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

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.forEach(...) - these are two different calls.

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