cmd/compile: add transform functions for OXDOT and builtins

Pull out the tranformation part of the typechecking functions for:
 - selector expressions (OXDOT)
 - calls to builtin functions (which go through the typechecker loop
   twice, once for the call and once for each different kind of
   builtin).

Some of the transformation functions create new nodes that should have
the same type as the original node. For consistency, now each of the
transformation functions requires that the node passed in has its type
and typecheck flag set. If the transformation function replaces or adds
new nodes, it will set the type and typecheck flag for those new nodes.

As usual, passes all the gotests, even with -G=3 enabled.

Change-Id: Ic48b0ce5f58425f4a358afa78315bfc7c28066c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/304729
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

5 files changed, 342 insertions, 51 deletions

diff --git a/src/cmd/compile/internal/noder/helpers.go b/src/cmd/compile/internal/noder/helpers.go
index 82428daa4a..9ebf17aae6 100644
--- a/src/cmd/compile/internal/noder/helpers.go
+++ b/src/cmd/compile/internal/noder/helpers.go
@@ -81,8 +81,8 @@ func Binary(pos src.XPos, op ir.Op, typ *types.Type, x, y ir.Node) ir.Node {
 			n.SetTypecheck(3)
 			return n
 		}
-		n1 := transformAdd(n)
-		return typed(typ, n1)
+		typed(typ, n)
+		return transformAdd(n)
 	default:
 		return typed(x.Type(), ir.NewBinaryExpr(pos, op, x, y))
 	}
@@ -99,9 +99,8 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 			// the type.
 			return typed(typ, n)
 		}
-		n1 := transformConvCall(n)
-		n1.SetTypecheck(1)
-		return n1
+		typed(typ, n)
+		return transformConvCall(n)
 	}
 
 	if fun, ok := fun.(*ir.Name); ok && fun.BuiltinOp != 0 {
@@ -133,12 +132,8 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 			}
 		}
 
-		switch fun.BuiltinOp {
-		case ir.OCLOSE, ir.ODELETE, ir.OPANIC, ir.OPRINT, ir.OPRINTN:
-			return typecheck.Stmt(n)
-		default:
-			return typecheck.Expr(n)
-		}
+		typed(typ, n)
+		return transformBuiltin(n)
 	}
 
 	// Add information, now that we know that fun is actually being called.
@@ -176,8 +171,8 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 	if fun.Op() != ir.OFUNCINST {
 		// If no type params, do the normal call transformations. This
 		// will convert OCALL to OCALLFUNC.
-		transformCall(n)
 		typed(typ, n)
+		transformCall(n)
 		return n
 	}
 
@@ -195,8 +190,9 @@ func Compare(pos src.XPos, typ *types.Type, op ir.Op, x, y ir.Node) ir.Node {
 		n.SetTypecheck(3)
 		return n
 	}
+	typed(typ, n)
 	transformCompare(n)
-	return typed(typ, n)
+	return n
 }
 
 func Deref(pos src.XPos, x ir.Node) *ir.StarExpr {
@@ -291,8 +287,9 @@ func Slice(pos src.XPos, typ *types.Type, x, low, high, max ir.Node) ir.Node {
 		n.SetTypecheck(3)
 		return n
 	}
+	typed(typ, n)
 	transformSlice(n)
-	return typed(typ, n)
+	return n
 }
 
 func Unary(pos src.XPos, op ir.Op, x ir.Node) ir.Node {
diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index ba01f0424b..45864763d4 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -369,12 +369,12 @@ func (subst *subster) node(n ir.Node) ir.Node {
 		if x.Typecheck() == 3 {
 			// These are nodes whose transforms were delayed until
 			// their instantiated type was known.
+			m.SetTypecheck(1)
 			if typecheck.IsCmp(x.Op()) {
 				transformCompare(m.(*ir.BinaryExpr))
 			} else {
 				switch x.Op() {
-				case ir.OSLICE:
-				case ir.OSLICE3:
+				case ir.OSLICE, ir.OSLICE3:
 					transformSlice(m.(*ir.SliceExpr))
 
