vm: implement ISNULL opcode

pkg/vm/opcode/opcode.go

@@ -119,6 +119,8 @@ const (
 	SYSCALL  Opcode = 0x68
 	TAILCALL Opcode = 0x69
 
+	ISNULL Opcode = 0x70
+
 	// Stack
 	DUPFROMALTSTACK Opcode = 0x6A
 	TOALTSTACK      Opcode = 0x6B

pkg/vm/vm.go

@@ -563,6 +563,10 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 	case opcode.PUSHNULL:
 		v.estack.PushVal(NullItem{})
 
+	case opcode.ISNULL:
+		res := v.estack.Pop().value.Equals(NullItem{})
+		v.estack.PushVal(res)
+
 	// Stack operations.
 	case opcode.TOALTSTACK:
 		v.astack.Push(v.estack.Pop())

pkg/vm/vm_test.go

@@ -206,6 +206,22 @@ func TestPUSHNULL(t *testing.T) {
 	require.True(t, v.estack.Pop().Bool())
 }
 
+func TestISNULL(t *testing.T) {
+	t.Run("Integer", func(t *testing.T) {
+		prog := makeProgram(opcode.PUSH1, opcode.ISNULL)
+		v := load(prog)
+		runVM(t, v)
+		require.False(t, v.estack.Pop().Bool())
+	})
+
+	t.Run("Null", func(t *testing.T) {
+		prog := makeProgram(opcode.PUSHNULL, opcode.ISNULL)
+		v := load(prog)
+		runVM(t, v)
+		require.True(t, v.estack.Pop().Bool())
+	})
+}
+
 // appendBigStruct returns a program which:
 // 1. pushes size Structs on stack
 // 2. packs them into a new struct