encoding/gob: add custom decoder buffer for performance

As we did with encoding, provide a trivial byte reader for
faster decoding. We can also reduce some of the copying
by doing the allocation all at once using a slightly different
interface from byte buffers.

benchmark                            old ns/op     new ns/op     delta
BenchmarkEndToEndPipe                13368         12902         -3.49%
BenchmarkEndToEndByteBuffer          5969          5642          -5.48%
BenchmarkEndToEndSliceByteBuffer     479485        470798        -1.81%
BenchmarkEncodeComplex128Slice       92367         92201         -0.18%
BenchmarkEncodeFloat64Slice          39990         38960         -2.58%
BenchmarkEncodeInt32Slice            30510         27938         -8.43%
BenchmarkEncodeStringSlice           33753         33365         -1.15%
BenchmarkDecodeComplex128Slice       232278        196704        -15.32%
BenchmarkDecodeFloat64Slice          150258        128191        -14.69%
BenchmarkDecodeInt32Slice            133806        115748        -13.50%
BenchmarkDecodeStringSlice           335117        300534        -10.32%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/154360049

3 files changed, 78 insertions, 39 deletions

diff --git a/src/encoding/gob/codec_test.go b/src/encoding/gob/codec_test.go
index b3749e352..56a7298fa 100644
--- a/src/encoding/gob/codec_test.go
+++ b/src/encoding/gob/codec_test.go
@@ -50,6 +50,12 @@ func testError(t *testing.T) {
 	return
 }
 
+func newDecBuffer(data []byte) *decBuffer {
+	return &decBuffer{
+		data: data,
+	}
+}
+
 // Test basic encode/decode routines for unsigned integers
 func TestUintCodec(t *testing.T) {
 	defer testError(t)
@@ -65,7 +71,7 @@ func TestUintCodec(t *testing.T) {
 	for u := uint64(0); ; u = (u + 1) * 7 {
 		b.Reset()
 		encState.encodeUint(u)
-		decState := newDecodeState(bytes.NewBuffer(b.Bytes()))
+		decState := newDecodeState(newDecBuffer(b.Bytes()))
 		v := decState.decodeUint()
 		if u != v {
 			t.Errorf("Encode/Decode: sent %#x received %#x", u, v)
@@ -81,7 +87,7 @@ func verifyInt(i int64, t *testing.T) {
 	var b = new(encBuffer)
 	encState := newEncoderState(b)
 	encState.encodeInt(i)
-	decState := newDecodeState(bytes.NewBuffer(b.Bytes()))
+	decState := newDecodeState(newDecBuffer(b.Bytes()))
 	decState.buf = make([]byte, 8)
 	j := decState.decodeInt()
 	if i != j {
@@ -118,7 +124,7 @@ var complexResult = []byte{0x07, 0xFE, 0x31, 0x40, 0xFE, 0x33, 0x40}
 // The result of encoding "hello" with field number 7
 var bytesResult = []byte{0x07, 0x05, 'h', 'e', 'l', 'l', 'o'}
 
-func newDecodeState(buf *bytes.Buffer) *decoderState {
+func newDecodeState(buf *decBuffer) *decoderState {
 	d := new(decoderState)
 	d.b = buf
 	d.buf = make([]byte, uint64Size)
@@ -328,7 +334,7 @@ func execDec(typ string, instr *decInstr, state *decoderState, t *testing.T, val
 }
 
 func newDecodeStateFromData(data []byte) *decoderState {
-	b := bytes.NewBuffer(data)
+	b := newDecBuffer(data)
 	state := newDecodeState(b)
 	state.fieldnum = -1
 	return state
diff --git a/src/encoding/gob/decode.go b/src/encoding/gob/decode.go
index f44838e4c..a5bef9314 100644
--- a/src/encoding/gob/decode.go
+++ b/src/encoding/gob/decode.go
@@ -7,7 +7,6 @@
 package gob
 
 import (
-	"bytes"
 	"encoding"
 	"errors"
 	"io"
@@ -29,15 +28,71 @@ type decoderState struct {
 	dec *Decoder
 	// The buffer is stored with an extra indirection because it may be replaced
 	// if we load a type during decode (when reading an interface value).
-	b        *bytes.Buffer
+	b        *decBuffer
 	fieldnum int // the last field number read.
 	buf      []byte
 	next     *decoderState // for free list
 }
 
