Array/List iteration without extra object allocations

Question

I'm working on a game written in Kotlin and was looking into improving GC churn. One of the major sources of churn are for-loops called in the main game/rendering loops that result in the allocation of iterators.

Turning to the documentation, I found this paragraph:

A for loop over an array is compiled to an index-based loop that does not create an iterator object.

If you want to iterate through an array or a list with an index, you can do it this way:

for (i in array.indices)
  print(array[i])

Note that this “iteration through a range” is compiled down to optimal implementation with no extra objects created.

https://kotlinlang.org/docs/reference/control-flow.html#for-loops

Is this really true? To verify, I took this simple Kotlin program and inspected the generated byte code:

fun main(args: Array<String>) {
    val arr = arrayOf(1, 2, 3)
    for (i in arr.indices) {
        println(arr[i])
    }
}

According to the quote above, this should not result in any objects allocated, but get compiled down to a good old pre-Java-5 style for-loop. However, what I got was this:

      41: aload_1
      42: checkcast     #23                 // class "[Ljava/lang/Object;"
      45: invokestatic  #31                 // Method kotlin/collections/ArraysKt.getIndices:([Ljava/lang/Object;)Lkotlin/ranges/IntRange;
      48: dup
      49: invokevirtual #37                 // Method kotlin/ranges/IntRange.getFirst:()I
      52: istore_2
      53: invokevirtual #40                 // Method kotlin/ranges/IntRange.getLast:()I
      56: istore_3
      57: iload_2
      58: iload_3
      59: if_icmpgt     93

This looks to me as if a method called getIndices is called that allocates a temporary IntRange object to back up bounds checking in this loop. How is this an "optimal implementation" with "no extra objects created", or am I missing something?

UPDATE: So, after toying around a bit more and looking at the answers, the following appears to be true for Kotlin 1.0.2:

Arrays:

• for (i in array.indices): range allocation
• for (i in 0..array.size): no allocation
• for (el in array): no allocation
• array.forEach: no allocation

Collections:

• for (i in coll.indices) range allocation
• for (i in 0..coll.size): no allocation
• for (el in coll): iterator allocation
• coll.forEach: iterator allocation

Answer 1

To iterate an array without allocating extra objects you can use one of the following ways.

1. for-loop

    for (e in arr) {
        println(e)
    }

2. forEach extension

    arr.forEach {
        println(it)
    }

3. forEachIndexed extension, if you need to know index of each element

    arr.forEachIndexed { index, e ->
        println("$e at $index")
    }

Answer 2

As far as I know the only allocation-less way to define a for loop is

for (i in 0..count - 1)

All other forms lead to either a Range allocation or an Iterator allocation. Unfortunately, you cannot even define an effective reverse for loop.

Answer 3

Here is an example of preparing a list and iterate with index and value.

val list = arrayListOf("1", "11", "111")
for ((index, value) in list.withIndex()) {
   println("$index: $value")
}

Output:

0:1

1:11

2:111

Also, following code works similar,

val simplearray = arrayOf(1, 2, 3, 4, 5)
for ((index, value) in simplearray.withIndex()) {
    println("$index: $value")
}