gob: allow transmission of things other than structs at the top level.

also fix a bug handling nil maps: before, would needlessly send empty map

R=rsc
CC=golang-dev
http://codereview.appspot.com/1739043

6 files changed, 234 insertions, 113 deletions

diff --git a/src/pkg/gob/codec_test.go b/src/pkg/gob/codec_test.go
index d8bdf2d2f..49a13e84d 100644
--- a/src/pkg/gob/codec_test.go
+++ b/src/pkg/gob/codec_test.go
@@ -1039,7 +1039,7 @@ func TestInvalidField(t *testing.T) {
 type Indirect struct {
 	a ***[3]int
 	s ***[]int
-	m ***map[string]int
+	m ****map[string]int
 }
 
 type Direct struct {
@@ -1059,10 +1059,11 @@ func TestIndirectSliceMapArray(t *testing.T) {
 	*i.s = new(*[]int)
 	**i.s = new([]int)
 	***i.s = []int{4, 5, 6}
-	i.m = new(**map[string]int)
-	*i.m = new(*map[string]int)
-	**i.m = new(map[string]int)
-	***i.m = map[string]int{"one": 1, "two": 2, "three": 3}
+	i.m = new(***map[string]int)
+	*i.m = new(**map[string]int)
+	**i.m = new(*map[string]int)
+	***i.m = new(map[string]int)
+	****i.m = map[string]int{"one": 1, "two": 2, "three": 3}
 	b := new(bytes.Buffer)
 	NewEncoder(b).Encode(i)
 	dec := NewDecoder(b)
@@ -1093,12 +1094,12 @@ func TestIndirectSliceMapArray(t *testing.T) {
 		t.Error("error: ", err)
 	}
 	if len(***i.a) != 3 || (***i.a)[0] != 11 || (***i.a)[1] != 22 || (***i.a)[2] != 33 {
-		t.Errorf("indirect to direct: ***i.a is %v not %v", ***i.a, d.a)
+		t.Errorf("direct to indirect: ***i.a is %v not %v", ***i.a, d.a)
 	}
 	if len(***i.s) != 3 || (***i.s)[0] != 44 || (***i.s)[1] != 55 || (***i.s)[2] != 66 {
-		t.Errorf("indirect to direct: ***i.s is %v not %v", ***i.s, ***i.s)
+		t.Errorf("direct to indirect: ***i.s is %v not %v", ***i.s, ***i.s)
 	}
-	if len(***i.m) != 3 || (***i.m)["four"] != 4 || (***i.m)["five"] != 5 || (***i.m)["six"] != 6 {
-		t.Errorf("indirect to direct: ***i.m is %v not %v", ***i.m, d.m)
+	if len(****i.m) != 3 || (****i.m)["four"] != 4 || (****i.m)["five"] != 5 || (****i.m)["six"] != 6 {
+		t.Errorf("direct to indirect: ****i.m is %v not %v", ****i.m, d.m)
 	}
 }
diff --git a/src/pkg/gob/decode.go b/src/pkg/gob/decode.go
index 0dbf81488..51e439900 100644
--- a/src/pkg/gob/decode.go
+++ b/src/pkg/gob/decode.go
@@ -13,15 +13,13 @@ import (
 	"math"
 	"os"
 	"reflect"
-	"runtime"
 	"unsafe"
 )
 
 var (
-	errBadUint   = os.ErrorString("gob: encoded unsigned integer out of range")
-	errBadType   = os.ErrorString("gob: unknown type id or corrupted data")
-	errRange     = os.ErrorString("gob: internal error: field numbers out of bounds")
-	errNotStruct = os.ErrorString("gob: TODO: can only handle structs")
+	errBadUint = os.ErrorString("gob: encoded unsigned integer out of range")
+	errBadType = os.ErrorString("gob: unknown type id or corrupted data")
+	errRange   = os.ErrorString("gob: internal error: field numbers out of bounds")
 )
 
 // The global execution state of an instance of the decoder.
@@ -389,18 +387,44 @@ type decEngine struct {
 	numInstr int // the number of active instructions
 }
 
