Slow range forEach in Kotlin

Question

I used the following code to measure performance of different syntax constructions in Kotlin

fun time(what: String, body: () -> Int) {
    val start =          


        

Answer 1

From documentation:

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

forEach as you can see at https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/for-each.html is special-cased for arrays, but has a single implementations for all Iterables, so it needs to create an iterator.

Answer 2

Thanks for this code. I was more interested in the performance difference between array and list and was a bit shocked to have 6 times as long running time. I thought the list is only slow in creation.

Above runtimes on my machine or comparison: for in range: 55
for in collection: 57
forEach: 55
range forEach: 223
sum: 57

//code change
//val arr = IntArray(n) { rand.nextInt() }
val arr = List(n) { rand.nextInt() }

for in range: 383
for in collection: 367
forEach: 367
range forEach: 486
sum: 371

Answer 3

The most interesting comparison is probably between these 2 cases:

Case A: taking 86 ms

time("forEach") {
    var sum = 0
    arr.forEach { sum += it }
    sum
}

Case B: taking 294 ms

time("range forEach") {
    var sum = 0
    (0 until n).forEach { sum += arr[it] }
    sum
}

While case A is actually calling IntArray.forEach(...) case B is calling Iterable<T>.forEach(...) - these are two different calls.

I guess the Kotlin compiler knows to optimize IntArray.forEach(...), but not Iterable<T>.forEach(...).