[dev.typeparams] cmd/compile: cleanup wrapper code for generics

Simple change - added some comments, but also removed some TODO comments
which are now done or no longer a question. Cleaned up the initial check
for generic methods.

The one remaining TODO that really needs to be done is generating
dictionary wrappers for any methods involving embedded fields. Given we
are not doing this, I think this would only affect if a struct with an
embedded field with methods was converted to an interface containing the
embedded method, and then the method was called via that interface.

Change-Id: I6a8a2885de33ad60b313c1899b659daef7550dfb
Reviewed-on: https://go-review.googlesource.com/c/go/+/340260
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -303,6 +303,7 @@ func MapIterType(t *types.Type) *types.Type {
 // Generates stub functions as needed.
 func methods(t *types.Type) []*typeSig {
 	if t.HasShape() {
+		// Shape types have no methods.
 		return nil
 	}
 	// method type
@@ -1228,9 +1229,8 @@ func InterfaceMethodOffset(ityp *types.Type, i int64) int64 {
 // NeedRuntimeType ensures that a runtime type descriptor is emitted for t.
 func NeedRuntimeType(t *types.Type) {
 	if t.HasTParam() {
-		// Generic types don't have a runtime type descriptor (but will
+		// Generic types don't really exist at run-time and have no runtime
-		// have a dictionary)
+		// type descriptor.  But we do write out shape types.
-		// TODO: also shape type here?
 		return
 	}
 	if _, ok := signatset[t]; !ok {
@@ -1786,26 +1786,28 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 	if forItab && !types.IsDirectIface(rcvr) {
 		rcvr = rcvr.PtrTo()
 	}
-	generic := false
-	if !types.IsInterfaceMethod(method.Type) &&
-		(len(rcvr.RParams()) > 0 ||
-			rcvr.IsPtr() && len(rcvr.Elem().RParams()) > 0) { // TODO: right detection?
-		// Don't need dictionary if we are reaching a method (possibly via
-		// an embedded field) which is an interface method.
-		// TODO: check that we do the right thing when method is an interface method.
-		generic = true
 
-		targs := rcvr.RParams()
+	generic := false
-		if rcvr.IsPtr() {
+	// We don't need a dictionary if we are reaching a method (possibly via an
-			targs = rcvr.Elem().RParams()
+	// embedded field) which is an interface method.
+	if !types.IsInterfaceMethod(method.Type) {
+		rcvr1 := rcvr
+		if rcvr1.IsPtr() {
+			rcvr1 = rcvr.Elem()
 		}
-		// TODO: why do shape-instantiated types exist?
+		if len(rcvr1.RParams()) > 0 {
-		for _, t := range targs {
+			// If rcvr has rparams, remember method as generic, which
-			if t.HasShape() {
+			// means we need to add a dictionary to the wrapper.
-				base.Fatalf("method on type instantiated with shapes targ:%+v rcvr:%+v", t, rcvr)
+			generic = true
+			targs := rcvr1.RParams()
+			for _, t := range targs {
+				if t.HasShape() {
+					base.Fatalf("method on type instantiated with shapes targ:%+v rcvr:%+v", t, rcvr)
+				}
 			}
 		}
 	}
+
 	newnam := ir.MethodSym(rcvr, method.Sym)
 	lsym := newnam.Linksym()
 	if newnam.Siggen() {
@@ -1818,6 +1820,8 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 		return lsym
 	}
 
+	// For generic methods, we need to generate the wrapper even if the receiver
+	// types are identical, because we want to add the dictionary.
 	if !generic && types.Identical(rcvr, method.Type.Recv().Type) {
 		return lsym
 	}
@@ -1890,7 +1894,7 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 		if generic {
 			var args []ir.Node
 			var targs []*types.Type
-			if rcvr.IsPtr() { // TODO: correct condition?
+			if rcvr.IsPtr() {
 				targs = rcvr.Elem().RParams()
 			} else {
 				targs = rcvr.RParams()
@@ -1899,10 +1903,9 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 				fmt.Printf("%s\n", ir.MethodSym(orig, method.Sym).Name)
 				panic("multiple .inst.")
 			}
-			// Temporary fix: the wrapper for an auto-generated
+			// The wrapper for an auto-generated pointer/non-pointer
-			// pointer/non-pointer receiver method should share the
+			// receiver method should share the same dictionary as the
-			// same dictionary as the corresponding original
+			// corresponding original (user-written) method.
-			// (user-written) method.
 			baseOrig := orig
 			if baseOrig.IsPtr() && !method.Type.Recv().Type.IsPtr() {
 				baseOrig = baseOrig.Elem()
@@ -2058,7 +2061,6 @@ func getDictionary(gf *types.Sym, targs []*types.Type) ir.Node {
 	// Note: treat dictionary pointers as uintptrs, so they aren't pointers
 	// with respect to GC. That saves on stack scanning work, write barriers, etc.
 	// We can get away with it because dictionaries are global variables.
-	// TODO: use a cast, or is typing directly ok?
 	np.SetType(types.Types[types.TUINTPTR])
 	np.SetTypecheck(1)
 	return np