-func decodeStruct(engine *decEngine, rtyp *reflect.StructType, b *bytes.Buffer, p uintptr, indir int) os.Error {
-	if indir > 0 {
-		up := unsafe.Pointer(p)
-		if indir > 1 {
-			up = decIndirect(up, indir)
-		}
-		if *(*unsafe.Pointer)(up) == nil {
-			// Allocate object.
-			*(*unsafe.Pointer)(up) = unsafe.New((*runtime.StructType)(unsafe.Pointer(rtyp)))
-		}
-		p = *(*uintptr)(up)
+// allocate makes sure storage is available for an object of underlying type rtyp
+// that is indir levels of indirection through p.
+func allocate(rtyp reflect.Type, p uintptr, indir int) uintptr {
+	if indir == 0 {
+		return p
+	}
+	up := unsafe.Pointer(p)
+	if indir > 1 {
+		up = decIndirect(up, indir)
 	}
+	if *(*unsafe.Pointer)(up) == nil {
+		// Allocate object.
+		*(*unsafe.Pointer)(up) = unsafe.New(rtyp)
+	}
+	return *(*uintptr)(up)
+}
+
+func decodeSingle(engine *decEngine, rtyp reflect.Type, b *bytes.Buffer, p uintptr, indir int) os.Error {
+	p = allocate(rtyp, p, indir)
+	state := newDecodeState(b)
+	state.fieldnum = singletonField
+	basep := p
+	delta := int(decodeUint(state))
+	if delta != 0 {
+		state.err = os.ErrorString("gob decode: corrupted data: non-zero delta for singleton")
+		return state.err
+	}
+	instr := &engine.instr[singletonField]
+	ptr := unsafe.Pointer(basep) // offset will be zero
+	if instr.indir > 1 {
+		ptr = decIndirect(ptr, instr.indir)
+	}
+	instr.op(instr, state, ptr)
+	return state.err
+}
+
+func decodeStruct(engine *decEngine, rtyp *reflect.StructType, b *bytes.Buffer, p uintptr, indir int) os.Error {
+	p = allocate(rtyp, p, indir)
 	state := newDecodeState(b)
 	state.fieldnum = -1
 	basep := p
@@ -468,12 +492,7 @@ func decodeArrayHelper(state *decodeState, p uintptr, elemOp decOp, elemWid uint
 
 func decodeArray(atyp *reflect.ArrayType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, length, indir, elemIndir int, ovfl os.ErrorString) os.Error {
 	if indir > 0 {
-		up := unsafe.Pointer(p)
-		if *(*unsafe.Pointer)(up) == nil {
-			// Allocate object.
-			*(*unsafe.Pointer)(up) = unsafe.New(atyp)
-		}
-		p = *(*uintptr)(up)
+		p = allocate(atyp, p, 1) // All but the last level has been allocated by dec.Indirect
 	}
 	if n := decodeUint(state); n != uint64(length) {
 		return os.ErrorString("gob: length mismatch in decodeArray")
@@ -493,12 +512,7 @@ func decodeIntoValue(state *decodeState, op decOp, indir int, v reflect.Value, o
 
 func decodeMap(mtyp *reflect.MapType, state *decodeState, p uintptr, keyOp, elemOp decOp, indir, keyIndir, elemIndir int, ovfl os.ErrorString) os.Error {
 	if indir > 0 {
-		up := unsafe.Pointer(p)
-		if *(*unsafe.Pointer)(up) == nil {
-			// Allocate object.
-			*(*unsafe.Pointer)(up) = unsafe.New(mtyp)
-		}
-		p = *(*uintptr)(up)
+		p = allocate(mtyp, p, 1) // All but the last level has been allocated by dec.Indirect
 	}
 	up := unsafe.Pointer(p)
 	if *(*unsafe.Pointer)(up) == nil { // maps are represented as a pointer in the runtime
@@ -806,18 +820,34 @@ func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId) bool {
 	return true
 }
 