+// decBuffer is an extremely simple, fast implementation of a read-only byte buffer.
+// It is initialized by calling Size and then copying the data into the slice returned by Bytes().
+type decBuffer struct {
+	data   []byte
+	offset int // Read offset.
+}
+
+func (d *decBuffer) Read(p []byte) (int, error) {
+	n := copy(p, d.data[d.offset:])
+	if n == 0 && len(p) != 0 {
+		return 0, io.EOF
+	}
+	d.offset += n
+	return n, nil
+}
+
+func (d *decBuffer) Drop(n int) {
+	if n > d.Len() {
+		panic("drop")
+	}
+	d.offset += n
+}
+
+// Size grows the buffer to exactly n bytes, so d.Bytes() will
+// return a slice of length n. Existing data is first discarded.
+func (d *decBuffer) Size(n int) {
+	d.Reset()
+	if cap(d.data) < n {
+		d.data = make([]byte, n)
+	} else {
+		d.data = d.data[0:n]
+	}
+}
+
+func (d *decBuffer) ReadByte() (byte, error) {
+	if d.offset >= len(d.data) {
+		return 0, io.EOF
+	}
+	c := d.data[d.offset]
+	d.offset++
+	return c, nil
+}
+
+func (d *decBuffer) Len() int {
+	return len(d.data) - d.offset
+}
+
+func (d *decBuffer) Bytes() []byte {
+	return d.data[d.offset:]
+}
+
+func (d *decBuffer) Reset() {
+	d.data = d.data[0:0]
+	d.offset = 0
+}
+
 // We pass the bytes.Buffer separately for easier testing of the infrastructure
 // without requiring a full Decoder.
-func (dec *Decoder) newDecoderState(buf *bytes.Buffer) *decoderState {
+func (dec *Decoder) newDecoderState(buf *decBuffer) *decoderState {
 	d := dec.freeList
 	if d == nil {
 		d = new(decoderState)
@@ -633,7 +688,7 @@ func (dec *Decoder) ignoreInterface(state *decoderState) {
 		error_(dec.err)
 	}
 	// At this point, the decoder buffer contains a delimited value. Just toss it.
-	state.b.Next(int(state.decodeUint()))
+	state.b.Drop(int(state.decodeUint()))
 }
 
 // decodeGobDecoder decodes something implementing the GobDecoder interface.
diff --git a/src/encoding/gob/decoder.go b/src/encoding/gob/decoder.go
index fe1494100..c453e9ba3 100644
--- a/src/encoding/gob/decoder.go
+++ b/src/encoding/gob/decoder.go
@@ -6,7 +6,6 @@ package gob
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"io"
 	"reflect"
@@ -23,13 +22,12 @@ const tooBig = 1 << 30
 type Decoder struct {
 	mutex        sync.Mutex                              // each item must be received atomically
 	r            io.Reader                               // source of the data
-	buf          bytes.Buffer                            // buffer for more efficient i/o from r
+	buf          decBuffer                               // buffer for more efficient i/o from r
 	wireType     map[typeId]*wireType                    // map from remote ID to local description
 	decoderCache map[reflect.Type]map[typeId]**decEngine // cache of compiled engines
 	ignorerCache map[typeId]**decEngine                  // ditto for ignored objects
 	freeList     *decoderState                           // list of free decoderStates; avoids reallocation
 	countBuf     []byte                                  // used for decoding integers while parsing messages
-	tmp          []byte                                  // temporary storage for i/o; saves reallocating
 	err          error
 }
 
@@ -90,37 +88,17 @@ func (dec *Decoder) recvMessage() bool {
 
 // readMessage reads the next nbytes bytes from the input.
 func (dec *Decoder) readMessage(nbytes int) {
-	// Allocate the dec.tmp buffer, up to 10KB.
-	const maxBuf = 10 * 1024
-	nTmp := nbytes
-	if nTmp > maxBuf {
-		nTmp = maxBuf
+	if dec.buf.Len() != 0 {
+		// The buffer should always be empty now.
+		panic("non-empty decoder buffer")
 	}
-	if cap(dec.tmp) < nTmp {
-		nAlloc := nTmp + 100 // A little extra for growth.
-		if nAlloc > maxBuf {
-			nAlloc = maxBuf
-		}
-		dec.tmp = make([]byte, nAlloc)
-	}
-	dec.tmp = dec.tmp[:nTmp]
-
 	// Read the data
-	dec.buf.Grow(nbytes)
-	for nbytes > 0 {
-		if nbytes < nTmp {
-			dec.tmp = dec.tmp[:nbytes]
-		}
-		var nRead int
-		nRead, dec.err = io.ReadFull(dec.r, dec.tmp)
-		if dec.err != nil {
-			if dec.err == io.EOF {
-				dec.err = io.ErrUnexpectedEOF
-			}
-			return
+	dec.buf.Size(nbytes)
+	_, dec.err = io.ReadFull(dec.r, dec.buf.Bytes())
+	if dec.err != nil {
+		if dec.err == io.EOF {
+			dec.err = io.ErrUnexpectedEOF
 		}
-		dec.buf.Write(dec.tmp)
-		nbytes -= nRead
 	}
 }