 				case ir.OADD:
@@ -400,7 +400,6 @@ func (subst *subster) node(n ir.Node) ir.Node {
 					base.Fatalf("Unexpected node with Typecheck() == 3")
 				}
 			}
-			m.SetTypecheck(1)
 		}
 
 		switch x.Op() {
@@ -421,15 +420,18 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			}
 
 		case ir.OXDOT:
-			// A method value/call via a type param will have been left as an
-			// OXDOT. When we see this during stenciling, finish the
-			// typechecking, now that we have the instantiated receiver type.
-			// We need to do this now, since the access/selection to the
-			// method for the real type is very different from the selection
-			// for the type param.
-			m.SetTypecheck(0)
-			// m will transform to an OCALLPART
-			typecheck.Expr(m)
+			// A method value/call via a type param will have been
+			// left as an OXDOT. When we see this during stenciling,
+			// finish the transformation, now that we have the
+			// instantiated receiver type. We need to do this now,
+			// since the access/selection to the method for the real
+			// type is very different from the selection for the type
+			// param. m will be transformed to an OCALLPART node. It
+			// will be transformed to an ODOTMETH or ODOTINTER node if
+			// we find in the OCALL case below that the method value
+			// is actually called.
+			transformDot(m.(*ir.SelectorExpr), false)
+			m.SetTypecheck(1)
 
 		case ir.OCALL:
 			call := m.(*ir.CallExpr)
@@ -437,15 +439,12 @@ func (subst *subster) node(n ir.Node) ir.Node {
 				// Transform the conversion, now that we know the
 				// type argument.
 				m = transformConvCall(m.(*ir.CallExpr))
-				m.SetTypecheck(1)
 			} else if call.X.Op() == ir.OCALLPART {
-				// Redo the typechecking of OXDOT, now that we
+				// Redo the transformation of OXDOT, now that we
 				// know the method value is being called. Then
 				// transform the call.
 				call.X.(*ir.SelectorExpr).SetOp(ir.OXDOT)
-				call.X.SetTypecheck(0)
-				call.X.SetType(nil)
-				typecheck.Callee(call.X)
+				transformDot(call.X.(*ir.SelectorExpr), true)
 				transformCall(call)
 			} else if call.X.Op() == ir.ODOT || call.X.Op() == ir.ODOTPTR {
 				// An OXDOT for a generic receiver was resolved to
@@ -456,11 +455,9 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			} else if name := call.X.Name(); name != nil {
 				switch name.BuiltinOp {
 				case ir.OMAKE, ir.OREAL, ir.OIMAG, ir.OLEN, ir.OCAP, ir.OAPPEND:
-					// Call old typechecker (to do any
-					// transformations) now that we know the
-					// type of the args.
-					m.SetTypecheck(0)
-					m = typecheck.Expr(m)
+					// Transform these builtins now that we
+					// know the type of the args.
+					m = transformBuiltin(call)
 				default:
 					base.FatalfAt(call.Pos(), "Unexpected builtin op")
 				}
diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index e90d374d0f..489a535231 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -11,6 +11,10 @@
 //    - Setting the actual type of existing nodes (already done based on
 //      type info from types2)
 //    - Dealing with untyped constants (which types2 has already resolved)
+//
+// Each of the transformation functions requires that node passed in has its type
+// and typecheck flag set. If the transformation function replaces or adds new
+// nodes, it will set the type and typecheck flag for those new nodes.
 
 package noder
 
@@ -19,6 +23,7 @@ import (
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"fmt"
 	"go/constant"
 )
 