+func (dec *Decoder) compileSingle(remoteId typeId, rt reflect.Type) (engine *decEngine, err os.Error) {
+	engine = new(decEngine)
+	engine.instr = make([]decInstr, 1) // one item
+	name := rt.String()                // best we can do
+	if !dec.compatibleType(rt, remoteId) {
+		return nil, os.ErrorString("gob: wrong type received for local value " + name)
+	}
+	op, indir, err := dec.decOpFor(remoteId, rt, name)
+	if err != nil {
+		return nil, err
+	}
+	ovfl := os.ErrorString(`value for "` + name + `" out of range`)
+	engine.instr[singletonField] = decInstr{op, singletonField, indir, 0, ovfl}
+	engine.numInstr = 1
+	return
+}
+
 func (dec *Decoder) compileDec(remoteId typeId, rt reflect.Type) (engine *decEngine, err os.Error) {
-	srt, ok1 := rt.(*reflect.StructType)
+	srt, ok := rt.(*reflect.StructType)
+	if !ok {
+		return dec.compileSingle(remoteId, rt)
+	}
 	var wireStruct *structType
 	// Builtin types can come from global pool; the rest must be defined by the decoder
 	if t, ok := builtinIdToType[remoteId]; ok {
 		wireStruct = t.(*structType)
 	} else {
-		w, ok2 := dec.wireType[remoteId]
-		if !ok1 || !ok2 {
-			return nil, errNotStruct
-		}
-		wireStruct = w.structT
+		wireStruct = dec.wireType[remoteId].structT
 	}
 	engine = new(decEngine)
 	engine.instr = make([]decInstr, len(wireStruct.field))
@@ -891,20 +921,19 @@ func (dec *Decoder) getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, er
 func (dec *Decoder) decode(wireId typeId, e interface{}) os.Error {
 	// Dereference down to the underlying struct type.
 	rt, indir := indirect(reflect.Typeof(e))
-	st, ok := rt.(*reflect.StructType)
-	if !ok {
-		return os.ErrorString("gob: decode can't handle " + rt.String())
-	}
 	enginePtr, err := dec.getDecEnginePtr(wireId, rt)
 	if err != nil {
 		return err
 	}
 	engine := *enginePtr
-	if engine.numInstr == 0 && st.NumField() > 0 && len(dec.wireType[wireId].structT.field) > 0 {
-		name := rt.Name()
-		return os.ErrorString("gob: type mismatch: no fields matched compiling decoder for " + name)
+	if st, ok := rt.(*reflect.StructType); ok {
+		if engine.numInstr == 0 && st.NumField() > 0 && len(dec.wireType[wireId].structT.field) > 0 {
+			name := rt.Name()
+			return os.ErrorString("gob: type mismatch: no fields matched compiling decoder for " + name)
+		}
+		return decodeStruct(engine, st, dec.state.b, uintptr(reflect.NewValue(e).Addr()), indir)
 	}
-	return decodeStruct(engine, st, dec.state.b, uintptr(reflect.NewValue(e).Addr()), indir)
+	return decodeSingle(engine, rt, dec.state.b, uintptr(reflect.NewValue(e).Addr()), indir)
 }
 
 func init() {
diff --git a/src/pkg/gob/decoder.go b/src/pkg/gob/decoder.go
index 90dc2e34c..caec51712 100644
--- a/src/pkg/gob/decoder.go
+++ b/src/pkg/gob/decoder.go
@@ -108,8 +108,9 @@ func (dec *Decoder) Decode(e interface{}) os.Error {
 		}
 
