reflect: make Value an opaque struct

Making Value opaque means we can drop the interface kludges
in favor of a significantly simpler and faster representation.
v.Kind() will be a prime candidate for inlining too.

On a Thinkpad X201s using -benchtime 10:

benchmark                           old ns/op    new ns/op    delta
json.BenchmarkCodeEncoder           284391780    157415960  -44.65%
json.BenchmarkCodeMarshal           286979140    158992020  -44.60%
json.BenchmarkCodeDecoder           717175800    388288220  -45.86%
json.BenchmarkCodeUnmarshal         734470500    404548520  -44.92%
json.BenchmarkCodeUnmarshalReuse    707172280    385258720  -45.52%
json.BenchmarkSkipValue              24630036     18557062  -24.66%

benchmark                            old MB/s     new MB/s  speedup
json.BenchmarkCodeEncoder                6.82        12.33    1.81x
json.BenchmarkCodeMarshal                6.76        12.20    1.80x
json.BenchmarkCodeDecoder                2.71         5.00    1.85x
json.BenchmarkCodeUnmarshal              2.64         4.80    1.82x
json.BenchmarkCodeUnmarshalReuse         2.74         5.04    1.84x
json.BenchmarkSkipValue                 77.92       103.42    1.33x

I cannot explain why BenchmarkSkipValue gets faster.
Maybe it is one of those code alignment things.

R=iant, r, gri, r
CC=golang-dev
https://golang.org/cl/5373101

src/pkg/reflect/all_test.go

@@ -16,6 +16,13 @@ import (
 	"unsafe"
 )
 
+func TestBool(t *testing.T) {
+	v := ValueOf(true)
+	if v.Bool() != true {
+		t.Fatal("ValueOf(true).Bool() = false")
+	}
+}
+
 type integer int
 type T struct {
 	a int
@@ -215,7 +222,8 @@ func TestTypes(t *testing.T) {
 
 func TestSet(t *testing.T) {
 	for i, tt := range valueTests {
-		v := ValueOf(tt.i).Elem()
+		v := ValueOf(tt.i)
+		v = v.Elem()
 		switch v.Kind() {
 		case Int:
 			v.SetInt(132)
@@ -1100,21 +1108,38 @@ func TestMethod(t *testing.T) {
 	}
 
 	// Curried method of value.
-	i = ValueOf(p).Method(1).Call([]Value{ValueOf(10)})[0].Int()
+	tfunc := TypeOf(func(int) int(nil))
+	v := ValueOf(p).Method(1)
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Value Method Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Value Method returned %d; want 250", i)
 	}
-	i = ValueOf(p).MethodByName("Dist").Call([]Value{ValueOf(10)})[0].Int()
+	v = ValueOf(p).MethodByName("Dist")
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Value MethodByName Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Value MethodByName returned %d; want 250", i)
 	}
 
 	// Curried method of pointer.
-	i = ValueOf(&p).Method(1).Call([]Value{ValueOf(10)})[0].Int()
+	v = ValueOf(&p).Method(1)
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Pointer Value Method Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Pointer Value Method returned %d; want 250", i)
 	}
-	i = ValueOf(&p).MethodByName("Dist").Call([]Value{ValueOf(10)})[0].Int()
+	v = ValueOf(&p).MethodByName("Dist")
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Pointer Value MethodByName Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Pointer Value MethodByName returned %d; want 250", i)
 	}
@@ -1129,11 +1154,19 @@ func TestMethod(t *testing.T) {
 		}
 	}{p}
 	pv := ValueOf(s).Field(0)
-	i = pv.Method(0).Call([]Value{ValueOf(10)})[0].Int()
+	v = pv.Method(0)
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Interface Method Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Interface Method returned %d; want 250", i)
 	}
-	i = pv.MethodByName("Dist").Call([]Value{ValueOf(10)})[0].Int()
+	v = pv.MethodByName("Dist")
+	if tt := v.Type(); tt != tfunc {
+		t.Errorf("Interface MethodByName Type is %s; want %s", tt, tfunc)
+	}
+	i = v.Call([]Value{ValueOf(10)})[0].Int()
 	if i != 250 {
 		t.Errorf("Interface MethodByName returned %d; want 250", i)
 	}

