cmd/compile, runtime: add go:yeswritebarrierrec pragma

This pragma cancels the effect of go:nowritebarrierrec. This is useful
in the scheduler because there are places where we enter a function
without a valid P (and hence cannot have write barriers), but then
obtain a P. This allows us to annotate the function with
go:nowritebarrierrec and split out the part after we've obtained a P
into a go:yeswritebarrierrec function.

Change-Id: Ic8ce4b6d3c074a1ecd8280ad90eaf39f0ffbcc2a
Reviewed-on: https://go-review.googlesource.com/30938
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 78 insertions, 0 deletions

diff --git a/test/nowritebarrier.go b/test/nowritebarrier.go
new file mode 100644
index 0000000000..23dce753b0
--- /dev/null
+++ b/test/nowritebarrier.go
@@ -0,0 +1,78 @@
+// errorcheck -+
+
+// Copyright 2016 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 go:nowritebarrier and related directives.
+
+package p
+
+type t struct {
+	f *t
+}
+
+var x t
+var y *t
+
+//go:nowritebarrier
+func a1() {
+	x.f = y // ERROR "write barrier prohibited"
+	a2()    // no error
+}
+
+//go:noinline
+func a2() {
+	x.f = y
+}
+
+//go:nowritebarrierrec
+func b1() {
+	b2()
+}
+
+//go:noinline
+func b2() {
+	x.f = y // ERROR "write barrier prohibited by caller"
+}
+
+// Test recursive cycles through nowritebarrierrec and yeswritebarrierrec.
+
+//go:nowritebarrierrec
+func c1() {
+	c2()
+}
+
+//go:yeswritebarrierrec
+func c2() {
+	c3()
+}
+
+func c3() {
+	x.f = y
+	c4()
+}
+
+//go:nowritebarrierrec
+func c4() {
+	c2()
+}
+
+//go:nowritebarrierrec
+func d1() {
+	d2()
+}
+
+func d2() {
+	d3()
+}
+
+func d3() {
+	x.f = y // ERROR "write barrier prohibited by caller"
+	d4()
+}
+
+//go:yeswritebarrierrec
+func d4() {
+	d2()
+}