 		// No, it's a value.
-		// Make sure the type has been defined already.
-		if dec.wireType[id] == nil {
+		// Make sure the type has been defined already or is a builtin type (for
+		// top-level singleton values).
+		if dec.wireType[id] == nil && builtinIdToType[id] == nil {
 			dec.state.err = errBadType
 			break
 		}
diff --git a/src/pkg/gob/encode.go b/src/pkg/gob/encode.go
index b48c1f698..a7d44ecc2 100644
--- a/src/pkg/gob/encode.go
+++ b/src/pkg/gob/encode.go
@@ -271,7 +271,7 @@ const uint64Size = unsafe.Sizeof(uint64(0))
 type encoderState struct {
 	b        *bytes.Buffer
 	err      os.Error             // error encountered during encoding.
-	inArray  bool                 // encoding an array element or map key/value pair
+	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.
 }
@@ -352,7 +352,7 @@ func encIndirect(p unsafe.Pointer, indir int) unsafe.Pointer {
 
 func encBool(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	b := *(*bool)(p)
-	if b || state.inArray {
+	if b || state.sendZero {
 		state.update(i)
 		if b {
 			encodeUint(state, 1)
@@ -364,7 +364,7 @@ func encBool(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encInt(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := int64(*(*int)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeInt(state, v)
 	}
@@ -372,7 +372,7 @@ func encInt(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUint(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := uint64(*(*uint)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -380,7 +380,7 @@ func encUint(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encInt8(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := int64(*(*int8)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeInt(state, v)
 	}
@@ -388,7 +388,7 @@ func encInt8(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUint8(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := uint64(*(*uint8)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -396,7 +396,7 @@ func encUint8(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encInt16(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := int64(*(*int16)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeInt(state, v)
 	}
@@ -404,7 +404,7 @@ func encInt16(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUint16(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := uint64(*(*uint16)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -412,7 +412,7 @@ func encUint16(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encInt32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := int64(*(*int32)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeInt(state, v)
 	}
@@ -420,7 +420,7 @@ func encInt32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUint32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := uint64(*(*uint32)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -428,7 +428,7 @@ func encUint32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encInt64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := *(*int64)(p)
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeInt(state, v)
 	}
@@ -436,7 +436,7 @@ func encInt64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUint64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := *(*uint64)(p)
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -444,7 +444,7 @@ func encUint64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encUintptr(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	v := uint64(*(*uintptr)(p))
-	if v != 0 || state.inArray {
+	if v != 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, v)
 	}
@@ -468,7 +468,7 @@ func floatBits(f float64) uint64 {
 
 func encFloat(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	f := *(*float)(p)
-	if f != 0 || state.inArray {
+	if f != 0 || state.sendZero {
 		v := floatBits(float64(f))
 		state.update(i)
 		encodeUint(state, v)
@@ -477,7 +477,7 @@ func encFloat(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encFloat32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	f := *(*float32)(p)
-	if f != 0 || state.inArray {
+	if f != 0 || state.sendZero {
 		v := floatBits(float64(f))
 		state.update(i)
 		encodeUint(state, v)
@@ -486,7 +486,7 @@ func encFloat32(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encFloat64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	f := *(*float64)(p)
-	if f != 0 || state.inArray {
+	if f != 0 || state.sendZero {
 		state.update(i)
 		v := floatBits(f)
 		encodeUint(state, v)
@@ -496,7 +496,7 @@ func encFloat64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 // Complex numbers are just a pair of floating-point numbers, real part first.
 func encComplex(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	c := *(*complex)(p)
-	if c != 0+0i || state.inArray {
+	if c != 0+0i || state.sendZero {
 		rpart := floatBits(float64(real(c)))
 		ipart := floatBits(float64(imag(c)))
 		state.update(i)
@@ -507,7 +507,7 @@ func encComplex(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encComplex64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	c := *(*complex64)(p)
-	if c != 0+0i || state.inArray {
+	if c != 0+0i || state.sendZero {
 		rpart := floatBits(float64(real(c)))
 		ipart := floatBits(float64(imag(c)))
 		state.update(i)
@@ -518,7 +518,7 @@ func encComplex64(i *encInstr, state *encoderState, p unsafe.Pointer) {
 
