cmd/compile: better documentation around checkwidth

Change-Id: I5c7ec9676b5573c883c196459acea85aa9ff8130
Reviewed-on: https://go-review.googlesource.com/c/146021
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -208,7 +208,7 @@ func dowidth(t *types.Type) {
 	}
 
 	t.Width = -2
-	t.Align = 0
+	t.Align = 0 // 0 means use t.Width, below
 
 	et := t.Etype
 	switch et {
@@ -222,7 +222,7 @@ func dowidth(t *types.Type) {
 		}
 	}
 
-	w := int64(0)
+	var w int64
 	switch et {
 	default:
 		Fatalf("dowidth: unknown type: %v", t)
@@ -366,7 +366,7 @@ func dowidth(t *types.Type) {
 
 	t.Width = w
 	if t.Align == 0 {
-		if w > 8 || w&(w-1) != 0 || w == 0 {
+		if w == 0 || w > 8 || w&(w-1) != 0 {
 			Fatalf("invalid alignment for %v", t)
 		}
 		t.Align = uint8(w)
@@ -423,12 +423,11 @@ func checkwidth(t *types.Type) {
 		return
 	}
 
-	if t.Deferwidth() {
-		return
+	// if type has not yet been pushed on deferredTypeStack yet, do it now
+	if !t.Deferwidth() {
+		t.SetDeferwidth(true)
+		deferredTypeStack = append(deferredTypeStack, t)
 	}
-	t.SetDeferwidth(true)
-
-	deferredTypeStack = append(deferredTypeStack, t)
 }
 
 func defercheckwidth() {
@@ -443,6 +442,7 @@ func resumecheckwidth() {
 	if defercalc == 0 {
 		Fatalf("resumecheckwidth")
 	}
+
 	for len(deferredTypeStack) > 0 {
 		t := deferredTypeStack[len(deferredTypeStack)-1]
 		deferredTypeStack = deferredTypeStack[:len(deferredTypeStack)-1]

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

@@ -141,7 +141,7 @@ type Type struct {
 	Extra interface{}
 
 	// Width is the width of this Type in bytes.
-	Width int64
+	Width int64 // valid if Align > 0
 
 	methods    Fields
 	allMethods Fields
@@ -156,16 +156,16 @@ type Type struct {
 	Vargen int32 // unique name for OTYPE/ONAME
 
 	Etype EType // kind of type
-	Align uint8 // the required alignment of this type, in bytes
+	Align uint8 // the required alignment of this type, in bytes (0 means Width and Align have not yet been computed)
 
 	flags bitset8
 }
 
 const (
-	typeNotInHeap = 1 << iota // type cannot be heap allocated
-	typeBroke                 // broken type definition
-	typeNoalg                 // suppress hash and eq algorithm generation
-	typeDeferwidth
+	typeNotInHeap  = 1 << iota // type cannot be heap allocated
+	typeBroke                  // broken type definition
+	typeNoalg                  // suppress hash and eq algorithm generation
+	typeDeferwidth             // width computation has been deferred and type is on deferredTypeStack
 	typeRecur
 )