src/pkg/reflect/type.go

@@ -188,7 +188,7 @@ type Type interface {
 
 // A Kind represents the specific kind of type that a Type represents.
 // The zero Kind is not a valid kind.
-type Kind uint8
+type Kind uint
 
 const (
 	Invalid Kind = iota
@@ -455,14 +455,15 @@ func (t *uncommonType) Method(i int) (m Method) {
 	if p.name != nil {
 		m.Name = *p.name
 	}
-	flag := uint32(0)
+	fl := flag(Func) << flagKindShift
 	if p.pkgPath != nil {
 		m.PkgPath = *p.pkgPath
-		flag |= flagRO
+		fl |= flagRO
 	}
-	m.Type = toType(p.typ)
+	mt := toCommonType(p.typ)
+	m.Type = mt
 	fn := p.tfn
-	m.Func = valueFromIword(flag, m.Type, iword(fn))
+	m.Func = Value{mt, fn, fl}
 	m.Index = i
 	return
 }
@@ -768,7 +769,7 @@ func (t *structType) Field(i int) (f StructField) {
 	if i < 0 || i >= len(t.fields) {
 		return
 	}
-	p := t.fields[i]
+	p := &t.fields[i]
 	f.Type = toType(p.typ)
 	if p.name != nil {
 		f.Name = *p.name
@@ -867,10 +868,12 @@ L:
 
 	if n == 1 {
 		// Found matching field.
-		if len(ff.Index) <= depth {
+		if depth >= len(ff.Index) {
 			ff.Index = make([]int, depth+1)
 		}
-		ff.Index[depth] = fi
+		if len(ff.Index) > 1 {
+			ff.Index[depth] = fi
+		}
 	} else {
 		// None or more than one matching field found.
 		fd = inf
@@ -906,9 +909,6 @@ func toCommonType(p *runtime.Type) *commonType {
 		t commonType
 	}
 	x := unsafe.Pointer(p)
-	if uintptr(x)&reflectFlags != 0 {
-		panic("reflect: invalid interface value")
-	}
 	return &(*hdr)(x).t
 }
 
@@ -944,10 +944,12 @@ func (t *commonType) runtimeType() *runtime.Type {
 // PtrTo returns the pointer type with element t.
 // For example, if t represents type Foo, PtrTo(t) represents *Foo.
 func PtrTo(t Type) Type {
-	// If t records its pointer-to type, use it.
-	ct := t.(*commonType)
+	return t.(*commonType).ptrTo()
+}
+
+func (ct *commonType) ptrTo() *commonType {
 	if p := ct.ptrToThis; p != nil {
-		return toType(p)
+		return toCommonType(p)
 	}
 
 	// Otherwise, synthesize one.
@@ -959,7 +961,7 @@ func PtrTo(t Type) Type {
 	if m := ptrMap.m; m != nil {
 		if p := m[ct]; p != nil {
 			ptrMap.RUnlock()
-			return p.commonType.toType()
+			return &p.commonType
 		}
 	}
 	ptrMap.RUnlock()
@@ -971,7 +973,7 @@ func PtrTo(t Type) Type {
 	if p != nil {
 		// some other goroutine won the race and created it
 		ptrMap.Unlock()
-		return p
+		return &p.commonType
 	}
 
 	var rt struct {
@@ -1003,7 +1005,7 @@ func PtrTo(t Type) Type {
 
 	ptrMap.m[ct] = p
 	ptrMap.Unlock()
-	return p.commonType.toType()
+	return &p.commonType
 }
 
 func (t *commonType) Implements(u Type) bool {