 func encComplex128(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	c := *(*complex128)(p)
-	if c != 0+0i || state.inArray {
+	if c != 0+0i || state.sendZero {
 		rpart := floatBits(real(c))
 		ipart := floatBits(imag(c))
 		state.update(i)
@@ -530,7 +530,7 @@ func encComplex128(i *encInstr, state *encoderState, p unsafe.Pointer) {
 // Byte arrays are encoded as an unsigned count followed by the raw bytes.
 func encUint8Array(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	b := *(*[]byte)(p)
-	if len(b) > 0 || state.inArray {
+	if len(b) > 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, uint64(len(b)))
 		state.b.Write(b)
@@ -540,7 +540,7 @@ func encUint8Array(i *encInstr, state *encoderState, p unsafe.Pointer) {
 // Strings are encoded as an unsigned count followed by the raw bytes.
 func encString(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	s := *(*string)(p)
-	if len(s) > 0 || state.inArray {
+	if len(s) > 0 || state.sendZero {
 		state.update(i)
 		encodeUint(state, uint64(len(s)))
 		io.WriteString(state.b, s)
@@ -560,6 +560,26 @@ type encEngine struct {
 	instr []encInstr
 }
 
+const singletonField = 0
+
+func encodeSingle(engine *encEngine, b *bytes.Buffer, basep uintptr) os.Error {
+	state := new(encoderState)
+	state.b = b
+	state.fieldnum = singletonField
+	// There is no surrounding struct to frame the transmission, so we must
+	// generate data even if the item is zero.  To do this, set sendZero.
+	state.sendZero = true
+	instr := &engine.instr[singletonField]
+	p := unsafe.Pointer(basep) // offset will be zero
+	if instr.indir > 0 {
+		if p = encIndirect(p, instr.indir); p == nil {
+			return nil
+		}
+	}
+	instr.op(instr, state, p)
+	return state.err
+}
+
 func encodeStruct(engine *encEngine, b *bytes.Buffer, basep uintptr) os.Error {
 	state := new(encoderState)
 	state.b = b
@@ -584,7 +604,7 @@ func encodeArray(b *bytes.Buffer, p uintptr, op encOp, elemWid uintptr, elemIndi
 	state := new(encoderState)
 	state.b = b
 	state.fieldnum = -1
-	state.inArray = true
+	state.sendZero = true
 	encodeUint(state, uint64(length))
 	for i := 0; i < length && state.err == nil; i++ {
 		elemp := p
@@ -607,22 +627,17 @@ func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir in
 		v = reflect.Indirect(v)
 	}
 	if v == nil {
-		state.err = os.ErrorString("gob: encodeMap: nil element")
+		state.err = os.ErrorString("gob: encodeReflectValue: nil element")
 		return
 	}
 	op(nil, state, unsafe.Pointer(v.Addr()))
 }
 
-func encodeMap(b *bytes.Buffer, rt reflect.Type, p uintptr, keyOp, elemOp encOp, keyIndir, elemIndir int) os.Error {
+func encodeMap(b *bytes.Buffer, mv *reflect.MapValue, keyOp, elemOp encOp, keyIndir, elemIndir int) os.Error {
 	state := new(encoderState)
 	state.b = b
 	state.fieldnum = -1
-	state.inArray = true
-	// Maps cannot be accessed by moving addresses around the way
-	// that slices etc. can.  We must recover a full reflection value for
-	// the iteration.
-	v := reflect.NewValue(unsafe.Unreflect(rt, unsafe.Pointer((p))))
-	mv := reflect.Indirect(v).(*reflect.MapValue)
+	state.sendZero = true
 	keys := mv.Keys()
 	encodeUint(state, uint64(len(keys)))
 	for _, key := range keys {
@@ -694,6 +709,10 @@ func encOpFor(rt reflect.Type) (encOp, int, os.Error) {
 				return nil, 0, err
 			}
 			op = func(i *encInstr, state *encoderState, p unsafe.Pointer) {
+				slice := (*reflect.SliceHeader)(p)
+				if slice.Len == 0 {
+					return
+				}
 				state.update(i)
 				state.err = encodeArray(state.b, uintptr(p), elemOp, t.Elem().Size(), indir, t.Len())
 			}
@@ -707,8 +726,16 @@ func encOpFor(rt reflect.Type) (encOp, int, os.Error) {
 				return nil, 0, err
 			}
 			op = func(i *encInstr, state *encoderState, p unsafe.Pointer) {
+				// Maps cannot be accessed by moving addresses around the way
+				// that slices etc. can.  We must recover a full reflection value for
+				// the iteration.
+				v := reflect.NewValue(unsafe.Unreflect(t, unsafe.Pointer((p))))
+				mv := reflect.Indirect(v).(*reflect.MapValue)
+				if mv.Len() == 0 {
+					return
+				}
 				state.update(i)
-				state.err = encodeMap(state.b, typ, uintptr(p), keyOp, elemOp, keyIndir, elemIndir)
+				state.err = encodeMap(state.b, mv, keyOp, elemOp, keyIndir, elemIndir)
 			}
 		case *reflect.StructType:
 			// Generate a closure that calls out to the engine for the nested type.
@@ -732,21 +759,27 @@ func encOpFor(rt reflect.Type) (encOp, int, os.Error) {
 
