkEncodeInt32Slice-4            32            32            +0.00%
BenchmarkEncodeStringSlice-4           36            36            +0.00%
BenchmarkEncodeInterfaceSlice-4        144555        347           -99.76%
BenchmarkDecodeComplex128Slice-4       28240         28097         -0.51%
BenchmarkDecodeFloat64Slice-4          11840         11697         -1.21%
BenchmarkDecodeInt32Slice-4            10817         10673         -1.33%
BenchmarkDecodeStringSlice-4           56128         39985         -28.76%
BenchmarkDecodeInterfaceSlice-4        132565        100421        -24.25%

Change-Id: Ief7c7706b1f2916486ab7190b81aafbb16b70f1e
Reviewed-on: https://go-review.googlesource.com/13660
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 17 insertions, 2 deletions

diff --git a/src/encoding/gob/encode.go b/src/encoding/gob/encode.go
index f66279f141..96052ef33b 100644
--- a/src/encoding/gob/encode.go
+++ b/src/encoding/gob/encode.go
@@ -10,6 +10,7 @@ import (
 	"encoding"
 	"math"
 	"reflect"
+	"sync"
 )
 
 const uint64Size = 8
@@ -36,6 +37,14 @@ type encBuffer struct {
 	scratch [64]byte
 }
 
+var encBufferPool = sync.Pool{
+	New: func() interface{} {
+		e := new(encBuffer)
+		e.data = e.scratch[0:0]
+		return e
+	},
+}
+
 func (e *encBuffer) WriteByte(c byte) {
 	e.data = append(e.data, c)
 }
@@ -58,7 +67,11 @@ func (e *encBuffer) Bytes() []byte {
 }
 
 func (e *encBuffer) Reset() {
-	e.data = e.data[0:0]
+	if len(e.data) >= tooBig {
+		e.data = e.scratch[0:0]
+	} else {
+		e.data = e.data[0:0]
+	}
 }
 
 func (enc *Encoder) newEncoderState(b *encBuffer) *encoderState {
@@ -407,7 +420,7 @@ func (enc *Encoder) encodeInterface(b *encBuffer, 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(encBuffer)
+	data := encBufferPool.Get().(*encBuffer)
 	data.Write(spaceForLength)
 	enc.encode(data, elem, ut)
 	if enc.err != nil {
@@ -415,6 +428,8 @@ func (enc *Encoder) encodeInterface(b *encBuffer, iv reflect.Value) {
 	}
 	enc.popWriter()
 	enc.writeMessage(b, data)
+	data.Reset()
+	encBufferPool.Put(data)
 	if enc.err != nil {
 		error_(enc.err)
 	}