If you’re an iOS or macOS developer, you’ve likely encountered the frustration of long build times. The question, “Why is Swift compile time so slow?” is a common refrain in development circles. Swift, Apple’s modern programming language, promises safety, speed, and expressiveness. However, its compilation speed often lags behind other languages, impacting developer productivity and lengthening iteration cycles. Understanding the underlying reasons for these slow compile times is crucial for optimizing your development workflow and mitigating the impact of lengthy builds. This article dives deep into the factors contributing to Swift’s compilation speed challenges and explores potential solutions.

Understanding Swift’s Compilation Process

To truly understand why is Swift compile time so slow, we need to examine the various stages of the compilation process. Swift’s compiler, like most compilers, takes source code as input and transforms it into executable machine code. This process involves several key phases, each contributing to the overall compilation time. These phases include parsing, semantic analysis, optimization, and code generation. Each of these phases can be a significant contributor to the overall compilation time, depending on the complexity of the project and the optimization level selected.

The parsing phase involves breaking down the source code into a structured representation. Semantic analysis checks the code for type errors and other inconsistencies. Optimization attempts to improve the performance of the generated code, and code generation translates the optimized code into machine instructions. The Swift compiler performs extensive type checking and inference, which, while beneficial for code safety, can be computationally expensive. Furthermore, Swift’s support for advanced language features like generics, protocol-oriented programming, and complex operators also adds to the complexity of the compilation process, impacting build times.

According to a study by JetBrains, the average compilation time for Swift projects tends to be higher compared to similar projects in Objective-C. JetBrains’ Developer Ecosystem Survey 2021 highlights this disparity, emphasizing the ongoing need for optimization in Swift’s compilation process. This slower speed has a direct impact on the speed of development, causing developers to be less efficient.

Factors Contributing to Slow Compile Times

Several specific factors contribute to the issue of why is Swift compile time so slow. These factors range from language features to project structure and hardware limitations. Addressing these factors can lead to significant improvements in build performance.

• Complex Type Inference: Swift’s powerful type inference system automatically determines the data type of variables and expressions, which can be incredibly convenient but also computationally intensive, especially with complex expressions or nested generics.
• Large Codebases: As your project grows in size, the compiler has more code to process, leading to longer compilation times. This is a universal problem, but Swift seems to be more affected than other languages.
• Bridging with Objective-C: Interoperability with Objective-C, while essential for many projects, introduces overhead due to the need for bridging headers and runtime compatibility layers.
• Excessive Use of Generics and Metatypes: While powerful, generics and metatypes can lead to increased compile times due to the generation of specialized code for each usage.

The excessive use of certain language features can exacerbate the problem. For instance, deeply nested optional chaining or extensive use of Any can hinder the compiler’s ability to optimize code effectively. Consider refactoring code to reduce complexity and improve type clarity. Moreover, inefficient use of libraries and frameworks can add unnecessary overhead. Regularly auditing your project’s dependencies can identify potential bottlenecks and areas for optimization. Ultimately, addressing these factors requires a combination of code optimization, project restructuring, and compiler improvements.

Strategies for Improving Swift Compile Time

While why is Swift compile time so slow might seem like an intractable problem, several strategies can help improve build performance. These strategies involve optimizing your code, configuring your build settings, and leveraging tools to identify and address bottlenecks.

1. Use Build Time Analyzers: Tools like SwiftLint and Build Time Analyzer can help identify code that contributes to slow compile times.
2. Reduce Generic Complexity: Limit the use of complex generics and consider using concrete types where possible.
3. Optimize Bridging Headers: Minimize the amount of code included in bridging headers.
4. Modularize Your Codebase: Break your project into smaller modules to enable parallel compilation.
5. Use Whole Module Optimization: Enable whole module optimization for release builds to allow the compiler to perform more aggressive optimizations.

Modularization can be particularly effective. By breaking your project into smaller, independent modules, you enable the compiler to process them in parallel, significantly reducing overall build time. Additionally, consider using precompiled headers for frequently used code to avoid recompilation. Another effective technique is to avoid force unwrapping optionals (!) as they bypass the compiler’s safety checks, potentially leading to runtime crashes and hindering optimization. Always prefer safe unwrapping techniques like optional binding (if let) or guard statements (guard let).