 // The local Type was compiled from the actual value, so we know it's compatible.
 func compileEnc(rt reflect.Type) (*encEngine, os.Error) {
-	srt, ok := rt.(*reflect.StructType)
-	if !ok {
-		panic("can't happen: non-struct")
-	}
+	srt, isStruct := rt.(*reflect.StructType)
 	engine := new(encEngine)
-	engine.instr = make([]encInstr, srt.NumField()+1) // +1 for terminator
-	for fieldnum := 0; fieldnum < srt.NumField(); fieldnum++ {
-		f := srt.Field(fieldnum)
-		op, indir, err := encOpFor(f.Type)
+	if isStruct {
+		engine.instr = make([]encInstr, srt.NumField()+1) // +1 for terminator
+		for fieldnum := 0; fieldnum < srt.NumField(); fieldnum++ {
+			f := srt.Field(fieldnum)
+			op, indir, err := encOpFor(f.Type)
+			if err != nil {
+				return nil, err
+			}
+			engine.instr[fieldnum] = encInstr{op, fieldnum, indir, uintptr(f.Offset)}
+		}
+		engine.instr[srt.NumField()] = encInstr{encStructTerminator, 0, 0, 0}
+	} else {
+		engine.instr = make([]encInstr, 1)
+		op, indir, err := encOpFor(rt)
 		if err != nil {
 			return nil, err
 		}
-		engine.instr[fieldnum] = encInstr{op, fieldnum, indir, uintptr(f.Offset)}
+		engine.instr[0] = encInstr{op, singletonField, indir, 0} // offset is zero
 	}
-	engine.instr[srt.NumField()] = encInstr{encStructTerminator, 0, 0, 0}
 	return engine, nil
 }
 
@@ -772,14 +805,14 @@ func encode(b *bytes.Buffer, e interface{}) os.Error {
 	for i := 0; i < indir; i++ {
 		v = reflect.Indirect(v)
 	}
-	if _, ok := v.(*reflect.StructValue); !ok {
-		return os.ErrorString("gob: encode can't handle " + v.Type().String())
-	}
 	typeLock.Lock()
 	engine, err := getEncEngine(rt)
 	typeLock.Unlock()
 	if err != nil {
 		return err
 	}
-	return encodeStruct(engine, b, v.Addr())
+	if _, ok := v.(*reflect.StructValue); ok {
+		return encodeStruct(engine, b, v.Addr())
+	}
+	return encodeSingle(engine, b, v.Addr())
 }
diff --git a/src/pkg/gob/encoder.go b/src/pkg/gob/encoder.go
index e24c18d20..28cf6f6e0 100644
--- a/src/pkg/gob/encoder.go
+++ b/src/pkg/gob/encoder.go
@@ -68,7 +68,7 @@ func (enc *Encoder) send() {
 	}
 }
 
-func (enc *Encoder) sendType(origt reflect.Type) {
+func (enc *Encoder) sendType(origt reflect.Type) (sent bool) {
 	// Drill down to the base type.
 	rt, _ := indirect(origt)
 
