cmd/compile: add missing write barriers for return statements

Copying return values to heap-escaped result parameters requires write
barriers.

Fixes #13587.

Change-Id: Ifa04ff7fa4adcc6393acdd82e527beb8f2a00a8b
Reviewed-on: https://go-review.googlesource.com/17762Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -322,6 +322,9 @@ func walkstmt(np **Node) {
 
 			ll := ascompatee(n.Op, rl, n.List, &n.Ninit)
 			n.List = reorder3(ll)
+			for lr := n.List; lr != nil; lr = lr.Next {
+				lr.N = applywritebarrier(lr.N, &n.Ninit)
+			}
 			break
 		}
 

test/fixedbugs/issue13587.go

+// errorcheck -0 -l -d=wb
+
+// Copyright 2015 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 write barrier for implicit assignments to result parameters
+// that have escaped to the heap.
+
+package issue13587
+
+import "errors"
+
+func escape(p *error)
+
+func F() (err error) {
+	escape(&err)
+	return errors.New("error") // ERROR "write barrier"
+}