@@ -27,6 +32,7 @@ import (
 // transformAdd transforms an addition operation (currently just addition of
 // strings). Corresponds to the "binary operators" case in typecheck.typecheck1.
 func transformAdd(n *ir.BinaryExpr) ir.Node {
+	assert(n.Type() != nil && n.Typecheck() == 1)
 	l := n.X
 	if l.Type().IsString() {
 		var add *ir.AddStringExpr
@@ -43,7 +49,7 @@ func transformAdd(n *ir.BinaryExpr) ir.Node {
 		} else {
 			add.List.Append(r)
 		}
-		add.SetType(l.Type())
+		typed(l.Type(), add)
 		return add
 	}
 	return n
@@ -74,7 +80,6 @@ func stringtoruneslit(n *ir.ConvExpr) ir.Node {
 func transformConv(n *ir.ConvExpr) ir.Node {
 	t := n.X.Type()
 	op, _ := typecheck.Convertop(n.X.Op() == ir.OLITERAL, t, n.Type())
-	assert(op != ir.OXXX)
 	n.SetOp(op)
 	switch n.Op() {
 	case ir.OCONVNOP:
@@ -103,15 +108,18 @@ func transformConv(n *ir.ConvExpr) ir.Node {
 // transformConvCall transforms a conversion call. Corresponds to the OTYPE part of
 // typecheck.tcCall.
 func transformConvCall(n *ir.CallExpr) ir.Node {
+	assert(n.Type() != nil && n.Typecheck() == 1)
 	arg := n.Args[0]
 	n1 := ir.NewConvExpr(n.Pos(), ir.OCONV, nil, arg)
-	n1.SetType(n.X.Type())
+	typed(n.X.Type(), n1)
 	return transformConv(n1)
 }
 
 // transformCall transforms a normal function/method call. Corresponds to last half
 // (non-conversion, non-builtin part) of typecheck.tcCall.
 func transformCall(n *ir.CallExpr) {
+	// n.Type() can be nil for calls with no return value
+	assert(n.Typecheck() == 1)
 	transformArgs(n)
 	l := n.X
 	t := l.Type()
@@ -160,6 +168,7 @@ func transformCall(n *ir.CallExpr) {
 // equals). Corresponds to the "comparison operators" case in
 // typecheck.typecheck1, including tcArith.
 func transformCompare(n *ir.BinaryExpr) {
+	assert(n.Type() != nil && n.Typecheck() == 1)
 	if (n.Op() == ir.OEQ || n.Op() == ir.ONE) && !types.Identical(n.X.Type(), n.Y.Type()) {
 		// Comparison is okay as long as one side is assignable to the
 		// other. The only allowed case where the conversion is not CONVNOP is
@@ -214,6 +223,7 @@ func implicitstar(n ir.Node) ir.Node {
 
 // transformIndex transforms an index operation.  Corresponds to typecheck.tcIndex.
 func transformIndex(n *ir.IndexExpr) {
+	assert(n.Type() != nil && n.Typecheck() == 1)
 	n.X = implicitstar(n.X)
 	l := n.X
 	t := l.Type()
@@ -230,6 +240,7 @@ func transformIndex(n *ir.IndexExpr) {
 
