cmd/gc: emit code for extern = <N>

https://golang.org/cl/152700045/ made it possible for struct literals assigned to globals to use <N> as the RHS. Normally, this is to zero out variables on first use. Because globals are already zero (or their linker initialized value), we just ignored this.

Now that <N> can occur from non-initialization code, we need to emit this code. We don't use <N> for initialization of globals any more, so this shouldn't cause any excessive zeroing.

Fixes #8961.

LGTM=rsc
R=golang-codereviews, rsc
CC=bradfitz, golang-codereviews
https://golang.org/cl/154540044

src/cmd/gc/gen.c

@@ -732,13 +732,9 @@ cgen_as(Node *nl, Node *nr)
 	}
 
 	if(nr == N || iszero(nr)) {
-		// externals and heaps should already be clear
-		if(nr == N) {
-			if(nl->class == PEXTERN)
-				return;
-			if(nl->class & PHEAP)
-				return;
-		}
+		// heaps should already be clear
+		if(nr == N && (nl->class & PHEAP))
+			return;
 
 		tl = nl->type;
 		if(tl == T)

test/fixedbugs/issue8961.go

@@ -0,0 +1,20 @@
+// 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.
+
+// Issue 8961. Empty composite literals to small globals were not filled in
+package main
+
+type small struct { a int }
+var foo small
+
+func main() {
+	foo.a = 1
+	foo = small{}
+	if foo.a != 0 {
+		println("expected foo.a to be 0, was", foo.a)
+		panic("composite literal not filled in")
+	}
+}