encoding/gob: custom array/slice decoders

Use go generate to write better loops for decoding arrays,
just as we did for encoding. It doesn't help as much,
relatively speaking, but it's still noticeable.

benchmark                          old ns/op     new ns/op     delta
BenchmarkDecodeComplex128Slice     202348        184529        -8.81%
BenchmarkDecodeFloat64Slice        135800        120979        -10.91%
BenchmarkDecodeInt32Slice          121200        105149        -13.24%
BenchmarkDecodeStringSlice         288129        278214        -3.44%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/154420044

7 files changed, 874 insertions, 29 deletions

diff --git a/src/encoding/gob/dec_helpers.go b/src/encoding/gob/dec_helpers.go
new file mode 100644
index 000000000..ae59ef0ac
--- /dev/null
+++ b/src/encoding/gob/dec_helpers.go
@@ -0,0 +1,468 @@
+// Created by decgen --output dec_helpers.go; DO NOT EDIT
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gob
+
+import (
+	"math"
+	"reflect"
+)
+
+var decArrayHelper = map[reflect.Kind]decHelper{
+	reflect.Bool:       decBoolArray,
+	reflect.Complex64:  decComplex64Array,
+	reflect.Complex128: decComplex128Array,
+	reflect.Float32:    decFloat32Array,
+	reflect.Float64:    decFloat64Array,
+	reflect.Int:        decIntArray,
+	reflect.Int16:      decInt16Array,
+	reflect.Int32:      decInt32Array,
+	reflect.Int64:      decInt64Array,
+	reflect.Int8:       decInt8Array,
+	reflect.String:     decStringArray,
+	reflect.Uint:       decUintArray,
+	reflect.Uint16:     decUint16Array,
+	reflect.Uint32:     decUint32Array,
+	reflect.Uint64:     decUint64Array,
+	reflect.Uintptr:    decUintptrArray,
+}
+
+var decSliceHelper = map[reflect.Kind]decHelper{
+	reflect.Bool:       decBoolSlice,
+	reflect.Complex64:  decComplex64Slice,
+	reflect.Complex128: decComplex128Slice,
+	reflect.Float32:    decFloat32Slice,
+	reflect.Float64:    decFloat64Slice,
+	reflect.Int:        decIntSlice,
+	reflect.Int16:      decInt16Slice,
+	reflect.Int32:      decInt32Slice,
+	reflect.Int64:      decInt64Slice,
+	reflect.Int8:       decInt8Slice,
+	reflect.String:     decStringSlice,
+	reflect.Uint:       decUintSlice,
+	reflect.Uint16:     decUint16Slice,
+	reflect.Uint32:     decUint32Slice,
+	reflect.Uint64:     decUint64Slice,
+	reflect.Uintptr:    decUintptrSlice,
+}
+
+func decBoolArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decBoolSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decBoolSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]bool)
+	if !ok {
+		// It is kind bool but not type bool. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding bool array or slice: length exceeds input size (%!d(string=Bool) elements)", length)
+		}
+		slice[i] = state.decodeUint() != 0
+	}
+	return true
+}
+
+func decComplex64Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decComplex64Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decComplex64Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]complex64)
+	if !ok {
+		// It is kind complex64 but not type complex64. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding complex64 array or slice: length exceeds input size (%!d(string=Complex64) elements)", length)
+		}
+		real := float32FromBits(state.decodeUint(), ovfl)
+		imag := float32FromBits(state.decodeUint(), ovfl)
+		slice[i] = complex(float32(real), float32(imag))
+	}
+	return true
+}
+
+func decComplex128Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decComplex128Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decComplex128Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]complex128)
+	if !ok {
+		// It is kind complex128 but not type complex128. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding complex128 array or slice: length exceeds input size (%!d(string=Complex128) elements)", length)
+		}
+		real := float64FromBits(state.decodeUint())
+		imag := float64FromBits(state.decodeUint())
+		slice[i] = complex(real, imag)
+	}
+	return true
+}
+
+func decFloat32Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decFloat32Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decFloat32Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]float32)
+	if !ok {
+		// It is kind float32 but not type float32. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding float32 array or slice: length exceeds input size (%!d(string=Float32) elements)", length)
+		}
+		slice[i] = float32(float32FromBits(state.decodeUint(), ovfl))
+	}
+	return true
+}
+
+func decFloat64Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decFloat64Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decFloat64Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]float64)
+	if !ok {
+		// It is kind float64 but not type float64. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding float64 array or slice: length exceeds input size (%!d(string=Float64) elements)", length)
+		}
+		slice[i] = float64FromBits(state.decodeUint())
+	}
+	return true
+}
+
+func decIntArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decIntSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decIntSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]int)
+	if !ok {
+		// It is kind int but not type int. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding int array or slice: length exceeds input size (%!d(string=Int) elements)", length)
+		}
+		x := state.decodeInt()
+		// MinInt and MaxInt
