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How to Set Up an Edit-Build-Test-Debug Loop

This document describes how to set up a development loop for people interested in contributing to Swift.

If you are only interested in building the toolchain as a one-off, there are a couple of differences:

  1. You can ignore the parts related to Sccache.
  2. You can stop reading after Building the project for the first time.

Table of Contents

System Requirements

  1. Operating system: The supported operating systems for developing the Swift toolchain are: macOS, Ubuntu Linux LTS, and the latest Ubuntu Linux release. At the moment, Windows is not supported as a host development operating system. Experimental instructions for Windows are available under Windows.md.
  2. Python 3: Several utility scripts are written in Python.
  3. Git 2.x to check out the sources. We find that older versions of Git can't successfully check out all of the required repositories or fail during a rebase when switching between checkout schemes.
  4. Disk space: Make sure that you have enough available disk space before starting. The source code, including full git history, requires about 3.5 GB. Build artifacts take anywhere between 5 GB to 100 GB, depending on the build settings. It is recommended to have at least 150 GB of available disk space.
  5. RAM: It is recommended to have at least 8 GB for building a toolchain and 16 GB for development. When building for development on a virtual machine or emulator, you might need more than 32 GB.
  6. Time: Depending on your machine and build settings, a from-scratch build can take a few minutes to several hours, so you might want to grab a beverage while you follow the instructions. Incremental builds are much faster.

Cloning the project

  1. Create a directory for the whole project:

    mkdir swift-project
cd swift-project

    Warning
    Make sure the absolute path to your swift-project directory does not contain spaces, since that might cause issues during the build step.

  2. Clone the sources:

    • Via SSH (recommended): If you plan on contributing regularly, cloning over SSH provides a better experience. After you've uploaded your SSH keys to GitHub: 
      git clone git@github.com:apple/swift.git swift
cd swift
utils/update-checkout --clone-with-ssh
    • Via HTTPS: If you want to check out the sources as read-only, or are not familiar with setting up SSH, you can use HTTPS instead: 
      git clone https://github.com/apple/swift.git swift
cd swift
utils/update-checkout --clone

    Note
    If you've already forked the project on GitHub at this stage, do not clone your fork to start off. We describe how to setup your fork in a subsection below.

  3. Double-check that swift's sibling directories are present.

    ls ..

    This should list directories like llvm-project, swiftpm and so on.

  4. Checkout the right branch/tag: If you are building the toolchain for local development, you can skip this step, as Step 2 will checkout swift's main branch and matching branches for other projects. If you are building the toolchain as a one-off, it is more likely that you want a specific branch or a tag, often corresponding to a specific release or a specific snapshot. You can update the branch/tag for all repositories as follows:

    utils/update-checkout --scheme mybranchname
# OR
utils/update-checkout --tag mytagname

    Detailed branching information, including names for release branches, can be found in Branches.md.

Note
The commands used in the rest of this guide assumes that the absolute path to your working directory is something like /path/to/swift-project/swift. Double-check that running pwd prints a path ending with swift.

Troubleshooting cloning issues

  • If update-checkout failed, double-check that the absolute path to your working directory does not have non-ASCII characters.
  • Before running update-checkout, double-check that swift is the only repository inside the swift-project directory. Otherwise, update-checkout may not clone the necessary dependencies.

Installing dependencies

macOS

  1. Install Xcode. The minimum required version is specified in the node information on https://ci.swift.org, may change frequently, and is often a beta release.
  2. Install CMake, Ninja and Sccache:

Linux

  1. The latest Linux dependencies are listed in the respective Dockerfiles:

    Note that a prebuilt Swift release toolchain is installed and added to the PATH in all these Docker containers: it is recommended that you do the same, in order to build the portions of the Swift compiler written in Swift.