 // transformSlice transforms a slice operation.  Corresponds to typecheck.tcSlice.
 func transformSlice(n *ir.SliceExpr) {
+	assert(n.Type() != nil && n.Typecheck() == 1)
 	l := n.X
 	if l.Type().IsArray() {
 		addr := typecheck.NodAddr(n.X)
@@ -521,3 +532,279 @@ func transformSelect(sel *ir.SelectStmt) {
 func transformAsOp(n *ir.AssignOpStmt) {
 	transformCheckAssign(n, n.X)
 }
+
+// transformDot transforms an OXDOT (or ODOT) or ODOT, ODOTPTR, ODOTMETH,
+// ODOTINTER, or OCALLPART, as appropriate. It adds in extra nodes as needed to
+// access embedded fields. Corresponds to typecheck.tcDot.
+func transformDot(n *ir.SelectorExpr, isCall bool) ir.Node {
+	assert(n.Type() != nil && n.Typecheck() == 1)
+	if n.Op() == ir.OXDOT {
+		n = typecheck.AddImplicitDots(n)
+		n.SetOp(ir.ODOT)
+	}
+
+	t := n.X.Type()
+
+	if n.X.Op() == ir.OTYPE {
+		return transformMethodExpr(n)
+	}
+
+	if t.IsPtr() && !t.Elem().IsInterface() {
+		t = t.Elem()
+		n.SetOp(ir.ODOTPTR)
+	}
+
+	f := typecheck.Lookdot(n, t, 0)
+	assert(f != nil)
+
+	if (n.Op() == ir.ODOTINTER || n.Op() == ir.ODOTMETH) && !isCall {
+		n.SetOp(ir.OCALLPART)
+		n.SetType(typecheck.MethodValueWrapper(n).Type())
+	}
+	return n
+}
+
+// Corresponds to typecheck.typecheckMethodExpr.
+func transformMethodExpr(n *ir.SelectorExpr) (res ir.Node) {
+	t := n.X.Type()
+
+	// Compute the method set for t.
+	var ms *types.Fields
+	if t.IsInterface() {
+		ms = t.Fields()
+	} else {
+		mt := types.ReceiverBaseType(t)
+		typecheck.CalcMethods(mt)
+		ms = mt.AllMethods()
+
+		// The method expression T.m requires a wrapper when T
+		// is different from m's declared receiver type. We
+		// normally generate these wrappers while writing out
+		// runtime type descriptors, which is always done for
+		// types declared at package scope. However, we need
+		// to make sure to generate wrappers for anonymous
+		// receiver types too.
+		if mt.Sym() == nil {
+			typecheck.NeedRuntimeType(t)
+		}
+	}
+
+	s := n.Sel
+	m := typecheck.Lookdot1(n, s, t, ms, 0)
+	assert(m != nil)
+
+	n.SetOp(ir.OMETHEXPR)
+	n.Selection = m
+	n.SetType(typecheck.NewMethodType(m.Type, n.X.Type()))
+	return n
+}
+
+// Corresponds to typecheck.tcAppend.
+func transformAppend(n *ir.CallExpr) ir.Node {
+	transformArgs(n)
+	args := n.Args
+	t := args[0].Type()
+	assert(t.IsSlice())
+
+	if n.IsDDD {
+		if t.Elem().IsKind(types.TUINT8) && args[1].Type().IsString() {
+			return n
+		}
+
+		args[1] = assignconvfn(args[1], t.Underlying())
+		return n
+	}
+
+	as := args[1:]
+	for i, n := range as {
+		assert(n.Type() != nil)
+		as[i] = assignconvfn(n, t.Elem())
+	}
+	return n
+}
+
+// Corresponds to typecheck.tcComplex.
+func transformComplex(n *ir.BinaryExpr) ir.Node {
+	l := n.X
+	r := n.Y
+
+	assert(types.Identical(l.Type(), r.Type()))
+
+	var t *types.Type
+	switch l.Type().Kind() {
+	case types.TFLOAT32:
+		t = types.Types[types.TCOMPLEX64]
+	case types.TFLOAT64:
+		t = types.Types[types.TCOMPLEX128]
+	default:
+		panic(fmt.Sprintf("transformComplex: unexpected type %v", l.Type()))
+	}
+
+	// Must set the type here for generics, because this can't be determined
+	// by substitution of the generic types.
+	typed(t, n)
+	return n
+}
+
+// Corresponds to typecheck.tcDelete.
+func transformDelete(n *ir.CallExpr) ir.Node {
+	transformArgs(n)
+	args := n.Args
+	assert(len(args) == 2)
+
+	l := args[0]
+	r := args[1]
+
+	args[1] = assignconvfn(r, l.Type().Key())
+	return n
+}
+
+// Corresponds to typecheck.tcMake.
+func transformMake(n *ir.CallExpr) ir.Node {
+	args := n.Args
+
+	n.Args = nil
+	l := args[0]
+	t := l.Type()
+	assert(t != nil)
+
+	i := 1
+	var nn ir.Node
+	switch t.Kind() {