+		if x < ^int64(^uint(0)>>1) || int64(^uint(0)>>1) < x {
+			error_(ovfl)
+		}
+		slice[i] = int(x)
+	}
+	return true
+}
+
+func decInt16Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decInt16Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decInt16Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]int16)
+	if !ok {
+		// It is kind int16 but not type int16. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding int16 array or slice: length exceeds input size (%!d(string=Int16) elements)", length)
+		}
+		x := state.decodeInt()
+		if x < math.MinInt16 || math.MaxInt16 < x {
+			error_(ovfl)
+		}
+		slice[i] = int16(x)
+	}
+	return true
+}
+
+func decInt32Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decInt32Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decInt32Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]int32)
+	if !ok {
+		// It is kind int32 but not type int32. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding int32 array or slice: length exceeds input size (%!d(string=Int32) elements)", length)
+		}
+		x := state.decodeInt()
+		if x < math.MinInt32 || math.MaxInt32 < x {
+			error_(ovfl)
+		}
+		slice[i] = int32(x)
+	}
+	return true
+}
+
+func decInt64Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decInt64Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decInt64Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]int64)
+	if !ok {
+		// It is kind int64 but not type int64. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding int64 array or slice: length exceeds input size (%!d(string=Int64) elements)", length)
+		}
+		slice[i] = state.decodeInt()
+	}
+	return true
+}
+
+func decInt8Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decInt8Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decInt8Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]int8)
+	if !ok {
+		// It is kind int8 but not type int8. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding int8 array or slice: length exceeds input size (%!d(string=Int8) elements)", length)
+		}
+		x := state.decodeInt()
+		if x < math.MinInt8 || math.MaxInt8 < x {
+			error_(ovfl)
+		}
+		slice[i] = int8(x)
+	}
+	return true
+}
+
+func decStringArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decStringSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decStringSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]string)
+	if !ok {
+		// It is kind string but not type string. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding string array or slice: length exceeds input size (%!d(string=String) elements)", length)
+		}
+		u := state.decodeUint()
+		n := int(u)
+		if n < 0 || uint64(n) != u || n > state.b.Len() {
+			errorf("length of string exceeds input size (%d bytes)", u)
+		}
+		if n > state.b.Len() {
+			errorf("string data too long for buffer: %d", n)
+		}
+		// Read the data.
+		data := make([]byte, n)
+		if _, err := state.b.Read(data); err != nil {
+			errorf("error decoding string: %s", err)
+		}
+		slice[i] = string(data)
+	}
+	return true
+}
+
+func decUintArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decUintSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decUintSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]uint)
+	if !ok {
+		// It is kind uint but not type uint. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding uint array or slice: length exceeds input size (%!d(string=Uint) elements)", length)
+		}
+		x := state.decodeUint()
+		/*TODO if math.MaxUint32 < x {
+			error_(ovfl)
+		}*/
+		slice[i] = uint(x)
+	}
+	return true
+}
+
+func decUint16Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decUint16Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decUint16Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]uint16)
+	if !ok {
+		// It is kind uint16 but not type uint16. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding uint16 array or slice: length exceeds input size (%!d(string=Uint16) elements)", length)
+		}
+		x := state.decodeUint()
+		if math.MaxUint16 < x {
+			error_(ovfl)
+		}
+		slice[i] = uint16(x)
+	}
+	return true
+}
+
+func decUint32Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decUint32Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decUint32Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]uint32)
+	if !ok {
+		// It is kind uint32 but not type uint32. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding uint32 array or slice: length exceeds input size (%!d(string=Uint32) elements)", length)
+		}
+		x := state.decodeUint()
+		if math.MaxUint32 < x {
+			error_(ovfl)
+		}
+		slice[i] = uint32(x)
+	}
+	return true
+}
+
+func decUint64Array(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decUint64Slice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decUint64Slice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]uint64)
+	if !ok {
+		// It is kind uint64 but not type uint64. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding uint64 array or slice: length exceeds input size (%!d(string=Uint64) elements)", length)
+		}
+		slice[i] = state.decodeUint()
+	}
+	return true
+}
+
+func decUintptrArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return decUintptrSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+
+func decUintptrSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]uintptr)
+	if !ok {
+		// It is kind uintptr but not type uintptr. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding uintptr array or slice: length exceeds input size (%!d(string=Uintptr) elements)", length)
+		}
+		x := state.decodeUint()
+		if uint64(^uintptr(0)) < x {
+			error_(ovfl)
+		}
+		slice[i] = uintptr(x)
+	}
+	return true
+}
diff --git a/src/encoding/gob/decgen.go b/src/encoding/gob/decgen.go
new file mode 100644
index 000000000..1cd1fb043
--- /dev/null
+++ b/src/encoding/gob/decgen.go
@@ -0,0 +1,240 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+// encgen writes the helper functions for encoding. Intended to be
+// used with go generate; see the invocation in encode.go.
+
+// TODO: We could do more by being unsafe. Add a -unsafe flag?
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/format"
+	"log"
+	"os"
+)
+
+var output = flag.String("output", "dec_helpers.go", "file name to write")
+
+type Type struct {
+	lower   string
+	upper   string
+	decoder string
+}
+
+var types = []Type{
+	{
+		"bool",
+		"Bool",
+		`slice[i] = state.decodeUint() != 0`,
+	},
+	{
+		"complex64",
+		"Complex64",
+		`real := float32FromBits(state.decodeUint(), ovfl)
+		imag := float32FromBits(state.decodeUint(), ovfl)
+		slice[i] = complex(float32(real), float32(imag))`,
+	},
+	{
+		"complex128",
+		"Complex128",
+		`real := float64FromBits(state.decodeUint())
+		imag := float64FromBits(state.decodeUint())
+		slice[i] = complex(real, imag)`,
+	},
+	{
+		"float32",
+		"Float32",
+		`slice[i] = float32(float32FromBits(state.decodeUint(), ovfl))`,
+	},
+	{
+		"float64",
+		"Float64",
+		`slice[i] = float64FromBits(state.decodeUint())`,
+	},
+	{
+		"int",
+		"Int",
+		`x := state.decodeInt()
+		// MinInt and MaxInt
+		if x < ^int64(^uint(0)>>1) || int64(^uint(0)>>1) < x {
+			error_(ovfl)
+		}
+		slice[i] = int(x)`,
+	},
+	{
+		"int16",
+		"Int16",
+		`x := state.decodeInt()
+		if x < math.MinInt16 || math.MaxInt16 < x {
+			error_(ovfl)
+		}
+		slice[i] = int16(x)`,
+	},
+	{
+		"int32",
+		"Int32",
+		`x := state.decodeInt()
+		if x < math.MinInt32 || math.MaxInt32 < x {
+			error_(ovfl)
+		}
+		slice[i] = int32(x)`,
+	},
+	{
+		"int64",
+		"Int64",
+		`slice[i] = state.decodeInt()`,
+	},
+	{
+		"int8",
+		"Int8",
+		`x := state.decodeInt()
+		if x < math.MinInt8 || math.MaxInt8 < x {
+			error_(ovfl)
+		}
+		slice[i] = int8(x)`,
+	},
+	{
+		"string",
+		"String",
+		`u := state.decodeUint()
+		n := int(u)
+		if n < 0 || uint64(n) != u || n > state.b.Len() {
+			errorf("length of string exceeds input size (%d bytes)", u)
+		}
+		if n > state.b.Len() {
+			errorf("string data too long for buffer: %d", n)
+		}
+		// Read the data.
+		data := make([]byte, n)
+		if _, err := state.b.Read(data); err != nil {
+			errorf("error decoding string: %s", err)
+		}
+		slice[i] = string(data)`,
+	},
+	{
+		"uint",
+		"Uint",
+		`x := state.decodeUint()
+		/*TODO if math.MaxUint32 < x {
+			error_(ovfl)
+		}*/
+		slice[i] = uint(x)`,
+	},
+	{
+		"uint16",
+		"Uint16",
+		`x := state.decodeUint()
+		if math.MaxUint16 < x {
+			error_(ovfl)
+		}
+		slice[i] = uint16(x)`,
+	},
+	{
+		"uint32",
+		"Uint32",
+		`x := state.decodeUint()
+		if math.MaxUint32 < x {
+			error_(ovfl)
+		}
+		slice[i] = uint32(x)`,
+	},
+	{
+		"uint64",
+		"Uint64",
+		`slice[i] = state.decodeUint()`,
+	},
+	{
+		"uintptr",
+		"Uintptr",
+		`x := state.decodeUint()
+		if uint64(^uintptr(0)) < x {
+			error_(ovfl)
+		}
+		slice[i] = uintptr(x)`,
+	},
+	// uint8 Handled separately.
+}
+
+func main() {
+	log.SetFlags(0)
+	log.SetPrefix("decgen: ")
+	flag.Parse()
+	if flag.NArg() != 0 {
+		log.Fatal("usage: decgen [--output filename]")
+	}
+	var b bytes.Buffer
+	fmt.Fprintf(&b, "// Created by decgen --output %s; DO NOT EDIT\n", *output)
+	fmt.Fprint(&b, header)
+	printMaps(&b, "Array")
+	fmt.Fprint(&b, "\n")
+	printMaps(&b, "Slice")
+	for _, t := range types {
+		fmt.Fprintf(&b, arrayHelper, t.lower, t.upper)
+		fmt.Fprintf(&b, sliceHelper, t.lower, t.upper, t.decoder)
+	}
+	source, err := format.Source(b.Bytes())
+	if err != nil {
+		log.Fatal("source format error:", err)
+	}
+	fd, err := os.Create(*output)
+	_, err = fd.Write(source)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func printMaps(b *bytes.Buffer, upperClass string) {
+	fmt.Fprintf(b, "var dec%sHelper = map[reflect.Kind]decHelper{\n", upperClass)
+	for _, t := range types {
+		fmt.Fprintf(b, "reflect.%s: dec%s%s,\n", t.upper, t.upper, upperClass)
+	}
+	fmt.Fprintf(b, "}\n")
+}
+
+const header = `
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gob
+
+import (
+	"math"
+	"reflect"
+)
+
+`
+
+const arrayHelper = `
+func dec%[2]sArray(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	// Can only slice if it is addressable.
+	if !v.CanAddr() {
+		return false
+	}
+	return dec%[2]sSlice(state, v.Slice(0, v.Len()), length, ovfl)
+}
+`
+
+const sliceHelper = `
+func dec%[2]sSlice(state *decoderState, v reflect.Value, length int, ovfl error) bool {
+	slice, ok := v.Interface().([]%[1]s)
+	if !ok {
+		// It is kind %[1]s but not type %[1]s. TODO: We can handle this unsafely.
+		return false
+	}
+	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding %[1]s array or slice: length exceeds input size (%d elements)", length)
+		}
+		%[3]s
+	}
+	return true
+}
+`
diff --git a/src/encoding/gob/decode.go b/src/encoding/gob/decode.go
index 6a9213fb3..f44838e4c 100644
--- a/src/encoding/gob/decode.go
+++ b/src/encoding/gob/decode.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:generate go run decgen.go -output dec_helpers.go
+
 package gob
 
