cmd/internal/gc: add and test write barrier debug output

We can expand the test cases as we discover problems.
This is some basic tests plus all the things I got wrong
in some recent work.

Change-Id: Id875fcfaf74eb087ae42b441fe47a34c5b8ccb39
Reviewed-on: https://go-review.googlesource.com/9158
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/gc/lex.go

@@ -35,6 +35,8 @@ var goarch string
 
 var goroot string
 
+var Debug_wb int
+
 // Debug arguments.
 // These can be specified with the -d flag, as in "-d nil"
 // to set the debug_checknil variable. In general the list passed
@@ -46,6 +48,7 @@ var debugtab = []struct {
 	{"nil", &Debug_checknil},          // print information about nil checks
 	{"typeassert", &Debug_typeassert}, // print information about type assertion inlining
 	{"disablenil", &Disable_checknil}, // disable nil checks
+	{"wb", &Debug_wb},                 // print information about write barriers
 }
 
 // Our own isdigit, isspace, isalpha, isalnum that take care

src/cmd/internal/gc/walk.go

@@ -2218,6 +2218,9 @@ func applywritebarrier(n *Node, init **NodeList) *Node {
 		if Curfn != nil && Curfn.Func.Nowritebarrier {
 			Yyerror("write barrier prohibited")
 		}
+		if Debug_wb > 0 {
+			Warnl(int(n.Lineno), "write barrier")
+		}
 		t := n.Left.Type
 		l := Nod(OADDR, n.Left, nil)
 		l.Etype = 1 // addr does not escape

test/writebarrier.go

@@ -0,0 +1,110 @@
+// 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 where write barriers are and are not emitted.
+
+package p
+
+import "unsafe"
+
+func f(x **byte, y *byte) {
+	*x = y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f1(x *[]byte, y []byte) {
+	*x = y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f1a(x *[]byte, y *[]byte) {
+	*x = *y // ERROR "write barrier"
+
+	z := *y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f2(x *interface{}, y interface{}) {
+	*x = y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f2a(x *interface{}, y *interface{}) {
+	*x = *y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f3(x *string, y string) {
+	*x = y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f3a(x *string, y *string) {
+	*x = *y // ERROR "write barrier"
+
+	z := *y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f4(x *[2]string, y [2]string) {
+	*x = y // ERROR "write barrier"
+
+	z := y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+func f4a(x *[2]string, y *[2]string) {
+	*x = *y // ERROR "write barrier"
+
+	z := *y // no barrier
+	*x = z // ERROR "write barrier"
+}
+
+type T struct {
+	X *int
+	Y int
+	M map[int]int
+}
+
+func f5(t, u *T) {
+	t.X = &u.Y // ERROR "write barrier"
+}
+
+func f6(t *T) {
+	t.M = map[int]int{1: 2} // ERROR "write barrier"
+}
+
+func f7(x, y *int) []*int {
+	var z [3]*int
+	i := 0
+	z[i] = x // ERROR "write barrier"
+	i++
+	z[i] = y // ERROR "write barrier"
+	i++
+	return z[:i]
+}
+
+func f9(x *interface{}, v *byte) {
+	*x = v // ERROR "write barrier"
+}
+
+func f10(x *byte, f func(interface{})) {
+	f(x)
+}
+
+func f11(x *unsafe.Pointer, y unsafe.Pointer) {
+	*x = unsafe.Pointer(uintptr(y) + 1) // ERROR "write barrier"
+}