vm: embed reference counter in compound items

```
name              old time/op  new time/op  delta
RefCounter_Add-8  44.8ns ± 4%  11.7ns ± 3%  -73.94%  (p=0.000 n=10+10)
```

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/vm/json_test.go

@@ -208,6 +208,28 @@ func compareItems(t *testing.T, a, b stackitem.Item) {
 		p, ok := b.(*stackitem.Pointer)
 		require.True(t, ok)
 		require.Equal(t, si.Position(), p.Position()) // there no script in test files
+	case *stackitem.Array, *stackitem.Struct:
+		require.Equal(t, a.Type(), b.Type())
+
+		as := a.Value().([]stackitem.Item)
+		bs := a.Value().([]stackitem.Item)
+		require.Equal(t, len(as), len(bs))
+
+		for i := range as {
+			compareItems(t, as[i], bs[i])
+		}
+
+	case *stackitem.Map:
+		require.Equal(t, a.Type(), b.Type())
+
+		as := a.Value().([]stackitem.MapElement)
+		bs := a.Value().([]stackitem.MapElement)
+		require.Equal(t, len(as), len(bs))
+
+		for i := range as {
+			compareItems(t, as[i].Key, bs[i].Key)
+			compareItems(t, as[i].Value, bs[i].Value)
+		}
 	default:
 		require.Equal(t, a, b)
 	}

pkg/vm/ref_counter.go

@@ -5,15 +5,19 @@ import (
 )
 
 // refCounter represents reference counter for the VM.
-type refCounter struct {
-	items map[stackitem.Item]int
-	size  int
+type refCounter int
+
+type (
+	rcInc interface {
+		IncRC() int
 	}
+	rcDec interface {
+		DecRC() int
+	}
+)
 
 func newRefCounter() *refCounter {
-	return &refCounter{
-		items: make(map[stackitem.Item]int),
-	}
+	return new(refCounter)
 }
 
 // Add adds an item to the reference counter.
@@ -21,14 +25,12 @@ func (r *refCounter) Add(item stackitem.Item) {
 	if r == nil {
 		return
 	}
-	r.size++
+	*r++
 
-	switch item.(type) {
-	case *stackitem.Array, *stackitem.Struct, *stackitem.Map:
-		if r.items[item]++; r.items[item] > 1 {
+	irc, ok := item.(rcInc)
+	if !ok || irc.IncRC() > 1 {
 		return
 	}
-
 	switch t := item.(type) {
 	case *stackitem.Array, *stackitem.Struct:
 		for _, it := range item.Value().([]stackitem.Item) {
@@ -40,24 +42,18 @@ func (r *refCounter) Add(item stackitem.Item) {
 		}
 	}
 }
-}
 
 // Remove removes item from the reference counter.
 func (r *refCounter) Remove(item stackitem.Item) {
 	if r == nil {
 		return
 	}
-	r.size--
+	*r--
 
-	switch item.(type) {
-	case *stackitem.Array, *stackitem.Struct, *stackitem.Map:
-		if r.items[item] > 1 {
-			r.items[item]--
+	irc, ok := item.(rcDec)
+	if !ok || irc.DecRC() > 0 {
 		return
 	}
-
-		delete(r.items, item)
-
 	switch t := item.(type) {
 	case *stackitem.Array, *stackitem.Struct:
 		for _, it := range item.Value().([]stackitem.Item) {
@@ -69,4 +65,3 @@ func (r *refCounter) Remove(item stackitem.Item) {
 		}
 	}
 }
-}

pkg/vm/ref_counter_test.go

@@ -10,24 +10,34 @@ import (
 func TestRefCounter_Add(t *testing.T) {
 	r := newRefCounter()
 
-	require.Equal(t, 0, r.size)
+	require.Equal(t, 0, int(*r))
 
 	r.Add(stackitem.Null{})
-	require.Equal(t, 1, r.size)
+	require.Equal(t, 1, int(*r))
 
 	r.Add(stackitem.Null{})
-	require.Equal(t, 2, r.size) // count scalar items twice
+	require.Equal(t, 2, int(*r)) // count scalar items twice
 
 	arr := stackitem.NewArray([]stackitem.Item{stackitem.NewByteArray([]byte{1}), stackitem.NewBool(false)})
 	r.Add(arr)
-	require.Equal(t, 5, r.size) // array + 2 elements
+	require.Equal(t, 5, int(*r)) // array + 2 elements
 
 	r.Add(arr)
-	require.Equal(t, 6, r.size) // count only array
+	require.Equal(t, 6, int(*r)) // count only array
 
 	r.Remove(arr)
-	require.Equal(t, 5, r.size)
+	require.Equal(t, 5, int(*r))
 
 	r.Remove(arr)
-	require.Equal(t, 2, r.size)
+	require.Equal(t, 2, int(*r))
+}
+
+func BenchmarkRefCounter_Add(b *testing.B) {
+	a := stackitem.NewArray(nil)
+	rc := newRefCounter()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		rc.Add(a)
+	}
 }

pkg/vm/stackitem/item.go

@@ -188,6 +188,7 @@ func convertPrimitive(item Item, typ Type) (Item, error) {
 // Struct represents a struct on the stack.
 type Struct struct {
 	value []Item
+	rc
 }
 