+	case types.TSLICE:
+		l = args[i]
+		i++
+		var r ir.Node
+		if i < len(args) {
+			r = args[i]
+			i++
+		}
+		nn = ir.NewMakeExpr(n.Pos(), ir.OMAKESLICE, l, r)
+
+	case types.TMAP:
+		if i < len(args) {
+			l = args[i]
+			i++
+		} else {
+			l = ir.NewInt(0)
+		}
+		nn = ir.NewMakeExpr(n.Pos(), ir.OMAKEMAP, l, nil)
+		nn.SetEsc(n.Esc())
+
+	case types.TCHAN:
+		l = nil
+		if i < len(args) {
+			l = args[i]
+			i++
+		} else {
+			l = ir.NewInt(0)
+		}
+		nn = ir.NewMakeExpr(n.Pos(), ir.OMAKECHAN, l, nil)
+	default:
+		panic(fmt.Sprintf("transformMake: unexpected type %v", t))
+	}
+
+	assert(i == len(args))
+	typed(n.Type(), nn)
+	return nn
+}
+
+// Corresponds to typecheck.tcPanic.
+func transformPanic(n *ir.UnaryExpr) ir.Node {
+	n.X = assignconvfn(n.X, types.Types[types.TINTER])
+	return n
+}
+
+// Corresponds to typecheck.tcPrint.
+func transformPrint(n *ir.CallExpr) ir.Node {
+	transformArgs(n)
+	return n
+}
+
+// Corresponds to typecheck.tcRealImag.
+func transformRealImag(n *ir.UnaryExpr) ir.Node {
+	l := n.X
+	var t *types.Type
+
+	// Determine result type.
+	switch l.Type().Kind() {
+	case types.TCOMPLEX64:
+		t = types.Types[types.TFLOAT32]
+	case types.TCOMPLEX128:
+		t = types.Types[types.TFLOAT64]
+	default:
+		panic(fmt.Sprintf("transformRealImag: unexpected type %v", l.Type()))
+	}
+
+	// Must set the type here for generics, because this can't be determined
+	// by substitution of the generic types.
+	typed(t, n)
+	return n
+}
+
+// Corresponds to typecheck.tcLenCap.
+func transformLenCap(n *ir.UnaryExpr) ir.Node {
+	n.X = implicitstar(n.X)
+	return n
+}
+
+// Corresponds to Builtin part of tcCall.
+func transformBuiltin(n *ir.CallExpr) ir.Node {
+	// n.Type() can be nil for builtins with no return value
+	assert(n.Typecheck() == 1)
+	fun := n.X.(*ir.Name)
+	op := fun.BuiltinOp
+
+	switch op {
+	case ir.OAPPEND, ir.ODELETE, ir.OMAKE, ir.OPRINT, ir.OPRINTN, ir.ORECOVER:
+		n.SetOp(op)
+		n.X = nil
+		switch op {
+		case ir.OAPPEND:
+			return transformAppend(n)
+		case ir.ODELETE:
+			return transformDelete(n)
+		case ir.OMAKE:
+			return transformMake(n)
+		case ir.OPRINT, ir.OPRINTN:
+			return transformPrint(n)
+		case ir.ORECOVER:
+			// nothing more to do
+			return n
+		}
+
+	case ir.OCAP, ir.OCLOSE, ir.OIMAG, ir.OLEN, ir.OPANIC, ir.OREAL:
+		transformArgs(n)
+		fallthrough
+
+	case ir.ONEW, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
+		u := ir.NewUnaryExpr(n.Pos(), op, n.Args[0])
+		u1 := typed(n.Type(), ir.InitExpr(n.Init(), u)) // typecheckargs can add to old.Init
+		switch op {
+		case ir.OCAP, ir.OLEN:
+			return transformLenCap(u1.(*ir.UnaryExpr))
+		case ir.OREAL, ir.OIMAG:
+			return transformRealImag(u1.(*ir.UnaryExpr))
+		case ir.OPANIC:
+			return transformPanic(u1.(*ir.UnaryExpr))
+		case ir.OCLOSE, ir.ONEW, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
+			// nothing more to do
+			return u1
+		}
+
+	case ir.OCOMPLEX, ir.OCOPY:
+		transformArgs(n)
+		b := ir.NewBinaryExpr(n.Pos(), op, n.Args[0], n.Args[1])
+		n1 := typed(n.Type(), ir.InitExpr(n.Init(), b))
+		if op == ir.OCOPY {
+			// nothing more to do
+			return n1
+		}
+		return transformComplex(n1.(*ir.BinaryExpr))
+
+	default:
+		panic(fmt.Sprintf("transformBuiltin: unexpected op %v", op))
+	}
+
+	return n
+}
diff --git a/src/cmd/compile/internal/typecheck/expr.go b/src/cmd/compile/internal/typecheck/expr.go
index 7ab1670a45..24d141e8a2 100644
--- a/src/cmd/compile/internal/typecheck/expr.go
+++ b/src/cmd/compile/internal/typecheck/expr.go
@@ -366,9 +366,9 @@ func tcCompLit(n *ir.CompLitExpr) (res ir.Node) {
 				}
 				l := l.(*ir.StructKeyExpr)
 