 import (
@@ -19,6 +21,8 @@ var (
 	errRange   = errors.New("gob: bad data: field numbers out of bounds")
 )
 
+type decHelper func(state *decoderState, v reflect.Value, length int, ovfl error) bool
+
 // decoderState is the execution state of an instance of the decoder. A new state
 // is created for nested objects.
 type decoderState struct {
@@ -257,7 +261,7 @@ func float64FromBits(u uint64) float64 {
 // number, and returns it. It's a helper function for float32 and complex64.
 // It returns a float64 because that's what reflection needs, but its return
 // value is known to be accurately representable in a float32.
-func float32FromBits(i *decInstr, u uint64) float64 {
+func float32FromBits(u uint64, ovfl error) float64 {
 	v := float64FromBits(u)
 	av := v
 	if av < 0 {
@@ -265,7 +269,7 @@ func float32FromBits(i *decInstr, u uint64) float64 {
 	}
 	// +Inf is OK in both 32- and 64-bit floats.  Underflow is always OK.
 	if math.MaxFloat32 < av && av <= math.MaxFloat64 {
-		error_(i.ovfl)
+		error_(ovfl)
 	}
 	return v
 }
@@ -273,7 +277,7 @@ func float32FromBits(i *decInstr, u uint64) float64 {
 // decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
 // number, and stores it in value.
 func decFloat32(i *decInstr, state *decoderState, value reflect.Value) {
-	value.SetFloat(float32FromBits(i, state.decodeUint()))
+	value.SetFloat(float32FromBits(state.decodeUint(), i.ovfl))
 }
 
 // decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
@@ -286,8 +290,8 @@ func decFloat64(i *decInstr, state *decoderState, value reflect.Value) {
 // pair of floating point numbers, and stores them as a complex64 in value.
 // The real part comes first.
 func decComplex64(i *decInstr, state *decoderState, value reflect.Value) {
-	real := float32FromBits(i, state.decodeUint())
-	imag := float32FromBits(i, state.decodeUint())
+	real := float32FromBits(state.decodeUint(), i.ovfl)
+	imag := float32FromBits(state.decodeUint(), i.ovfl)
 	value.SetComplex(complex(real, imag))
 }
 
@@ -450,7 +454,10 @@ func (dec *Decoder) ignoreSingle(engine *decEngine) {
 }
 
 // decodeArrayHelper does the work for decoding arrays and slices.
-func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error) {
+func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
+	if helper != nil && helper(state, value, length, ovfl) {
+		return
+	}
 	instr := &decInstr{elemOp, 0, nil, ovfl}
 	isPtr := value.Type().Elem().Kind() == reflect.Ptr
 	for i := 0; i < length; i++ {
@@ -468,11 +475,11 @@ func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value,
 // decodeArray decodes an array and stores it in value.
 // The length is an unsigned integer preceding the elements.  Even though the length is redundant
 // (it's part of the type), it's a useful check and is included in the encoding.
-func (dec *Decoder) decodeArray(atyp reflect.Type, state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error) {
+func (dec *Decoder) decodeArray(atyp reflect.Type, state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
 	if n := state.decodeUint(); n != uint64(length) {
 		errorf("length mismatch in decodeArray")
 	}
-	dec.decodeArrayHelper(state, value, elemOp, length, ovfl)
+	dec.decodeArrayHelper(state, value, elemOp, length, ovfl, helper)
 }
 
 // decodeIntoValue is a helper for map decoding.
@@ -534,7 +541,7 @@ func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
 
 // decodeSlice decodes a slice and stores it in value.
 // Slices are encoded as an unsigned length followed by the elements.
-func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp decOp, ovfl error) {
+func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp decOp, ovfl error, helper decHelper) {
 	u := state.decodeUint()
 	typ := value.Type()
 	size := uint64(typ.Elem().Size())
@@ -551,7 +558,7 @@ func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp
 	} else {
 		value.Set(value.Slice(0, n))
 	}
-	dec.decodeArrayHelper(state, value, elemOp, n, ovfl)
+	dec.decodeArrayHelper(state, value, elemOp, n, ovfl, helper)
 }
 
 // ignoreSlice skips over the data for a slice value with no destination.
@@ -720,8 +727,9 @@ func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProg
 			elemId := dec.wireType[wireId].ArrayT.Elem
 			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
 			ovfl := overflow(name)
+			helper := decArrayHelper[t.Elem().Kind()]
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
-				state.dec.decodeArray(t, state, value, *elemOp, t.Len(), ovfl)
+				state.dec.decodeArray(t, state, value, *elemOp, t.Len(), ovfl, helper)
 			}
 
 		case reflect.Map:
@@ -748,8 +756,9 @@ func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProg
 			}
 			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
 			ovfl := overflow(name)
+			helper := decSliceHelper[t.Elem().Kind()]
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
-				state.dec.decodeSlice(state, value, *elemOp, ovfl)
+				state.dec.decodeSlice(state, value, *elemOp, ovfl, helper)
 			}
 
 		case reflect.Struct:
diff --git a/src/encoding/gob/enc_helpers.go b/src/encoding/gob/enc_helpers.go
index 1e6f30718..804e539d8 100644
--- a/src/encoding/gob/enc_helpers.go
+++ b/src/encoding/gob/enc_helpers.go
@@ -10,7 +10,7 @@ import (
 	"reflect"
 )
 
