[swift-users] String initializers and developer ergonomics

[Dennis Weissmann]

Hi Austin,

I further “swiftyfied” the code to this (swift-DEVELOPMENT-SNAPSHOT-2016-05-03-a):

import Foundation

let N = 1_000_000

func generateTestData() -> [String] {
  return (0..<N).map { _ in ",,abc, 123  ,x, , more more more,\u{A0}and yet more, " }
}

func splitAndTrim(s: String, sep: Character) -> [String] {
  return s.characters.split(separator: sep, omittingEmptySubsequences: false)
                     .map(String.init)
                     .map { $0.trimmingCharacters(in: .whitespacesAndNewlines()) }
}

func doSplits(data: [String]) -> Int {
  return data.reduce(0) { $0 + splitAndTrim(s: $1, sep: Character(",")).count }
}

let data = generateTestData()
let start = NSDate()
let sum = doSplits(data: data)
print("elapsed: \(NSDate().timeIntervalSince(start))")
print("sum: \(sum)")

I didn’t expect it to run as fast as the original (or even faster) but the code above takes 13 seconds to run… But that’s just an aside :)

To actually come back to the problem you mentioned:

You can disambiguate the initializer to call in the following ways (probably there are other ways):

Add an ! after the init call since the one which should be used is failable (the one that uses reflection does not), so the compiler chooses the “right” one.

result.append(begin == eos ? "" : String(cs[begin..<end.successor()])!) 

Or you can reference the initializer and use it afterwards (more future-proof):

let utf16View = cs[begin..<end.successor()]
let initializer = String.init as (String.UTF16View -> String?)
result.append(begin == eos ? "" : initializer(utf16View)!)

This reduces the time to 2.2 seconds on my machine (from ~2.9), but that’s still far from 0.8.

> What do other people think? Is this something worth worrying about, or is it so rare that it shouldn't matter? Also, any suggestions as to how that code sample might be improved would be appreciated - my naive first attempt wasn't any better.


It might be very surprising behavior and we could think about adding an external label to all those initializers that cause trouble. The performance is of course a little worrying for a zero-cost abstractions language, but I think there are currently more important goals (getting the design right, etc.). We are still at the early stages with a lot of stuff changing and I think when the syntax, ABI, etc. are settled, performance / optimization will get more attention :)

- Dennis

> On May 7, 2016, at 7:39 PM, Austin Zheng via swift-users <swift-users at swift.org> wrote:
> 
> Hello Swift users,
> 
> I wanted to run something past you folks and get some opinions/feedback.
> 
> About a month ago on Hacker News I saw someone commenting about how Swift's string-handling code was unbearably slow (3 seconds to run a code sample, vs. 0.8 in Java). I asked him to provide the code, and he obliged. Unfortunately, I didn't have time to dig into it until this morning. The code in its entirety can be found here: https://gist.github.com/austinzheng/d6c674780a58cb63832c4df3f809e683 <https://gist.github.com/austinzheng/d6c674780a58cb63832c4df3f809e683>
> 
> At line 26 we have the following code:
> 
> result.append(begin == eos ? "" : String(cs[begin..<end.successor()]))
> 
> 'cs' is a UTF16 view into an input string, while 'result' is a [String]. When I profiled the code in Instruments, I noticed that it was spending significant time within the reflection machinery.
> 
> It turns out that the initializer to make a String out of a utf16 view looks like this, and I believe this is the initializer the author intended to call:
> 
> init?(_: String.UTF16View) <>
> 
> However, the actual initializer being called was this String initializer in the Mirror code:
> 
> public init<Subject>(_ instance: Subject)
> 
> This seems like a tricky gotcha for developers who aren't extremely familiar with both the String and reflection APIs. His code looked reasonable at a first glance and I didn't suspect anything was wrong until I profiled it. Even so, I only made the connection because I recognized the name of the standard library function from poking around inside the source files.
> 
> What do other people think? Is this something worth worrying about, or is it so rare that it shouldn't matter? Also, any suggestions as to how that code sample might be improved would be appreciated - my naive first attempt wasn't any better.
> 
> Best,
> Austin
> 
> 
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