Why is for-in slower than while in swift debugging mode?

Question

Why is for-in slower than while in swift debugging mode ? If you think, yes It was ran in no optimization.

⬇️Below code, Time is compare of for-in and while in no optimization

49999995000000 for-in -- time = 3.3352

4999999950000000 while -- time = 0.3613

⬇️but, If using optimization for speed

49999995000000 for-in -- time = 0.0037

49999995000000 while -- time = 0.0035

I wonder that "why is for-in slower than while in no optimization? and why are for-in and while so fast in optimization? "

import Foundation

func processTime(_ title: String, blockFunction: () -> ()) {
    print()
    let startTime = CFAbsoluteTimeGetCurrent()
    blockFunction()
    let processTime = CFAbsoluteTimeGetCurrent() - startTime
    print(title, " -- time = \(String(format : "%.4f",processTime))")
}

processTime("for-in") {
    var sum = 0
    for i in 0..<10000000 {
        sum += i
    }
    print(sum)
}

processTime("while") {
    var sum = 0
    var i = 0
    while i<10000000 {
        sum += i
        i += 1
    }
    print(sum)
}

Answer 1

From a Swift perspective, your for loop actually translates to something like this:

let range = 0..<10000000
var iterator = range.makeIterator()
while let next = iterator.next() {
    ...
}

Notice that's a lot of calls to next on the range's iterator, which has its own state that has to be kept track of, and IndexingIterator.next calls a bunch of protocol methods, dispatching which takes some time too as it has to lookup the witness table. See exactly what calls Iterator.next is going to make here.

If you are in debug mode, none of that is going to be optimised.

Compare that to your while loop, which is basically set something to 0, compare, do the thing in the loop, add 1 to it, repeat. Clearly this is much simpler to do than calling all those methods.

If you enable optimisations however, the compiler can see that the for loop is doing what the while loop does anyway.

---

Because I find it interesting, I did a little time profile of a loop such as:

var s = ""
for i in 0...10000000 {
    s += "\(i)"
}

80% of the time is spent on next(), and look at how many things it does! My screenshot can't even contain everything. The string concatenating only takes up about 6% (not in the screenshot).