-var arrayHelper = map[reflect.Kind]encHelper{
+var encArrayHelper = map[reflect.Kind]encHelper{
 	reflect.Bool:       encBoolArray,
 	reflect.Complex64:  encComplex64Array,
 	reflect.Complex128: encComplex128Array,
@@ -29,7 +29,7 @@ var arrayHelper = map[reflect.Kind]encHelper{
 	reflect.Uintptr:    encUintptrArray,
 }
 
-var sliceHelper = map[reflect.Kind]encHelper{
+var encSliceHelper = map[reflect.Kind]encHelper{
 	reflect.Bool:       encBoolSlice,
 	reflect.Complex64:  encComplex64Slice,
 	reflect.Complex128: encComplex128Slice,
diff --git a/src/encoding/gob/encgen.go b/src/encoding/gob/encgen.go
index fa500e3da..efdd92829 100644
--- a/src/encoding/gob/encgen.go
+++ b/src/encoding/gob/encgen.go
@@ -144,7 +144,7 @@ var types = []Type{
 
 func main() {
 	log.SetFlags(0)
-	log.SetPrefix("helpergen: ")
+	log.SetPrefix("encgen: ")
 	flag.Parse()
 	if flag.NArg() != 0 {
 		log.Fatal("usage: encgen [--output filename]")
@@ -152,9 +152,9 @@ func main() {
 	var b bytes.Buffer
 	fmt.Fprintf(&b, "// Created by encgen --output %s; DO NOT EDIT\n", *output)
 	fmt.Fprint(&b, header)
-	printMaps(&b, "array", "Array")
+	printMaps(&b, "Array")
 	fmt.Fprint(&b, "\n")
-	printMaps(&b, "slice", "Slice")
+	printMaps(&b, "Slice")
 	for _, t := range types {
 		fmt.Fprintf(&b, arrayHelper, t.lower, t.upper)
 		fmt.Fprintf(&b, sliceHelper, t.lower, t.upper, t.zero, t.encoder)
@@ -170,8 +170,8 @@ func main() {
 	}
 }
 
-func printMaps(b *bytes.Buffer, lowerClass, upperClass string) {
-	fmt.Fprintf(b, "var %sHelper = map[reflect.Kind]encHelper{\n", lowerClass)
+func printMaps(b *bytes.Buffer, upperClass string) {
+	fmt.Fprintf(b, "var enc%sHelper = map[reflect.Kind]encHelper{\n", upperClass)
 	for _, t := range types {
 		fmt.Fprintf(b, "reflect.%s: enc%s%s,\n", t.upper, t.upper, upperClass)
 	}
diff --git a/src/encoding/gob/encode.go b/src/encoding/gob/encode.go
index 3b8d0b427..3da848c85 100644
--- a/src/encoding/gob/encode.go
+++ b/src/encoding/gob/encode.go
@@ -508,7 +508,7 @@ func encOpFor(rt reflect.Type, inProgress map[reflect.Type]*encOp, building map[
 			}
 			// Slices have a header; we decode it to find the underlying array.
 			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
-			helper := sliceHelper[t.Elem().Kind()]
+			helper := encSliceHelper[t.Elem().Kind()]
 			op = func(i *encInstr, state *encoderState, slice reflect.Value) {
 				if !state.sendZero && slice.Len() == 0 {
 					return
@@ -519,7 +519,7 @@ func encOpFor(rt reflect.Type, inProgress map[reflect.Type]*encOp, building map[
 		case reflect.Array:
 			// True arrays have size in the type.
 			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
-			helper := arrayHelper[t.Elem().Kind()]
+			helper := encArrayHelper[t.Elem().Kind()]
 			op = func(i *encInstr, state *encoderState, array reflect.Value) {
 				state.update(i)
 				state.enc.encodeArray(state.b, array, *elemOp, elemIndir, array.Len(), helper)
diff --git a/src/encoding/gob/timing_test.go b/src/encoding/gob/timing_test.go
index abfe936e8..940e5ad41 100644
--- a/src/encoding/gob/timing_test.go
+++ b/src/encoding/gob/timing_test.go
@@ -132,13 +132,14 @@ func TestCountDecodeMallocs(t *testing.T) {
 	}
 }
 
-func BenchmarkComplex128Slice(b *testing.B) {
+func BenchmarkEncodeComplex128Slice(b *testing.B) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
 	a := make([]complex128, 1000)
 	for i := range a {
 		a[i] = 1.2 + 3.4i
 	}
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		buf.Reset()
 		err := enc.Encode(a)
@@ -148,13 +149,14 @@ func BenchmarkComplex128Slice(b *testing.B) {
 	}
 }
 