  2. To install Sccache (optional):

    • If you're not building within a Docker container:

      sudo snap install sccache --candidate --classic

    • If you're building within a Docker container, you'll have to install sccache manually, since snap is not available in environments without systemd:

      SCCACHE_VERSION=v0.3.0
curl -L "https://github.com/mozilla/sccache/releases/download/${SCCACHE_VERSION}/sccache-${SCCACHE_VERSION}-$(uname -m)-unknown-linux-musl.tar.gz" -o sccache.tar.gz
tar xzpvf sccache.tar.gz
sudo cp "sccache-${SCCACHE_VERSION}-$(uname -m)-unknown-linux-musl/sccache" /usr/local/bin
sudo chmod +x /usr/local/bin/sccache

Note
LLDB currently requires at least swig-1.3.40 but will successfully build with version 2 shipped with Ubuntu.

Building the project for the first time

Spot check dependencies

  • Run cmake --version; this should be at least 3.19.6 (3.24.2 if you want to use Xcode for editing on macOS).
  • Run python3 --version; this should be at least 3.6.
  • Run ninja --version; check that this succeeds.
  • If you installed and want to use Sccache: Run sccache --version; check that this succeeds.

Note
If you are running on Apple Silicon hardware (M1, M2, etc), ensure you have the native arm64 build of these dependencies installed and configured in your PATH.

e.g. running file $(which python3) should print "arm64".

If it prints "x86_64", you are running Python in compatibility mode (Rosetta), and building Swift will fail. Running uname -m should also print "arm64", otherwise your terminal is running in Rosetta mode.

The roles of different tools

At this point, it is worthwhile to pause for a moment to understand what the different tools do:

  1. On macOS and Windows, IDEs (Xcode and Visual Studio resp.) serve as an easy way to install development dependencies such as a C++ compiler, a linker, header files, etc. The IDE's build system need not be used to build Swift. On Linux, these dependencies are installed by the distribution's package manager.

  2. CMake is a cross-platform build system for C and C++. It forms the core infrastructure used to configure builds of Swift and its companion projects.

  3. Ninja is a low-level build system that can be used to build the project, as an alternative to Xcode's build system. Ninja is somewhat faster, especially for incremental builds, and supports more build environments.

  4. Sccache is a caching tool: If you ever delete your build directory and rebuild from scratch (i.e. do a "clean build"), Sccache can accelerate the new build significantly. There are few things more satisfying than seeing Sccache cut through build times.

    Note Sccache defaults to a cache size of 10GB, which is relatively small compared to build artifacts. You can bump it up, say, by setting export SCCACHE_CACHE_SIZE="50G" in your dotfile(s).

  5. utils/update-checkout is a script to help you work with all the individual git repositories together, instead of manually cloning/updating each one.

  6. utils/build-script (we will introduce this shortly) is a high-level automation script that handles configuration (via CMake), building (via Ninja), caching (via Sccache), running tests and more.

Pro Tip: Most tools support --help flags describing the options they support. Additionally, both Clang and the Swift compiler have hidden flags (clang --help-hidden/swiftc --help-hidden) and frontend flags (clang -cc1 --help/swiftc -frontend --help) and the Swift compiler even has hidden frontend flags (swiftc -frontend --help-hidden). Sneaky!

Phew, that's a lot to digest! Now let's proceed to the actual build itself!

The actual build

Build the toolchain with optimizations, debuginfo, and assertions, using Ninja:

  • macOS:

    utils/build-script --skip-build-benchmarks \
  --skip-ios --skip-watchos --skip-tvos --swift-darwin-supported-archs "$(uname -m)" \
  --sccache --release-debuginfo --swift-disable-dead-stripping \
  --bootstrapping=hosttools

  • Linux:

    utils/build-script --release-debuginfo
    • If you want to additionally build the Swift core libraries, i.e., swift-corelibs-libdispatch, swift-corelibs-foundation, and swift-corelibs-xctest, add --xctest to the invocation.

  • If you installed and want to use Sccache, add --sccache to the invocation.

