cmd/compile: encapsulate and document two types.Type internal fields

Change-Id: I5f7d2155c2c3a47dabdf16fe46b122ede81de4fc
Reviewed-on: https://go-review.googlesource.com/c/147284Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/reflect.go

@@ -812,7 +812,7 @@ func dcommontype(lsym *obj.LSym, t *types.Type) int {
 
 	sptrWeak := true
 	var sptr *obj.LSym
-	if !t.IsPtr() || t.PtrBase != nil {
+	if !t.IsPtr() || t.IsPtrElem() {
 		tptr := types.NewPtr(t)
 		if t.Sym != nil || methods(tptr) != nil {
 			sptrWeak = false

src/cmd/compile/internal/gc/typecheck.go

@@ -3671,8 +3671,7 @@ func copytype(n *Node, t *types.Type) {
 	embedlineno := n.Type.ForwardType().Embedlineno
 	l := n.Type.ForwardType().Copyto
 
-	ptrBase := n.Type.PtrBase
-	sliceOf := n.Type.SliceOf
+	cache := n.Type.Cache
 
 	// TODO(mdempsky): Fix Type rekinding.
 	*n.Type = *t
@@ -3693,8 +3692,7 @@ func copytype(n *Node, t *types.Type) {
 
 	t.Nod = asTypesNode(n)
 	t.SetDeferwidth(false)
-	t.PtrBase = ptrBase
-	t.SliceOf = sliceOf
+	t.Cache = cache
 
 	// Propagate go:notinheap pragma from the Name to the Type.
 	if n.Name != nil && n.Name.Param != nil && n.Name.Param.Pragma&NotInHeap != 0 {

src/cmd/compile/internal/types/type.go

@@ -149,8 +149,11 @@ type Type struct {
 	Nod  *Node // canonical OTYPE node
 	Orig *Type // original type (type literal or predefined type)
 
-	SliceOf *Type
-	PtrBase *Type
+	// Cache of composite types, with this type being the element type.
+	Cache struct {
+		ptr   *Type // *T, or nil
+		slice *Type // []T, or nil
+	}
 
 	Sym    *Sym  // symbol containing name, for named types
 	Vargen int32 // unique name for OTYPE/ONAME
@@ -488,7 +491,7 @@ func NewArray(elem *Type, bound int64) *Type {
 
 // NewSlice returns the slice Type with element type elem.
 func NewSlice(elem *Type) *Type {
-	if t := elem.SliceOf; t != nil {
+	if t := elem.Cache.slice; t != nil {
 		if t.Elem() != elem {
 			Fatalf("elem mismatch")
 		}
@@ -497,7 +500,7 @@ func NewSlice(elem *Type) *Type {
 
 	t := New(TSLICE)
 	t.Extra = Slice{Elem: elem}
-	elem.SliceOf = t
+	elem.Cache.slice = t
 	return t
 }
 
@@ -551,7 +554,7 @@ func NewPtr(elem *Type) *Type {
 		Fatalf("NewPtr: pointer to elem Type is nil")
 	}
 
-	if t := elem.PtrBase; t != nil {
+	if t := elem.Cache.ptr; t != nil {
 		if t.Elem() != elem {
 			Fatalf("NewPtr: elem mismatch")
 		}
@@ -563,7 +566,7 @@ func NewPtr(elem *Type) *Type {
 	t.Width = int64(Widthptr)
 	t.Align = uint8(Widthptr)
 	if NewPtrCacheEnabled {
-		elem.PtrBase = t
+		elem.Cache.ptr = t
 	}
 	return t
 }
@@ -1258,6 +1261,11 @@ func (t *Type) IsPtr() bool {
 	return t.Etype == TPTR
 }
 
+// IsPtrElem reports whether t is the element of a pointer (to t).
+func (t *Type) IsPtrElem() bool {
+	return t.Cache.ptr != nil
+}
+
 // IsUnsafePtr reports whether t is an unsafe pointer.
 func (t *Type) IsUnsafePtr() bool {
 	return t.Etype == TUNSAFEPTR