-func BenchmarkInt32Slice(b *testing.B) {
+func BenchmarkEncodeFloat64Slice(b *testing.B) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
-	a := make([]int32, 1000)
+	a := make([]float64, 1000)
 	for i := range a {
-		a[i] = 1234
+		a[i] = 1.23e4
 	}
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		buf.Reset()
 		err := enc.Encode(a)
@@ -164,13 +166,14 @@ func BenchmarkInt32Slice(b *testing.B) {
 	}
 }
 
-func BenchmarkFloat64Slice(b *testing.B) {
+func BenchmarkEncodeInt32Slice(b *testing.B) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
-	a := make([]float64, 1000)
+	a := make([]int32, 1000)
 	for i := range a {
-		a[i] = 1.23e4
+		a[i] = 1234
 	}
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		buf.Reset()
 		err := enc.Encode(a)
@@ -180,13 +183,14 @@ func BenchmarkFloat64Slice(b *testing.B) {
 	}
 }
 
-func BenchmarkStringSlice(b *testing.B) {
+func BenchmarkEncodeStringSlice(b *testing.B) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
 	a := make([]string, 1000)
 	for i := range a {
 		a[i] = "now is the time"
 	}
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		buf.Reset()
 		err := enc.Encode(a)
@@ -195,3 +199,127 @@ func BenchmarkStringSlice(b *testing.B) {
 		}
 	}
 }
+
+// benchmarkBuf is a read buffer we can reset
+type benchmarkBuf struct {
+	offset int
+	data   []byte
+}
+
+func (b *benchmarkBuf) Read(p []byte) (n int, err error) {
+	n = copy(p, b.data[b.offset:])
+	if n == 0 {
+		return 0, io.EOF
+	}
+	b.offset += n
+	return
+}
+
+func (b *benchmarkBuf) ReadByte() (c byte, err error) {
+	if b.offset >= len(b.data) {
+		return 0, io.EOF
+	}
+	c = b.data[b.offset]
+	b.offset++
+	return
+}
+
+func (b *benchmarkBuf) reset() {
+	b.offset = 0
+}
+
+func BenchmarkDecodeComplex128Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]complex128, 1000)
+	for i := range a {
+		a[i] = 1.2 + 3.4i
+	}
+	err := enc.Encode(a)
+	if err != nil {
+		b.Fatal(err)
+	}
+	x := make([]complex128, 1000)
+	bbuf := benchmarkBuf{data: buf.Bytes()}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bbuf.reset()
+		dec := NewDecoder(&bbuf)
+		err := dec.Decode(&x)
+		if err != nil {
+			b.Fatal(i, err)
+		}
+	}
+}
+
+func BenchmarkDecodeFloat64Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]float64, 1000)
+	for i := range a {
+		a[i] = 1.23e4
+	}
+	err := enc.Encode(a)
+	if err != nil {
+		b.Fatal(err)
+	}
+	x := make([]float64, 1000)
+	bbuf := benchmarkBuf{data: buf.Bytes()}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bbuf.reset()
+		dec := NewDecoder(&bbuf)
+		err := dec.Decode(&x)
+		if err != nil {
+			b.Fatal(i, err)
+		}
+	}
+}
+
+func BenchmarkDecodeInt32Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]int32, 1000)
+	for i := range a {
+		a[i] = 1234
+	}
+	err := enc.Encode(a)
+	if err != nil {
+		b.Fatal(err)
+	}
+	x := make([]int32, 1000)
+	bbuf := benchmarkBuf{data: buf.Bytes()}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bbuf.reset()
+		dec := NewDecoder(&bbuf)
+		err := dec.Decode(&x)
+		if err != nil {
+			b.Fatal(i, err)
+		}
+	}
+}
+
+func BenchmarkDecodeStringSlice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]string, 1000)
+	for i := range a {
+		a[i] = "now is the time"
+	}
+	err := enc.Encode(a)
+	if err != nil {
+		b.Fatal(err)
+	}
+	x := make([]string, 1000)
+	bbuf := benchmarkBuf{data: buf.Bytes()}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bbuf.reset()
+		dec := NewDecoder(&bbuf)
+		err := dec.Decode(&x)
+		if err != nil {
+			b.Fatal(i, err)
+		}
+	}
+}