  • If you want to use a debugger such as LLDB on compiler sources, add --debug-swift to the invocation: a fruitful debugging experience warrants non-optimized code besides debug information.

This will create a directory swift-project/build/Ninja-RelWithDebInfoAssert containing the Swift compiler and standard library and clang/LLVM build artifacts. If the build fails, see Troubleshooting build issues.

In the following sections, for simplicity, we will assume that you are using a Ninja-RelWithDebInfoAssert build on macOS, unless explicitly mentioned otherwise. You will need to slightly tweak the paths for other build configurations.

Troubleshooting build issues

Editing code

Setting up your fork

If you are building the toolchain for development and submitting patches, you will need to setup a GitHub fork.

First fork the apple/swift repository, using the "Fork" button in the web UI, near the top-right. This will create a repository username/swift for your GitHub username. Next, add it as a remote:

# Using 'my-remote' as a placeholder name.

# If you set up SSH in step 2
git remote add my-remote git@github.com:username/swift.git

# If you used HTTPS in step 2
git remote add my-remote https://github.com/username/swift.git

Finally, create a new branch.

# Using 'my-branch' as a placeholder name
git checkout -b my-branch
git push --set-upstream my-remote my-branch

Using Ninja with Xcode

This workflow enables you to edit, build, run, and debug in Xcode. The following steps assume that you have already built the toolchain with Ninja.

Note
A seamless LLDB debugging experience requires that your build-script invocation for Ninja is tuned to produce build rules for the debug variant of the component you intend to debug.

  • Generate the Xcode project with 

    utils/build-script --swift-darwin-supported-archs "$(uname -m)" --xcode --clean

    This can take a few minutes due to metaprogrammed sources that depend on LLVM tools that are built from source.

  • Create an empty Xcode workspace.

  • Add build/Xcode-*/swift-macosx-*/Swift.xcodeproj to the workspace. If Xcode prompts to autocreate schemes, select Manually Manage Schemes and don't create any schemes just yet.

    This project includes the sources for almost everything in the repository, including the compiler, standard library and runtime. If you intend to work on a compiler subcomponent that is written in Swift and has a Package.swift file (e.g. lib/ASTGen), first choose Product > Scheme > Manage Schemes... and select the Autocreate schemes checkbox, then add the package directory to the workspace by choosing File > Add Files to "<workspace name>". Xcode will automatically create schemes for package manifest.

  • Create an Xcode project using the External Build System template, and add it to the workspace.

  • Create a target in the new project, using the External Build System template.

  • In the Info pane of the target settings, set

    • Build Tool to the absolute path of the ninja executable (the output of which ninja on the command line)
    • Arguments to a Ninja target (e.g. bin/swift-frontend is the compiler)
    • Directory to the absolute path of the build/Ninja-*/swift-macosx-* directory

  • Create a scheme in the workspace, making sure to select the target you just created. Be extra careful not to choose a target from the generated Xcode project you added to the workspace.

  • Spot-check your target in the settings for the Build scheme action.

  • If the target is executable, adjust the settings for the Run scheme action:

    • In the Info pane, select the Executable produced by the Ninja target from build/Ninja-*/swift-macosx-*/bin (e.g. swift-frontend).
    • In the Arguments pane, add command line arguments that you want to pass to the executable on launch (e.g. path/to/file.swift -typecheck for bin/swift-frontend).
    • Optionally set a custom working directory in the Options pane.

  • Follow the previous steps to create more targets and schemes per your line of work.

Regenerating the Xcode project

The structure of the generated Xcode project is distinct from the underlying organization of the files on disk, and does not adapt to changes in the file system, such as file/directory additions/deletions/renames. Over the course of multiple update-checkout rounds, the resulting divergence is likely to begin affecting your editing experience. To fix this, regenerate the project by running the invocation from the first step.

