cmd/compile: correctly import labels, gotos, and fallthroughs

The importer had several bugs with respect to labels and gotos:
- it didn't create a new ONAME node for label names (label dcl,
  goto, continue, and break)
- it overwrote the symbol for gotos with the dclstack
- it didn't set the dclstack for labels

In the process changed export format slightly to always assume
a label name for labels and gotos, and never assume a label for
fallthroughs.

For fallthroughs and switch cases, now also set Xoffset like in
the parser. (Not setting it, i.e., using 0 was ok since this is
only used for verifying correct use of fallthroughs, which was
checked already. But it's an extra level of verification of the
import.)

Fixes #15838.

Change-Id: I3637f6314b8651c918df0c8cd70cd858c92bd483
Reviewed-on: https://go-review.googlesource.com/23445
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/bexport.go

@@ -1488,19 +1488,18 @@ func (p *exporter) stmt(n *Node) {
 		p.stmtList(n.List)
 		p.stmtList(n.Nbody)
 
-	case OFALL:
-		op = OXFALL
-		fallthrough
+	case OFALL, OXFALL:
+		p.op(OXFALL)
 
-	case OBREAK, OCONTINUE, OGOTO, OXFALL:
+	case OBREAK, OCONTINUE:
 		p.op(op)
 		p.exprsOrNil(n.Left, nil)
 
 	case OEMPTY:
 		// nothing to emit
 
-	case OLABEL:
-		p.op(OLABEL)
+	case OGOTO, OLABEL:
+		p.op(op)
 		p.expr(n.Left)
 
 	default:

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

@@ -1038,6 +1038,7 @@ func (p *importer) node() *Node {
 	case OXCASE:
 		markdcl()
 		n := Nod(OXCASE, nil, nil)
+		n.Xoffset = int64(block)
 		n.List.Set(p.exprList())
 		// TODO(gri) eventually we must declare variables for type switch
 		// statements (type switch statements are not yet exported)
@@ -1048,16 +1049,24 @@ func (p *importer) node() *Node {
 	// case OFALL:
 	// 	unreachable - mapped to OXFALL case below by exporter
 
-	case OBREAK, OCONTINUE, OGOTO, OXFALL:
+	case OXFALL:
+		n := Nod(OXFALL, nil, nil)
+		n.Xoffset = int64(block)
+		return n
+
+	case OBREAK, OCONTINUE:
 		left, _ := p.exprsOrNil()
+		if left != nil {
+			left = newname(left.Sym)
+		}
 		return Nod(op, left, nil)
 
 	// case OEMPTY:
 	// 	unreachable - not emitted by exporter
 
-	case OLABEL:
-		n := Nod(OLABEL, p.expr(), nil)
-		n.Left.Sym = dclstack // context, for goto restrictions
+	case OGOTO, OLABEL:
+		n := Nod(op, newname(p.expr().Sym), nil)
+		n.Sym = dclstack // context, for goto restrictions
 		return n
 
 	case OEND:

test/fixedbugs/issue15838.dir/a.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+func F1() {
+L:
+	goto L
+}
+
+func F2() {
+L:
+	for {
+		break L
+	}
+}
+
+func F3() {
+L:
+	for {
+		continue L
+	}
+}
+
+func F4() {
+	switch {
+	case true:
+		fallthrough
+	default:
+	}
+}
+
+type T struct{}
+
+func (T) M1() {
+L:
+	goto L
+}
+
+func (T) M2() {
+L:
+	for {
+		break L
+	}
+}
+
+func (T) M3() {
+L:
+	for {
+		continue L
+	}
+}
+
+func (T) M4() {
+	switch {
+	case true:
+		fallthrough
+	default:
+	}
+}

test/fixedbugs/issue15838.dir/b.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "./a"
+
+type T struct{ a.T }

test/fixedbugs/issue15838.go

+// compiledir
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for issue #15838, and related failures.
+// Make sure the importer correctly sets up nodes for
+// label decls, goto, continue, break, and fallthrough
+// statements.
+
+package ignored