cmd/compile: move writebarrier pass after dse

This avoids generating writeBarrier.enabled
blocks for dead stores.

Change-Id: Ib11d8e2ba952f3f1f01d16776e40a7200a7683cf
Reviewed-on: https://go-review.googlesource.com/42012
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/compile.go

@@ -348,9 +348,9 @@ var passes = [...]pass{
 	{name: "late opt", fn: opt, required: true}, // TODO: split required rules and optimizing rules
 	{name: "generic deadcode", fn: deadcode},
 	{name: "check bce", fn: checkbce},
-	{name: "writebarrier", fn: writebarrier, required: true}, // expand write barrier ops
 	{name: "fuse", fn: fuse},
 	{name: "dse", fn: dse},
+	{name: "writebarrier", fn: writebarrier, required: true}, // expand write barrier ops
 	{name: "insert resched checks", fn: insertLoopReschedChecks,
 		disabled: objabi.Preemptibleloops_enabled == 0}, // insert resched checks in loops.
 	{name: "tighten", fn: tighten}, // move values closer to their uses

test/writebarrier.go

@@ -11,14 +11,14 @@ package p
 import "unsafe"
 
 func f(x **byte, y *byte) {
-	*x = y // ERROR "write barrier"
+	*x = y // no barrier (dead store)
 
 	z := y // no barrier
 	*x = z // ERROR "write barrier"
 }
 
 func f1(x *[]byte, y []byte) {
-	*x = y // ERROR "write barrier"
+	*x = y // no barrier (dead store)
 
 	z := y // no barrier
 	*x = z // ERROR "write barrier"
@@ -32,21 +32,21 @@ func f1a(x *[]byte, y *[]byte) {
 }
 
 func f2(x *interface{}, y interface{}) {
-	*x = y // ERROR "write barrier"
+	*x = y // no barrier (dead store)
 
 	z := y // no barrier
 	*x = z // ERROR "write barrier"
 }
 
 func f2a(x *interface{}, y *interface{}) {
-	*x = *y // ERROR "write barrier"
+	*x = *y // no barrier (dead store)
 
 	z := y // no barrier
 	*x = z // ERROR "write barrier"
 }
 
 func f3(x *string, y string) {
-	*x = y // ERROR "write barrier"
+	*x = y // no barrier (dead store)
 
 	z := y // no barrier
 	*x = z // ERROR "write barrier"
@@ -204,12 +204,18 @@ var y21 struct {
 }
 var z21 int
 
-func f21(x *int) {
-	// Global -> heap pointer updates must have write barriers.
+// f21x: Global -> heap pointer updates must have write barriers.
+func f21a(x *int) {
 	x21 = x   // ERROR "write barrier"
 	y21.x = x // ERROR "write barrier"
+}
+
+func f21b(x *int) {
 	x21 = &z21   // ERROR "write barrier"
 	y21.x = &z21 // ERROR "write barrier"
+}
+
+func f21c(x *int) {
 	y21 = struct{ x *int }{x} // ERROR "write barrier"
 }
 
@@ -229,10 +235,15 @@ type T23 struct {
 var t23 T23
 var i23 int
 
-func f23() {
-	// zeroing global needs write barrier for the hybrid barrier.
+// f23x: zeroing global needs write barrier for the hybrid barrier.
+func f23a() {
 	t23 = T23{} // ERROR "write barrier"
 	// also test partial assignments
 	t23 = T23{a: 1} // ERROR "write barrier"
+}
+
+func f23b() {
+	t23 = T23{} // no barrier (dead store)
+	// also test partial assignments
 	t23 = T23{p: &i23} // ERROR "write barrier"
 }