Troubleshooting editing issues in Xcode

  • If a syntax highlighting or code action issue does not resolve itself after regenerating the Xcode project, select a scheme that covers the affected area and try Product > Analyze.
  • Xcode has been seen to sometimes get stuck on indexing after switching back and forth between distant branches. To sort things out, close the workspace and delete the Index directory from its derived data.

Other IDEs setup

You can also use other editors and IDEs to work on Swift.

IntelliJ CLion

CLion supports CMake and Ninja. In order to configure it properly, build the swift project first using the build-script, then open the swift directory with CLion and proceed to project settings (cmd + ,).

In project settings, locate Build, Execution, Deployment > CMake. You will need to create a new profile named RelWithDebInfoAssert (or Debug if going to point it at the debug build). Enter the following information:

  • Name: mirror the name of the build configuration here, e.g. RelWithDebInfoAssert or Debug
  • Build type: This corresponds to CMAKE_BUILD_TYPE so should be e.g. RelWithDebInfoAssert or Debug
    • latest versions of the IDE suggest valid values here. Generally RelWithDebInfoAssert is a good one to work with
  • Toolchain: Default should be fine
  • Generator: Ninja
  • CMake options: You want to duplicate the essential CMake flags that build-script had used here, so CLion understands the build configuration. You can get the full list of CMake arguments from build-script by providing the -n dry-run flag; look for the last cmake command with a -G Ninja. Here is a minimal list of what you should provide to CLion here for this setting:
    • -D SWIFT_PATH_TO_CMARK_BUILD=SOME_PATH/swift-project/build/Ninja-RelWithDebInfoAssert/cmark-macosx-arm64 -D LLVM_DIR=SOME_PATH/swift-project/build/Ninja-RelWithDebInfoAssert/llvm-macosx-arm64/lib/cmake/llvm -D Clang_DIR=SOME_PATH/swift-project/build/Ninja-RelWithDebInfoAssert/llvm-macosx-arm64/lib/cmake/clang -D CMAKE_BUILD_TYPE=RelWithDebInfoAssert -D SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE=SOME_PATH/swift-project/swift-syntax -G Ninja -S .
    • replace the SOME_PATH to the path where your swift-project directory is
    • the CMAKE_BUILD_TYPE should match the build configuration name, so if you named this profile RelWithDebInfo the CMAKE_BUILD_TYPE should also be RelWithDebInfo
    • Note: If you're using an Intel machine to build swift, you'll need to replace the architecture in the options. (ex: arm64 with x86_64)
  • Build Directory: change this to the Swift build directory corresponding to the build-script run you did earlier, for example, SOME_PATH/swift-project/build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64.

With this done, CLion should be able to successfully import the project and have full autocomplete and code navigation powers.

Editing

Make changes to the code as appropriate. Implement a shiny new feature! Or fix a nasty bug! Update the documentation as you go! The codebase is your oyster!

🚧👷🏗️

Now that you have made some changes, you will need to rebuild...

Incremental builds with Ninja

Subsequent steps in this and the next subsections are specific to the platform you're building on, so we'll try to detect it first and reuse as a shell variable:

platform=$([[ $(uname) == Darwin ]] && echo macosx || echo linux)

After setting that variable you can rebuild the compiler incrementally with this command:

ninja -C ../build/Ninja-RelWithDebInfoAssert/swift-${platform}-$(uname -m) bin/swift-frontend

To rebuild everything that has its sources located in the swift repository, including the standard library:

ninja -C ../build/Ninja-RelWithDebInfoAssert/swift-${platform}-$(uname -m)

Similarly, you can rebuild other projects like Foundation or Dispatch by substituting their respective subdirectories in the commands above.

Spot checking an incremental build

As a quick test, go to lib/Basic/Version.cpp and tweak the version printing code slightly. Next, do an incremental build as above. This incremental build should be much faster than the from-scratch build at the beginning. Now check if the version string has been updated (assumes you have platform shell variable defined as specified in the previous subsection:

../build/Ninja-RelWithDebInfoAssert/swift-$(platform)-$(uname -m)/bin/swift-frontend --version

This should print your updated version string.

Reproducing an issue

Good first issues typically have small code examples that fit within a single file. You can reproduce such an issue in various ways, such as compiling it from the command line using /path/to/swiftc MyFile.swift, pasting the code into Compiler Explorer (aka godbolt) or using an Xcode Playground.

For files using frameworks from an SDK bundled with Xcode, you need the pass the SDK explicitly. Here are a couple of examples:

# Compile a file to an executable for your local machine.
xcrun -sdk macosx /path/to/swiftc MyFile.swift

# Say you are trying to compile a file importing an iOS-only framework.
xcrun -sdk iphoneos /path/to/swiftc -target arm64-apple-ios13.0 MyFile.swift

You can see the full list of -sdk options using xcodebuild -showsdks, and check some potential -target options for different operating systems by skimming the compiler's test suite under test/.

Sometimes bug reports come with SwiftPM packages or Xcode projects as minimal reproducers. While we do not add packages or projects to the compiler's test suite, it is generally helpful to first reproduce the issue in context before trying to create a minimal self-contained test case. If that's the case with the bug you're working on, check out our instructions on building packages and Xcode projects with a locally built compiler.

Running tests

There are two main ways to run tests:

  1. utils/run-test: By default, run-test builds the tests' dependencies before running them. 
    # Rebuild all test dependencies and run all tests under test/.
utils/run-test --lit ../llvm-project/llvm/utils/lit/lit.py \
  ../build/Ninja-RelWithDebInfoAssert/swift-macosx-$(uname -m)/test-macosx-$(uname -m)

# Rebuild all test dependencies and run tests containing "MyTest".
utils/run-test --lit ../llvm-project/llvm/utils/lit/lit.py \
  ../build/Ninja-RelWithDebInfoAssert/swift-macosx-$(uname -m)/test-macosx-$(uname -m) \
  --filter="MyTest"
  2. lit.py: lit doesn't know anything about dependencies. It just runs tests. 
    # Run all tests under test/.
../llvm-project/llvm/utils/lit/lit.py -s -vv \
  ../build/Ninja-RelWithDebInfoAssert/swift-macosx-$(uname -m)/test-macosx-$(uname -m)

# Run tests containing "MyTest"
../llvm-project/llvm/utils/lit/lit.py -s -vv \
  ../build/Ninja-RelWithDebInfoAssert/swift-macosx-$(uname -m)/test-macosx-$(uname -m) \
  --filter="MyTest"
    The -s and -vv flags print a progress bar and the executed commands respectively.

If you are making small changes to the compiler or some other component, you'll likely want to incrementally rebuild only the relevant target and use lit.py with --filter. One potential failure mode with this approach is accidental use of stale binaries. For example, say that you want to rerun a SourceKit test but you only incrementally rebuilt the compiler. Then your changes will not be reflected when the test runs because the sourcekitd binary was not rebuilt. Using run-test instead is the safer option, but it will lead to a longer feedback loop due to more things getting rebuilt.

In the rare event that a local test failure happens to be unrelated to your changes (is not due to stale binaries and reproduces without your changes), there is a good chance that it has already been caught by our continuous integration infrastructure, and it may be ignored.

If you want to rerun all the tests, you can either rebuild the whole project and use lit.py without --filter or use run-test to handle both aspects.

For more details on running tests and understanding the various Swift-specific lit customizations, see Testing.md. Also check out the lit documentation to understand how the different lit commands work.

Debugging issues

In this section, we briefly describe two common ways of debugging: print debugging and using LLDB.

Depending on the code you're interested in, LLDB may be significantly more effective when using a debug build. Depending on what components you are working on, you could turn off optimizations for only a few things. Here are some example invocations:

# optimized Stdlib + debug Swiftc + optimized Clang/LLVM
utils/build-script --release-debuginfo --debug-swift # other flags...

