const maxOpenDefers = 8

// backingArrayPtrLen extracts the pointer and length from a slice or string.
// This constructs two nodes referring to n, so n must be a cheapExpr.
func backingArrayPtrLen(n ir.Node) (ptr, length ir.Node) {
	var init ir.Nodes
	c := cheapExpr(n, &init)
	if c != n || len(init) != 0 {
		base.Fatalf("backingArrayPtrLen not cheap: %v", n)
	}

		fn := typecheck.LookupRuntime("typedslicecopy", l1.Type().Elem(), l2.Type().Elem())
		ptr1, len1 := backingArrayPtrLen(cheapExpr(slice, &nodes))
		ptr2, len2 := backingArrayPtrLen(l2)
		ncopy = mkcall1(fn, types.Types[types.TINT], &nodes, reflectdata.AppendElemRType(base.Pos, n), ptr1, len1, ptr2, len2)
	} else if base.Flag.Cfg.Instrumenting && !base.Flag.CompilingRuntime {

		ir.CurFunc.SetWBPos(n.Pos())
		fn := writebarrierfn("typedslicecopy", n.X.Type().Elem(), n.Y.Type().Elem())
		n.X = cheapExpr(n.X, init)
		ptrL, lenL := backingArrayPtrLen(n.X)
		n.Y = cheapExpr(n.Y, init)
		ptrR, lenR := backingArrayPtrLen(n.Y)
		return mkcall1(fn, n.Type(), init, reflectdata.CopyElemRType(base.Pos, n), ptrL, lenL, ptrR, lenR)
	}

	if runtimecall {
		// rely on runtime to instrument: