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Thinking in Swift (Part II) – Compiler

In Part I, we discussed some of the motivations behind Swift and a little bit of the history that led up to its release. In this article, we’ll talk about the different environments available for writing Swift. Then, we’ll go behind the scenes and look at how Swift code is transformed into something that can run on different processors (e.g. x86-64 for OS X and ARM for iOS).


XcodeIf you’re looking to build an OS X or iOS application with Swift, you will probably end up spending most of your time in Apple’s Xcode application. Like other development environments (e.g. Eclipse, Visual Studio, etc.), Xcode includes many features to help you be more productive. However, Xcode has its own learning curve and hides many of the interesting things that are going on under the covers. A tool that shields you from the internal details can be a good thing when you’re trying to get work done. However, when learning a new language like Swift, it is a worthwhile exercise to step away from the big fancy tool and get your hands a little dirty.

The folks that work on the official Swift blog have put together a short video titled Building Your First Swift App Video. This video demonstrates building a simple iOS app with Swift and Xcode.


PlaygroundAnother option that is available for writing Swift code is to create a Swift Playground (feature included with Xcode 6). A Playground provides a place where you can write Swift code and have it executed immediately as you type it. This type of environment is great for exploring the language and trying out pieces of code before you put them into an actual application.

There are a huge number of tutorials available on the Web about creating and using a Swift Playground. Simple type “swift playground” into your favorite search engine and take your pick. There is also a great video from Apple about Playground here.

Swift REPL

If you want to go a little more low-level or if you’re one of those people that just feels more at home on the command-line, you can fire up the Swift REPL (also installed as part of Xcode 6). The Swift REPL is a command-line tool that provides an interactive Read Eval Print Loop. What that means is that each line of code is executed as you type it. Sound familiar? A Swift Playground is really just a pretty wrapper around the Swift REPL.

For a great introduction to the Swift REPL, go check out another article from our friends over on the official Apple Swift blog.

The Swift Compiler

The time has now come when we’ll look to answer some questions that are often asked but rarely answered in blog articles and tutorials about Swift.

  • What happens to the Swift code that I write to make the magic happen?
  • Do Xcode, Playgrounds, and the REPL all use the same Swift compiler?
  • Can I invoke the Swift compiler manually and examine what it’s doing?
  • Is the final end product of my Swift code as efficient as what I get when using a language like C?

At a high-level, the job of any compiler is to take source code and turn it into something that can run on a processor (machine code). As part of that process, the source code is typically turned into an intermediate form, optimized, and then transformed into machine code. Those jobs can be split up into three separate components – the frontend, the optimizer, and the backend.

Figure 1: Traditional compiler architecture

Figure 1: Traditional compiler architecture

In Part I of this series, I talked about some of the history that led up to the development of Swift. During that discussion, the LLVM Compiler Infrastructure was mentioned. LLVM is not a compiler in the traditional sense. It’s more appropriate to think of it as scaffolding that can be used for building actual compilers (hence the “infrastructure” part of the name). The result is a system that can support multiple languages and multiple hardware platforms without each compiler having to reinvent the wheel.

Figure 2: LLVM compiler architecture

Figure 2: LLVM compiler architecture

For this design to work, each frontend must translate the original source code into a common language that can be consumed by the optimizer. LLVM defines a new language for this purpose called LLVM Intermediate Representation (IR). LLVM IR is not something that can be packaged for deployment like Java bytecode or .NET IL. It is simply a step taken by the compiler on the road from source code to machine code. In the case of Swift, your code actually ends up getting transformed into two other formats before LLVM IR. Here is the whole picture for Swift:

Figure 3: Swift language transitions

Figure 3: Swift language transitions

This is all very interesting but still very abstract. Let’s try to improve that situation by examining an actual Swift program during each step of the process.

Peeling Back the Layers of the Onion

For our adventure into the Swift compiler, we’ll simply need a text editor and the trusty command line.

  1. Use your favorite text editor to create a file with the following Swift code:
    let a = 3
    let b = 4
    let c = a + b
  2. Save this file as onion.swift to your desktop or some other convenient location.
  3. Launch the OS X Terminal application (or your own personal favorite terminal substitute) and change to the directory where onion.swift is located.
  4. Run the following command and examine the output.
    swiftc –help

    If you look at some of the “modes” that are listed near the top of the output, you’ll see options that correspond to what’s shown in Figure 3.

    -dump-ast      Parse and ... dump AST(s)
    -emit-sil      Emit canonical SIL file(s)
    -emit-ir       Emit LLVM IR file(s)
    -emit-assemblyEmit assembly file(s)

    Before we use these modes to examine the individual steps, we can use a mode that will combine all of the steps and build a working executable.

  5. Run the following command to create an OS X executable from the Swift code
    swiftc –emit-executable onion.swift

    This will create a new executable file in the same directory.

  6. Run the executable and check that the output is indeed 7

Great! We can, in fact, eat the onion that we created. Now, we can start peeling back the layers.

Swift Abstract Syntax Tree (AST)

The first layer we’ll look at is the abstract syntax tree (AST). This is the first step for most compilers – to convert the source code from a form that is “human friendly” into a syntax tree that is easier to work with programmatically.

Run the following command to output the AST for our simple Swift application:

swiftc –dump-ast onion.swift

The AST isn’t the prettiest to look at, but it isn’t meant to be. However, with this simple program, you can probably follow what’s there and maybe even pick up a few interesting nuggets. One nugget we’ll focus on right now is the content that corresponds to the addition operation (let c = a + b).

(declref_expr type='(Int, Int) -> Int'
       location=onion.swift:3:11 range=[onion.swift:3:11 –
       line:3:11] decl=Swift.(file).+ specialized=no)

Nothing really special here – it’s an expression that takes two Swift integers and returns a Swift integer.

Swift Intermediate Language (SIL)

The next layer of our onion is the Swift Intermediate Language that is generated from the AST. This intermediate form is used by the Swift optimizer to perform Swift language specific optimizations prior to generating the LLVM IR.

To view the SIL for our Swift application, execute the following command:

swiftc –emit-sil onion.swift

Well, this is even uglier to look at but that’s sort of expected since we’re moving closer to the machine code representation of our program. Once again, let’s see if we can spot the code that corresponds to “let c = a + b”.

%11 = builtin_function_ref "sadd_with_overflow_Word" : $@thin (Builtin.Word, Builtin.Word, Builtin.Int1) -> (Builtin.Word, Builtin.Int1)

What’s interesting here is that the addition operation is being performed through the use of a “builtin” function named “sadd_with_overflow_Word”. Builtin functions are special since they are available only to the Swift standard library and typically map directly to LLVM IR instructions.

The use of “Word” as the data type reflects the fact that the Int data type in Swift corresponds to the native word size of the target architecture (32 or 64 bits).

LLVM Intermediate Representation (LLVM IR)

It’s time for the next layer of the onion. This is the point where the SIL is translated into LLVM IR. This is the representation that the LLVM optimizer can work with to perform language-independent optimizations such as inlining and loop optimizations.

To peel back this layer, execute the following command to view the LLVM IR for our Swift program:

swiftc –emit-ir onion.swift

Now things are really starting to look low-level. Once again, let’s see if we can spot our addition operation.

%3 = call { i64, i1 } @llvm.sadd.with.overflow.i64(i64 %1, i64 %2)

So, now our addition operation has been transformed into a call to the llvm.sadd.with.overflow.i64 instruction. One thing that’s interesting here is that if we were compiling a different language (e.g. Objective-C), we might very see the same LLVM IR.

If learning more about LLVM IR is something you’re really into, you’re in luck. LLVM is extensively documented here. If you want to invent your own language and use LLVM, you’ll need to build a frontend that can generate LLVM IR. The good news – that’s all you’ll have to do. You’ll get the LLVM optimizer and backend code generators for free.

Assembly Language

If you’ve been able to follow along up until now – congratulations. We’re now at the last layer of the onion. It’s time to see the platform-specific assembly language for our Swift program.

To view the assembly language that’s created from the LLVM IR, execute the following command:

swiftc –emit-assembly onion.swift

There is some very good news contained within this output. Specifically, we see that our high-level Swift code that adds two Swift integers does in fact translate into the proper (and very efficient) x86-64 addq instruction for adding two 64-bit integers.

Admittedly, we have been examining a very simple case (adding two integers). However, you can use this same procedure to examine every layer of the onion for any piece of Swift code.

One important caveat is that we have been performing all of these operations with optimizations enabled. If you decide to perform some additional experiments with more complex code, you can include the –O option when executing the commands in this article to enable optimizations so that you can see output closer to what you would receive when compiling a project with Xcode in release mode.


In this article, we’ve looked at how the Swift code you write gets transformed into platform-specific machine code. There are times when Swift code can be interpreted, as we see with Playgrounds and the Swift REPL. However, when you build a project with Xcode, your code is compiled into a native executable and the compiler does a pretty good job. In fact, in many cases, your Swift code will execute faster than the equivalent Objective-C code.

As the saying goes though – garbage in, garbage out. The Swift and LLVM optimizers will do the best they can with your code but they can’t determine your intent if you don’t express it clearly in Swift. In Part III, we’ll look at a syntax-level feature of Swift that causes a lot of developers trouble – optionals.

Author: Jason Bell, Accelebrate’s Director of Technology and senior instructor.

Accelebrate offers private Swift training and iOS training and for groups and instructor-led online Swift classes for individuals.


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