Unnecessary load and store instruction in scala's byte code - scala

Question

I just did some inverstigation on pattern match and its corresponding byte code.
val a = Array(1,2,3,4)
a.map {
case i => i + 1
}
For above code, I use javap and got the byte code for the annonymous function inside map:
public int apply$mcII$sp(int);
Code:
0: iload_1
1: istore_2
2: iload_2
3: iconst_1
4: iadd
5: ireturn
So it seems to me that in line 0 we push an int (the parameter), then in line 1 we load the int and in line 2 we push it back ... What's the purpose here?
Thanks!

Answer 1

Dude, try -optimise.
public int apply$mcII$sp(int);
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: iload_1
1: iconst_1
2: iadd
3: ireturn
Use
scala> :javap -prv -
and then something like
scala> :javap -prv $line4/$read$$iw$$iw$$anonfun$1

Answer 2

This is not really an answer, since I couldn't figure out why this happens. I'm hoping that these observations will be at least helpful :)
I'm seeing the following bytecode in Scala 2.10:
public int apply$mcII$sp(int);
Code:
0: iload_1 ; var1 -> stack
1: istore_2 ; var2 <- stack
2: iload_2 ; var2 -> stack
3: iconst_1 ; 1 -> stack
4: iadd
5: istore_3 ; var3 <- stack
6: iload_3 ; var3 -> stack
7: ireturn ; return <- stack
The first two instructions seem to simply move the value of var1 to var2, then move var2 to the stack as a parameter. The same can be observed after iadd, where the result is stored in var3 for no apparent reason, since ireturn returns the value from the stack anyway.