Swift Performance - iOSDevUK

I presented today at iOSDevUK in Aberystwyth with some more material on Swift Performance and how to profile Swift. Also on wrapping classes in value types. Unfortunately much of the presentation was a demo and there is no video but I'm posting the materials here and will try to make a follow up post covering some more of the details in the next week or two (no promises).

Slides (note that slides a terrible communication mechanism and caveats and subtleties in the talk may be lost):

How Swift is Swift?

Slides, video and links related to my Swift Summit talk on Swift performance. Video should be available later is now available and it might be worth watching Airspeed's talk that preceded mine before watching it when it is available I'm also expecting a blog post from him shortly on his static vs. compile time talk which is also highly relevant to optimisation.

The key point in Swift is that as the compiler gets better there is no need to choose between nice code and fast code. With a little thought and knowledge it is usually possible to get close to the speed of lowish level C code with nice abstractions. Some may say that C++ is there already but I enjoy writing Swift code much more and C++ is hampered by it's C legacy and choices made many years ago (for good reasons) from becoming a truly nice language in my view (non-Nullable by default would be hard to retrofit for example).

There were also many other excellent talks at Swift Summit, and all were at least good. I'm looking forward to watching several again.

Swift 1.2 Update (Xcode 6.3 beta 2) - Performance

Apple have shipped another major update (release notes - registered devs only) only two weeks after shipping the beta 1. There are major updates to Swift Playgrounds, additional syntax support for more flexible `if let`, added a zip function, various other tweaks and fixed a tonne of bugs. This is all on top of the major changes in 1.2 beta 1 that I discussed at Swift London.

Erica Sadun has already blogged about the Playgrounds and `if let` changes and I'm sure that there will be plenty more over the next week. I don't intend to go over that ground but instead discuss the performance changes in Swift 1.2 versions. [Update: I should also have mentioned Jamesson Quave's post that covers the new zip function.]

The performance jumps are very significant in Swift 1.2 and certainly in Beta 2 the steps necessary to optimise your code are significantly changed. This post will cover some information on the improvements and how to get the best out of the Swift compiler. In most cases I have encountered the performance is now very close to C/C++ code and may be faster at times.

Swift 1.2 beta 2 Big Performance News

High performance can be achieved in Xcode 6.3 beta 2 without making code changes to achieve the result that were required to get close previously. Specifically the performance gains of these changes seem to have been largely eliminated:

  1. `final` methods/properties which used to be massive (although I still recommend it where possible for design/safety reasons).
  2. unsafeMutableBuffer instead of array (at least in -Ounchecked builds)
  3. moving code into the same file to allow the compiler to inline (with -whole-module-optimizations build option new in Xcode 6.3 beta 2).

There also seems to have been a slight general performance gain (about 5% above beta 1 from my initial tests).

Is Swift Fast Yet?

Yes. I've always believed that the language had potential to be as fast or faster than C because of strict compile time type checking, immutability and a lack of pointer aliasing concerns. With the Xcode 6.3 betas that promise is being delivered on. I'm sure the work is continuing and there is more to come but it is already fast in most cases. My thanks to the Apple team for making a fast platform but you have just taken away my ability to look impressive by speeding up code significantly with a few simple changes - you bastards.

The Importance of Being `final`

[UPDATE - Swift 1.2 from Xcode 6.3 Beta 2 brings performance benefits to non-final properties and methods making final unnecessary for performance from that time - see my initial reaction to Beta 2. I still recommend final where possible as it avoids need to consider effects of an object being subclassed and methods overwritten that change the behaviour.]

This post is intended to quantify, explain and show the performance hit that you take from not adding the word final to your classes (or their properties and methods). The post was triggered by a blog post complaining about Swift performance and showing some performance figures where Swift was significantly slower than Objective-C. In the optimised builds that gap could be closed just by adding a single final keyword. I'm grateful to David Owens for showing the code he was having trouble and giving me a base to demonstrate the difference final can make without it being me cherry picking any code of my choosing.

As with most performance issues this doesn't matter most of the time. Most code is waiting on user input, network responses or other slow things and it really doesn't matter that much if the code is being accessed a few times a second. However when you have got performance critical code and in particular that inner loop that is being executed hundreds of thousands of times a second it can make a huge difference.

What does final do