Is multiple assignment of tuples slower than multiple assignment statements? - scala

Question

Is there a difference between assigning multiple variables using a tuple, and assigning them in multiple statements?
For example, is there a difference between the following code snippets?
// multiple assignment using tuples
val (x, y) = (str.length, str.substring(1, 2))
// multiple-statement assignment
val x = str.length
val y = str.substring(1, 2)

Answer 1

There is a difference. The approach of using tuples is actually invoking an extractor (the unapply method), which would incur a cost at runtime. The second approach is certainly faster.
To get an idea about the difference, here is a decompilation of two methods showing both approaches. You can see clearly how the first approach causes much more operations.
An important point to note here is that the first expression requires auto-boxing to a java.lang.Integer (because Tuple2 accepts objects), while the second expression uses the value without boxing.
public void m1(java.lang.String);
Code:
0: new #16 // class scala/Tuple2
3: dup
4: aload_1
5: invokevirtual #22 // Method java/lang/String.length:()I
8: invokestatic #28 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
11: aload_1
12: iconst_1
13: iconst_2
14: invokevirtual #32 // Method java/lang/String.substring:(II)Ljava/lang/String;
17: invokespecial #35 // Method scala/Tuple2."<init>":(Ljava/lang/Object;Ljava/lang/Object;)V
20: astore_3
21: aload_3
22: ifnull 75
25: aload_3
26: invokevirtual #38 // Method scala/Tuple2._1$mcI$sp:()I
29: istore 4
31: aload_3
32: invokevirtual #42 // Method scala/Tuple2._2:()Ljava/lang/Object;
35: checkcast #18 // class java/lang/String
38: astore 5
40: new #16 // class scala/Tuple2
43: dup
44: iload 4
46: invokestatic #28 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
49: aload 5
51: invokespecial #35 // Method scala/Tuple2."<init>":(Ljava/lang/Object;Ljava/lang/Object;)V
54: astore 6
56: aload 6
58: astore_2
59: aload_2
60: invokevirtual #38 // Method scala/Tuple2._1$mcI$sp:()I
63: istore 7
65: aload_2
66: invokevirtual #42 // Method scala/Tuple2._2:()Ljava/lang/Object;
69: checkcast #18 // class java/lang/String
72: astore 8
74: return
75: new #44 // class scala/MatchError
78: dup
79: aload_3
80: invokespecial #47 // Method scala/MatchError."<init>":(Ljava/lang/Object;)V
83: athrow
public void m2(java.lang.String);
Code:
0: aload_1
1: invokevirtual #22 // Method java/lang/String.length:()I
4: istore_2
5: aload_1
6: iconst_1
7: iconst_2
8: invokevirtual #32 // Method java/lang/String.substring:(II)Ljava/lang/String;
11: astore_3
12: return
}