-				f := lookdot1(nil, l.Field, t, t.Fields(), 0)
+				f := Lookdot1(nil, l.Field, t, t.Fields(), 0)
 				if f == nil {
-					if ci := lookdot1(nil, l.Field, t, t.Fields(), 2); ci != nil { // Case-insensitive lookup.
+					if ci := Lookdot1(nil, l.Field, t, t.Fields(), 2); ci != nil { // Case-insensitive lookup.
 						if visible(ci.Sym) {
 							base.Errorf("unknown field '%v' in struct literal of type %v (but does have %v)", l.Field, t, ci.Sym)
 						} else if nonexported(l.Field) && l.Field.Name == ci.Sym.Name { // Ensure exactness before the suggestion.
@@ -496,7 +496,7 @@ func tcDot(n *ir.SelectorExpr, top int) ir.Node {
 		return n
 	}
 
-	if lookdot(n, t, 0) == nil {
+	if Lookdot(n, t, 0) == nil {
 		// Legitimate field or method lookup failed, try to explain the error
 		switch {
 		case t.IsEmptyInterface():
@@ -506,12 +506,12 @@ func tcDot(n *ir.SelectorExpr, top int) ir.Node {
 			// Pointer to interface is almost always a mistake.
 			base.Errorf("%v undefined (type %v is pointer to interface, not interface)", n, n.X.Type())
 
-		case lookdot(n, t, 1) != nil:
+		case Lookdot(n, t, 1) != nil:
 			// Field or method matches by name, but it is not exported.
 			base.Errorf("%v undefined (cannot refer to unexported field or method %v)", n, n.Sel)
 
 		default:
-			if mt := lookdot(n, t, 2); mt != nil && visible(mt.Sym) { // Case-insensitive lookup.
+			if mt := Lookdot(n, t, 2); mt != nil && visible(mt.Sym) { // Case-insensitive lookup.
 				base.Errorf("%v undefined (type %v has no field or method %v, but does have %v)", n, n.X.Type(), n.Sel, mt.Sym)
 			} else {
 				base.Errorf("%v undefined (type %v has no field or method %v)", n, n.X.Type(), n.Sel)
diff --git a/src/cmd/compile/internal/typecheck/typecheck.go b/src/cmd/compile/internal/typecheck/typecheck.go
index f06a8623d0..54f7cd9efa 100644
--- a/src/cmd/compile/internal/typecheck/typecheck.go
+++ b/src/cmd/compile/internal/typecheck/typecheck.go
@@ -1058,7 +1058,11 @@ func needTwoArgs(n *ir.CallExpr) (ir.Node, ir.Node, bool) {
 	return n.Args[0], n.Args[1], true
 }
 