@@ -147,11 +147,6 @@ func (enc *Encoder) Encode(e interface{}) os.Error {
 
 	enc.state.err = nil
 	rt, _ := indirect(reflect.Typeof(e))
-	// Must be a struct
-	if _, ok := rt.(*reflect.StructType); !ok {
-		enc.badType(rt)
-		return enc.state.err
-	}
 
 	// Sanity check only: encoder should never come in with data present.
 	if enc.state.b.Len() > 0 || enc.countState.b.Len() > 0 {
@@ -163,10 +158,23 @@ func (enc *Encoder) Encode(e interface{}) os.Error {
 	// First, have we already sent this type?
 	if _, alreadySent := enc.sent[rt]; !alreadySent {
 		// No, so send it.
-		enc.sendType(rt)
+		sent := enc.sendType(rt)
 		if enc.state.err != nil {
 			return enc.state.err
 		}
+		// If the type info has still not been transmitted, it means we have
+		// a singleton basic type (int, []byte etc.) at top level.  We don't
+		// need to send the type info but we do need to update enc.sent.
+		if !sent {
+			typeLock.Lock()
+			info, err := getTypeInfo(rt)
+			typeLock.Unlock()
+			if err != nil {
+				enc.setError(err)
+				return err
+			}
+			enc.sent[rt] = info.id
+		}
 	}
 
 	// Identify the type of this top-level value.
diff --git a/src/pkg/gob/encoder_test.go b/src/pkg/gob/encoder_test.go
index 4250b8a9d..b578cd0f8 100644
--- a/src/pkg/gob/encoder_test.go
+++ b/src/pkg/gob/encoder_test.go
@@ -131,17 +131,10 @@ func TestBadData(t *testing.T) {
 	corruptDataCheck("\x03now is the time for all good men", errBadType, t)
 }
 
-// Types not supported by the Encoder (only structs work at the top level).
-// Basic types work implicitly.
+// Types not supported by the Encoder.
 var unsupportedValues = []interface{}{
-	3,
-	"hi",
-	7.2,
-	[]int{1, 2, 3},
-	[3]int{1, 2, 3},
 	make(chan int),
 	func(a int) bool { return true },
-	make(map[string]int),
 	new(interface{}),
 }
 
@@ -275,3 +268,59 @@ func TestDefaultsInArray(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+var testInt int
+var testFloat32 float32
+var testString string
+var testSlice []string
+var testMap map[string]int
+
+type SingleTest struct {
+	in  interface{}
+	out interface{}
+	err string
+}
+
+var singleTests = []SingleTest{
+	SingleTest{17, &testInt, ""},
+	SingleTest{float32(17.5), &testFloat32, ""},
+	SingleTest{"bike shed", &testString, ""},
+	SingleTest{[]string{"bike", "shed", "paint", "color"}, &testSlice, ""},
+	SingleTest{map[string]int{"seven": 7, "twelve": 12}, &testMap, ""},
+
+	// Decode errors
+	SingleTest{172, &testFloat32, "wrong type"},
+}
+
+func TestSingletons(t *testing.T) {
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	dec := NewDecoder(b)
+	for _, test := range singleTests {
+		b.Reset()
+		err := enc.Encode(test.in)
+		if err != nil {
+			t.Errorf("error encoding %v: %s", test.in, err)
+			continue
+		}
+		err = dec.Decode(test.out)
+		switch {
+		case err != nil && test.err == "":
+			t.Errorf("error decoding %v: %s", test.in, err)
+			continue
+		case err == nil && test.err != "":
+			t.Errorf("expected error decoding %v: %s", test.in, test.err)
+			continue
+		case err != nil && test.err != "":
+			if strings.Index(err.String(), test.err) < 0 {
+				t.Errorf("wrong error decoding %v: wanted %s, got %v", test.in, test.err, err)
+			}
+			continue
+		}
+		// Get rid of the pointer in the rhs
+		val := reflect.NewValue(test.out).(*reflect.PtrValue).Elem().Interface()
+		if !reflect.DeepEqual(test.in, val) {
+			t.Errorf("decoding int: expected %v got %v", test.in, val)
+		}
+	}
+}