# debug Stdlib + optimized Swiftc + optimized Clang/LLVM
utils/build-script --release-debuginfo --debug-swift-stdlib # other flags...

# optimized Stdlib + debug Swiftc (except typechecker) + optimized Clang/LLVM
utils/build-script --release-debuginfo --debug-swift --force-optimized-typechecker

# Last resort option, it is highly unlikely that you will need this
# debug Stdlib + debug Swiftc + debug Clang/LLVM
utils/build-script --debug # other flags...

Debug builds have two major drawbacks:

  • A debug compiler is much slower, leading to longer feedback loops in case you need to repeatedly compile the Swift standard library and/or run a large number of tests.
  • The build artifacts consume a lot more disk space.

DebuggingTheCompiler.md goes into a LOT more detail on how you can level up your debugging skills! Make sure you check it out in case you're trying to debug a tricky issue and aren't sure how to go about it.

Print debugging

A large number of types have dump(..)/print(..) methods which can be used along with llvm::errs() or other LLVM streams. For example, if you have a variable std::vector<CanType> canTypes that you want to print, you could do:

auto &e = llvm::errs();
e << "canTypes = [";
llvm::interleaveComma(canTypes, e, [&](auto ty) { ty.dump(e); });
e << "]\n";

You can also crash the compiler using assert/llvm_unreachable/ llvm::report_fatal_error, after accumulating the result in a stream:

std::string msg; llvm::raw_string_ostream os(msg);
os << "unexpected canTypes = [";
llvm::interleaveComma(canTypes, os, [&](auto ty) { ty.dump(os); });
os << "] !!!\n";
llvm::report_fatal_error(os.str());

Debugging using LLDB

When the compiler crashes, the commandline arguments passed to it will be printed to stderr. It will likely look something like:

/path/to/swift-frontend <args>

  • Using LLDB on the commandline: Copy the entire invocation and pass it to LLDB.

    lldb -- /path/to/swift-frontend <args>

    Now you can use the usual LLDB commands like run, breakpoint set and so on. If you are new to LLDB, check out the official LLDB documentation and nesono's LLDB cheat sheet.

  • Using LLDB within Xcode: Select the current scheme 'swift-frontend' → Edit Scheme → Run phase → Arguments tab. Under "Arguments Passed on Launch", copy-paste the <args> and make sure that "Expand Variables Based On" is set to swift-frontend. Close the scheme editor. If you now run the compiler (⌘+R or Product → Run), you will be able to use the Xcode debugger.

    Xcode also has the ability to attach to and debug Swift processes launched elsewhere. Under Debug → Attach to Process by PID or name..., you can enter a compiler process's PID or name (swift-frontend) to debug a compiler instance invoked elsewhere. This can be helpful if you have a single compiler process being invoked by another tool, such as SwiftPM or another open Xcode project.

    Pro Tip: Xcode 12's terminal does not support colors, so you may see explicit color codes printed by dump() methods on various types. To avoid color codes in dumped output, run expr llvm::errs().enable_color(false).

Next steps

Make sure you check out the following resources:

  • LLVM Coding Standards: A style guide followed by both LLVM and Swift. If there is a mismatch between the LLVM Coding Standards and the surrounding code that you are editing, please match the style of existing code.
  • LLVM Programmer's Manual: A guide describing common programming idioms and data types used by LLVM and Swift.
  • docs/README.md: Provides a bird's eye view of the available documentation.
  • Lexicon.md: Provides definitions for jargon. If you run into a term frequently that you don't recognize, it's likely that this file has a definition for it.
  • Testing.md and DebuggingTheCompiler.md: These cover more ground on testing and debugging respectively.
  • Development Tips: Tips for being more productive.

If you see mistakes in the documentation (including typos, not just major errors) or identify gaps that you could potentially improve the contributing experience, please start a discussion on the forums, submit a pull request or file a bug report on Swift repository 'Issues' tab. Thanks!