Tools and Techniques for Analysis

To effectively address why is Swift compile time so slow, you need tools that can help you identify the specific areas of your code that are contributing to the problem. Several profiling and analysis tools are available to assist in this process.

Xcode provides built-in tools like the Build Time Analyzer, which can pinpoint the functions and files that take the longest to compile. This allows you to focus your optimization efforts on the most problematic areas. Additionally, tools like SwiftLint can help enforce coding standards and identify potential performance bottlenecks. SwiftLint, for example, enforces style and conventions, potentially leading to more efficient code and faster compilation times. These checks can also help make code easier to read and debug, further improving developer productivity.

Furthermore, consider using the xcodebuild command-line tool with the -showBuildTimingSummary flag to get a detailed breakdown of the build process. This can provide valuable insights into where the compiler is spending its time. For example, you might discover that a particular library is taking an unexpectedly long time to compile, prompting you to investigate alternative libraries or optimization strategies. Utilizing these tools and techniques empowers you to make data-driven decisions and effectively address the root causes of slow compile times.

FAQ: Addressing Common Concerns

Why does Swift compile slower than Objective-C?

Swift's advanced features like type inference, generics, and protocol-oriented programming add complexity to the compilation process compared to Objective-C's more straightforward approach.

Does hardware affect Swift compile time?

Yes, faster processors, more RAM, and solid-state drives (SSDs) can significantly reduce compile times. The compiler needs resources to perform its tasks, and faster hardware can provide those resources more readily.

How can I measure my Swift compile time accurately?

Use Xcode's Build Time Analyzer or the xcodebuild -showBuildTimingSummary command-line tool to get a detailed breakdown of the build process.

Is Swift getting faster to compile?

Apple is continually working to improve the Swift compiler's performance. Each new Swift release often includes optimizations that reduce compile times. This is a continuous process that improves with each release.

Ultimately, the key to faster builds lies in a proactive approach to code quality and build configuration. By continuously monitoring your compile times and addressing potential bottlenecks, you can ensure a smoother and more efficient development experience. Don’t let slow compile times hold you back – take control and optimize your Swift projects today! Explore related topics like “Best Practices for Swift Performance Optimization” or “Advanced Xcode Build Configuration Techniques” to further enhance your knowledge and skills.

Question & Answer :
I’m using Xcode 6 Beta 6.

This is something that’s been bugging me for some time now, but it’s reaching a point where it’s barely usable now.

My project is starting to have a decent size of 65 Swift files and a few bridged Objective-C files (which are really not the cause of the problem).

It seems like any slight modification to any Swift file (like adding a simple white space in a class that’s barely used in the app) will cause the entire Swift files for the specified target to be recompiled.

After a deeper investigation, I’ve found that what is taking pretty much 100% of the compiler time is the CompileSwift phase where Xcode runs the swiftc command on all Swift files of your target.

I did some further investigation, and if I only keep the app delegate with a default controller the compilation is very fast, but as I was adding more and more of my project files, the compile time was starting to get really slow.

Now with only 65 source files, it takes about 8/10 seconds to compile each time. Not very swift at all.

I haven’t seen any post talking about this issue except this one, but it was an old version of Xcode 6. So I’m wondering if I’m the only one in that case.

UPDATE

I’ve checked a few Swift projects on GitHub like Alamofire, Euler and CryptoSwift, but none of them had enough Swift files to actually compare. The only project I found that was starting a have decent size was SwiftHN, and even though it only had a dozen source files I was still able to verify the same thing, one simple space and the whole project needed recompilation which was starting to take a little time (2/3 seconds).

Compared to Objective-C code where both analyzer and compilation are blazing fast, this really feels like Swift will never be able to handle big projects, but please tell me I’m wrong.

UPDATE With Xcode 6 Beta 7

Still no improvement whatsoever. This is starting to get ridiculous. With the lack of #import in Swift, I really don’t see how Apple will ever be able to optimize this.

UPDATE With Xcode 6.3 and Swift 1.2

Apple has added incremental builds (and many other compiler optimizations). You have to migrate your code to Swift 1.2 to see those benefits, but Apple added a tool in Xcode 6.3 to help you do so:

HOWEVER

Don’t rejoice too quickly as I did. The graph solver that they use to make the build incremental is not very well optimised yet.

