cmd/gc: record &x[0] as taking address of x, if x is an array

Not recording the address being taken was causing
the liveness analysis not to preserve x in the absence
of direct references to x, which in turn was making the
net test fail with GOGC=0.

In addition to the test, this fixes a bug wherein
        GOGC=0 go test -short net
crashed if liveness analysis was in use (like at tip, not like Go 1.2).

TBR=ken2
CC=golang-codereviews
https://golang.org/cl/64470043

src/cmd/gc/go.h

@@ -1452,6 +1452,7 @@ void	walkstmt(Node **np);
 void	walkstmtlist(NodeList *l);
 Node*	conv(Node*, Type*);
 int	candiscard(Node*);
+Node*	outervalue(Node*);
 
 /*
  *	arch-specific ggen.c/gsubr.c/gobj.c/pgen.c/plive.c

src/cmd/gc/typecheck.c

@@ -721,7 +721,8 @@ reswitch:
 		if(n->left->type == T)
 			goto error;
 		checklvalue(n->left, "take the address of");
-		for(l=n->left; l->op == ODOT; l=l->left)
+		r = outervalue(n->left);
+		for(l = n->left; l != r; l = l->left)
 			l->addrtaken = 1;
 		if(l->orig != l && l->op == ONAME)
 			fatal("found non-orig name node %N", l);

src/cmd/gc/walk.c

@@ -2205,7 +2205,7 @@ reorder3save(Node **np, NodeList *all, NodeList *stop, NodeList **early)
  * what's the outer value that a write to n affects?
  * outer value means containing struct or array.
  */
-static Node*
+Node*
 outervalue(Node *n)
 {	
 	for(;;) {

test/fixedbugs/bug483.go

+// run
+
+// Copyright 2014 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 for a garbage collection bug involving not
+// marking x as having its address taken by &x[0]
+// when x is an array value.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"runtime"
+)
+
+func main() {
+	var x = [4]struct{ x, y interface{} }{
+		{"a", "b"},
+		{"c", "d"},
+		{"e", "f"},
+		{"g", "h"},
+	}
+
+	var buf bytes.Buffer
+	for _, z := range x {
+		runtime.GC()
+		fmt.Fprintf(&buf, "%s %s ", z.x.(string), z.y.(string))
+	}
+
+	if buf.String() != "a b c d e f g h " {
+		println("BUG wrong output\n", buf.String())
+	}
+}