cmd/compile: don't deadcode eliminate labels

Dead-code eliminating labels is tricky because there might
be gotos that can still reach them.

Bug probably introduced with CL 91056

Fixes #28616

Change-Id: I6680465134e3486dcb658896f5172606cc51b104
Reviewed-on: https://go-review.googlesource.com/c/147817
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Iskander Sharipov <iskander.sharipov@intel.com>

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

@@ -4084,6 +4084,12 @@ func deadcode(fn *Node) {
 }
 
 func deadcodeslice(nn Nodes) {
+	var lastLabel = -1
+	for i, n := range nn.Slice() {
+		if n != nil && n.Op == OLABEL {
+			lastLabel = i
+		}
+	}
 	for i, n := range nn.Slice() {
 		// Cut is set to true when all nodes after i'th position
 		// should be removed.
@@ -4106,10 +4112,14 @@ func deadcodeslice(nn Nodes) {
 				// If "then" or "else" branch ends with panic or return statement,
 				// it is safe to remove all statements after this node.
 				// isterminating is not used to avoid goto-related complications.
+				// We must be careful not to deadcode-remove labels, as they
+				// might be the target of a goto. See issue 28616.
 				if body := body.Slice(); len(body) != 0 {
 					switch body[(len(body) - 1)].Op {
 					case ORETURN, ORETJMP, OPANIC:
-						cut = true
+						if i > lastLabel {
+							cut = true
+						}
 					}
 				}
 			}

test/fixedbugs/issue28616.go

+// compile
+
+// Copyright 2018 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.
+
+// Make sure we don't dead code eliminate a label.
+
+package p
+
+var i int
+
+func f() {
+
+	if true {
+
+		if i == 1 {
+			goto label
+		}
+
+		return
+	}
+
+label:
+}