 // NewStruct returns an new Struct object.
@@ -311,7 +312,7 @@ func (i *Struct) Clone() (*Struct, error) {
 }
 
 func (i *Struct) clone(limit *int) (*Struct, error) {
-	ret := &Struct{make([]Item, len(i.value))}
+	ret := &Struct{value: make([]Item, len(i.value))}
 	for j := range i.value {
 		*limit--
 		if *limit < 0 {
@@ -624,6 +625,7 @@ func (i *ByteArray) Convert(typ Type) (Item, error) {
 // Array represents a new Array object.
 type Array struct {
 	value []Item
+	rc
 }
 
 // NewArray returns a new Array object.
@@ -724,6 +726,7 @@ type MapElement struct {
 // if need be.
 type Map struct {
 	value []MapElement
+	rc
 }
 
 // NewMap returns new Map object.

pkg/vm/stackitem/reference.go

@@ -0,0 +1,15 @@
+package stackitem
+
+type rc struct {
+	count int
+}
+
+func (r *rc) IncRC() int {
+	r.count++
+	return r.count
+}
+
+func (r *rc) DecRC() int {
+	r.count--
+	return r.count
+}

pkg/vm/vm.go

@@ -105,7 +105,6 @@ func NewWithTrigger(t trigger.Type) *VM {
 		Invocations:    make(map[util.Uint160]int),
 	}
 
-	vm.refs.items = make(map[stackitem.Item]int)
 	initStack(&vm.istack, "invocation", nil)
 	vm.estack = newStack("evaluation", &vm.refs)
 	return vm
@@ -520,7 +519,7 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		if errRecover := recover(); errRecover != nil {
 			v.state = FaultState
 			err = newError(ctx.ip, op, errRecover)
-		} else if v.refs.size > MaxStackSize {
+		} else if v.refs > MaxStackSize {
 			v.state = FaultState
 			err = newError(ctx.ip, op, "stack is too big")
 		}

pkg/vm/vm_test.go

@@ -415,7 +415,7 @@ func TestStackLimit(t *testing.T) {
 	require.NoError(t, vm.Step(), "failed to initialize static slot")
 	for i := range expected {
 		require.NoError(t, vm.Step())
-		require.Equal(t, expected[i].size, vm.refs.size, "i: %d", i)
+		require.Equal(t, expected[i].size, int(vm.refs), "i: %d", i)
 	}
 }
 
@@ -829,7 +829,7 @@ func getTestFuncForVM(prog []byte, result interface{}, args ...interface{}) func
 	if result != nil {
 		f = func(t *testing.T, v *VM) {
 			require.Equal(t, 1, v.estack.Len())
-			require.Equal(t, stackitem.Make(result), v.estack.Pop().value)
+			require.Equal(t, stackitem.Make(result).Value(), v.estack.Pop().Value())
 		}
 	}
 	return getCustomTestFuncForVM(prog, f, args...)
@@ -1761,7 +1761,7 @@ func TestPACK_UNPACK_MaxSize(t *testing.T) {
 	vm.estack.PushVal(len(elements))
 	runVM(t, vm)
 	// check reference counter = 1+1+1024
-	assert.Equal(t, 1+1+len(elements), vm.refs.size)
+	assert.Equal(t, 1+1+len(elements), int(vm.refs))
 	assert.Equal(t, 1+1+len(elements), vm.estack.Len()) // canary + length + elements
 	assert.Equal(t, int64(len(elements)), vm.estack.Peek(0).Value().(*big.Int).Int64())
 	for i := 0; i < len(elements); i++ {
@@ -1784,7 +1784,7 @@ func TestPACK_UNPACK_PACK_MaxSize(t *testing.T) {
 	vm.estack.PushVal(len(elements))
 	runVM(t, vm)
 	// check reference counter = 1+1+1024
-	assert.Equal(t, 1+1+len(elements), vm.refs.size)
+	assert.Equal(t, 1+1+len(elements), int(vm.refs))
 	assert.Equal(t, 2, vm.estack.Len())
 	a := vm.estack.Peek(0).Array()
 	assert.Equal(t, len(elements), len(a))
@@ -1959,7 +1959,7 @@ func testCLEARITEMS(t *testing.T, item stackitem.Item) {
 	v.estack.PushVal(item)
 	runVM(t, v)
 	require.Equal(t, 2, v.estack.Len())
-	require.EqualValues(t, 2, v.refs.size) // empty collection + it's size
+	require.EqualValues(t, 2, int(v.refs)) // empty collection + it's size
 	require.EqualValues(t, 0, v.estack.Pop().BigInt().Int64())
 }