cmd/compile: allow static init for unsafe.Pointer(&x) where x is global

This avoids both a write barrier and then dynamic initialization
globals of the form

	var x something
	var xp = unsafe.Pointer(&x)

Using static initialization avoids emitting a relocation for &x,
which helps cgo.

Fixes #9411.

Change-Id: I0dbf480859cce6ab57ab805d1b8609c45b48f156
Reviewed-on: https://go-review.googlesource.com/11693Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>

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

@@ -302,6 +302,10 @@ func staticcopy(l *Node, r *Node, out **NodeList) bool {
 	orig := r
 	r = r.Name.Defn.Right
 
+	for r.Op == OCONVNOP {
+		r = r.Left
+	}
+
 	switch r.Op {
 	case ONAME:
 		if staticcopy(l, r, out) {
@@ -395,6 +399,10 @@ func staticcopy(l *Node, r *Node, out **NodeList) bool {
 func staticassign(l *Node, r *Node, out **NodeList) bool {
 	var n1 Node
 
+	for r.Op == OCONVNOP {
+		r = r.Left
+	}
+
 	switch r.Op {
 	//dump("not static", r);
 	default:

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

@@ -2194,13 +2194,20 @@ func needwritebarrier(l *Node, r *Node) bool {
 		return false
 	}
 
-	// No write barrier for implicit or explicit zeroing.
-	if r == nil || iszero(r) {
+	// No write barrier for implicit zeroing.
+	if r == nil {
 		return false
 	}
 
-	// No write barrier for initialization to constant.
-	if r.Op == OLITERAL {
+	// Ignore no-op conversions when making decision.
+	// Ensures that xp = unsafe.Pointer(&x) is treated
+	// the same as xp = &x.
+	for r.Op == OCONVNOP {
+		r = r.Left
+	}
+
+	// No write barrier for zeroing or initialization to constant.
+	if iszero(r) || r.Op == OLITERAL {
 		return false
 	}
 

test/sinit.go

@@ -10,6 +10,8 @@
 
 package p
 
+import "unsafe"
+
 // Should be no init func in the assembly.
 // All these initializations should be done at link time.
 
@@ -284,3 +286,6 @@ type Mer interface {
 }
 
 var _ Mer = (*T1)(nil)
+
+var Byte byte
+var PtrByte unsafe.Pointer = unsafe.Pointer(&Byte)