-func lookdot1(errnode ir.Node, s *types.Sym, t *types.Type, fs *types.Fields, dostrcmp int) *types.Field {
+// Lookdot1 looks up the specified method s in the list fs of methods, returning
+// the matching field or nil. If dostrcmp is 0, it matches the symbols. If
+// dostrcmp is 1, it matches by name exactly. If dostrcmp is 2, it matches names
+// with case folding.
+func Lookdot1(errnode ir.Node, s *types.Sym, t *types.Type, fs *types.Fields, dostrcmp int) *types.Field {
 	var r *types.Field
 	for _, f := range fs.Slice() {
 		if dostrcmp != 0 && f.Sym.Name == s.Name {
@@ -1123,9 +1127,9 @@ func typecheckMethodExpr(n *ir.SelectorExpr) (res ir.Node) {
 	}
 
 	s := n.Sel
-	m := lookdot1(n, s, t, ms, 0)
+	m := Lookdot1(n, s, t, ms, 0)
 	if m == nil {
-		if lookdot1(n, s, t, ms, 1) != nil {
+		if Lookdot1(n, s, t, ms, 1) != nil {
 			base.Errorf("%v undefined (cannot refer to unexported method %v)", n, s)
 		} else if _, ambig := dotpath(s, t, nil, false); ambig {
 			base.Errorf("%v undefined (ambiguous selector)", n) // method or field
@@ -1155,20 +1159,26 @@ func derefall(t *types.Type) *types.Type {
 	return t
 }
 
-func lookdot(n *ir.SelectorExpr, t *types.Type, dostrcmp int) *types.Field {
+// Lookdot looks up field or method n.Sel in the type t and returns the matching
+// field. It transforms the op of node n to ODOTINTER or ODOTMETH, if appropriate.
+// It also may add a StarExpr node to n.X as needed for access to non-pointer
+// methods. If dostrcmp is 0, it matches the field/method with the exact symbol
+// as n.Sel (appropriate for exported fields). If dostrcmp is 1, it matches by name
+// exactly. If dostrcmp is 2, it matches names with case folding.
+func Lookdot(n *ir.SelectorExpr, t *types.Type, dostrcmp int) *types.Field {
 	s := n.Sel
 
 	types.CalcSize(t)
 	var f1 *types.Field
 	if t.IsStruct() || t.IsInterface() {
-		f1 = lookdot1(n, s, t, t.Fields(), dostrcmp)
+		f1 = Lookdot1(n, s, t, t.Fields(), dostrcmp)
 	}
 
 	var f2 *types.Field
 	if n.X.Type() == t || n.X.Type().Sym() == nil {
 		mt := types.ReceiverBaseType(t)
 		if mt != nil {
-			f2 = lookdot1(n, s, mt, mt.Methods(), dostrcmp)
+			f2 = Lookdot1(n, s, mt, mt.Methods(), dostrcmp)
 		}
 	}
 
@@ -1181,7 +1191,7 @@ func lookdot(n *ir.SelectorExpr, t *types.Type, dostrcmp int) *types.Field {
 			base.Errorf("%v is both field and method", n.Sel)
 		}
 		if f1.Offset == types.BADWIDTH {
-			base.Fatalf("lookdot badwidth t=%v, f1=%v@%p", t, f1, f1)
+			base.Fatalf("Lookdot badwidth t=%v, f1=%v@%p", t, f1, f1)
 		}
 		n.Selection = f1
 		n.SetType(f1.Type)