Indeed first, it doesn’t look at function signature changes so if you add a space in the block of one method, all files depending on that class will be recompiled.

Second, it seems to create the tree based on the files that were recompiled even if a change doesn’t affect them. For example, if you move these three classes into different files

class FileA: NSObject { var foo:String? } class FileB: NSObject { var bar:FileA? } class FileC: NSObject { var baz:FileB? } 

Now if you modify FileA, the compiler will obviously mark FileA to be recompiled. It will also recompile FileB (that would be OK based on the changes to FileA), but also FileC because FileB is recompiled, and that is pretty bad because FileC never uses FileA here.

So I hope they improve that dependency tree solver… I’ve opened a radar with this sample code.

UPDATE With Xcode 7 beta 5 and Swift 2.0

Yesterday Apple released the beta 5 and inside the release notes we could see:

Swift Language & Compiler • Incremental builds: changing just the body of a function should no longer cause dependent files to be rebuilt. (15352929)

I have given it a try and I must say it is working really (really !) well now. They greatly optimized the incremental builds in swift.

I highly recommend you create a swift2.0 branch and keep your code up to date using XCode 7 beta 5. You will be pleased by the enhancements of the compiler (however I’d say the global state of XCode 7 is still slow & buggy)

UPDATE With Xcode 8.2

It’s been a while since my last update on this issue so here it is.

Our app is now about 20k lines of almost exclusively Swift code, which is decent but not outstanding. It underwent swift 2 and than swift 3 migration. It takes about 5/6m to compile on a mid 2014 Macbook pro (2.5 GHz Intel Core i7) which is okay on a clean build.

However the incremental build is still a joke despite Apple claiming that:

Xcode will not rebuild an entire target when only small changes have occurred. (28892475)

Obviously I think many of us just laughed after checking out this nonsense (adding one private (private!) property to any file of my project will recompile the whole thing…)

I would like to point you guys to this thread on Apple developer forums which has some more information about the issue (as well as appreciated Apple dev communication on the matter once in a while)

Basically people have come up with a few things to try to improve the incremental build:

1. Add a HEADER_MAP_USES_VFS project setting set to true
2. Disable Find implicit dependencies from your scheme
3. Create a new project and move your files hierarchy to the new one.

I will try solution 3 but solution 1/2 didn’t work for us.

What’s ironically funny in this whole situation is that looking at the first post on this issue we were using Xcode 6 with I believe swift 1 or swift 1.1 code when we reached the first compilations slugginess and now about two years later despite actual improvements from Apple the situation is just as bad as it was with Xcode 6. How ironic.

I actually REALLY regret choosing Swift over Obj/C for our project because of the daily frustration it involves. (I even switch to AppCode but that’s another story)

Anyways I see this SO post has 32k+ views and 143 ups as of this writing so I guess I’m not the only one. Hang in there guys despite being pessimistic over this situation there might be some light at the end of the tunnel.

If you have the time (and courage!) I guess Apple welcomes radar about this.

UPDATE With Xcode 9

Stumble upon this today. Xcode quietly introduced a new build system to improve on the current awful performance. You have to enable it through the workspace settings.

Have given a try yet but will update this post after it’s done. Looks promising though.

Well, it turned out that Rob Napier was right. It was one single file (actually one method) that was causing the compiler to go berzek.

Now don’t get me wrong. Swift does recompile all your files each time, but the great thing now, is that Apple added real-time compilation feedback over the files it compiles, so Xcode 6 GM now shows which Swift files are being compiled and the status of compilation in real time as you can see in this screenshot:

So this comes very handy to know which of your files is taking so long. In my case it was this piece of code:

var dic = super.json().mutableCopy() as NSMutableDictionary dic.addEntriesFromDictionary([ "url" : self.url?.absoluteString ?? "", "title" : self.title ?? "" ]) return dic.copy() as NSDictionary 

because the property title was of type var title:String? and not NSString. The compiler was going crazy when adding it to the NSMutableDictionary.

Changing it to:

var dic = super.json().mutableCopy() as NSMutableDictionary dic.addEntriesFromDictionary([ "url" : self.url?.absoluteString ?? "", "title" : NSString(string: self.title ?? "") ]) return dic.copy() as NSDictionary 

made the compilation go from 10/15 seconds (maybe even more) down to a single second… amazing.