cmd/compile: omit write barriers for slice clears of go:notinheap pointers

Currently,

  for i := range a {
    a[i] = nil
  }

will compile to have write barriers even if a is a slice of pointers
to go:notinheap types. This happens because the optimization that
transforms this into a memclr only asks it a's element type has
pointers, and not if it specifically has heap pointers.

Fix this by changing arrayClear to use HasHeapPointer instead of
types.Haspointers. We probably shouldn't have both of these functions,
since a pointer to a notinheap type is effectively a uintptr, but
that's not going to change in this CL.

Change-Id: I284b85bdec6ae1e641f894e8f577989facdb0cf1
Reviewed-on: https://go-review.googlesource.com/c/152723
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -586,7 +586,7 @@ func arrayClear(n, v1, v2, a *Node) bool {
 	n.Nbody.Append(nod(OAS, hn, tmp))
 
 	var fn *Node
-	if types.Haspointers(a.Type.Elem()) {
+	if a.Type.Elem().HasHeapPointer() {
 		// memclrHasPointers(hp, hn)
 		Curfn.Func.setWBPos(stmt.Pos)
 		fn = mkcall("memclrHasPointers", nil, nil, hp, hn)

test/notinheap3.go

@@ -58,3 +58,19 @@ func h() {
 	_ = append(v1s, v1s...) // no barrier
 	_ = append(v2s, v2s...) // ERROR "write barrier"
 }
+
+// Slice clearing
+
+var (
+	sliceIH  []*ih
+	sliceNIH []*nih
+)
+
+func sliceClear() {
+	for i := range sliceIH {
+		sliceIH[i] = nil // ERROR "write barrier"
+	}
+	for i := range sliceNIH {
+		sliceNIH[i] = nil // no barrier
+	}
+}