Swift compiler development on a remote Linux machine

Building Swift is intensive on CPU, memory, and diskspace. Working on the compiler in parallel with app development wouldn’t have been possible on my Macbook Air. So, I’ve been doing my Swift compiler work on a Linode server instead.

If you’re interested in starting on Swift compiler development, renting out a remote server like this can be a great way to get your feet wet.

So, let’s get started on setting up a remote server for Swift compiler development.

Choose a remote Linux machine

Choosing a machine configuration depends on the system requirements for building Swift.

For hacking on Swift compiler features, the Swift repository’s README recommends building with --release-debuginfo --debug-swift, which includes debug information when compiling llvm, the Swift compiler, and the Swift standard library. A build like this would require at least 8 GB of RAM.

However, we can reduce the RAM required – and thereby adopt a less expensive server configuration – by building just the compiler part with debug symbols, with --release --debug-swift. Doing that would enable us build Swift on a 4 GB RAM machine. As long as we’re only going to work on compiler features, this build shall suffice.

In terms of disk space, if we build only Swift with debug symbols, we’ll need about 30 GB per build; if we build everything with debug symbols, we’ll need about 80 GB per build.

Apart from memory and diskspace, it’s a good idea to pick a server close to our location to reduce latency.

Once the Linux server is created and booted, we can start setting it up.

Set up the remote Linux machine

Set up a user

The freshly created server only has a root login now. We can login to that machine using the root password specified or obtained while creating the VPS:

$ ssh root@<ip-address> # Enter password

Once logged in, create a new user. Specify a password when prompted. You will need this password later when you use sudo commands.

# adduser <user> # Specify and remember password. Other info can be empty.
# usermod -aG sudo <user> # Give <user> sudo privileges
# su <user> # Switch to that user

Set up SSH keys

Create the .ssh directory:

$ mkdir ~/.ssh
$ chmod 700 ~/.ssh

Assuming you have an SSH keypair handy (they should be in ~/.ssh on your local machine), add your public key as an authorized key on the remote server.

$ cat >> ~/.ssh/authorized_keys # Paste your public key
$ chmod 600 ~/.ssh/authorized_keys

This should enable you to ssh into the remote server without having to type the server password.

Now, it’s a good idea to disable password-based login:

$ sudo vim /etc/ssh/sshd_config # set 'PasswordAuthentication' to 'no'
$ sudo systemctl reload sshd # Reload the SSH daemon

Set up tmux

We use tmux to keep terminal sessions alive on the server even when our SSH connection breaks. tmux is a terminal multiplexer, so one tmux session can encapsulate multiple terminal sessions.

Install tmux on the server, if it’s not installed already.

$ sudo apt-get install tmux

Create a new tmux session called “swift” in detatched mode.

$ tmux new-session -s swift -d

The newly created tmux session will have one tmux window, and therefore one terminal session. We can list the active tmux sessions by saying:

$ tmux list-sessions

We’ll see that we have one tmux session with one window. We can create more tmux windows and thereby, more terminal sessions. There are tmux commands to do that, but an easier way to work with tmux is through iTerm2 from your Mac.

Set up the local macOS machine

Set up iTerm2

If you don’t have iTerm2 on your Mac, download and install it. The following instructions assume iTerm2 version 3.3.7.

  1. Open iTerm2

  2. Go to iTerm2 > Preferences > Profiles and add a new profile (say “Swift dev”) using the “+” at the bottom left.

  3. Select that profile on the left, then select the General tab on the right.

    Under the Command section, select Command and enter the command as:

    ssh -t <user>@<ip-address> "tmux -CC attach -t swift"

    substituting your remote server username and ip address.

Using iTerm2’s tmux integration

Once we’ve created the iTerm2 profile, if we right-click on iTerm2 on the dock, and click on New Window … > Swift dev (or whatever you named your profile), iTerm2 will:

  • Open a new window and run the ssh command
  • Once the ssh connection is established, run the tmux attach command on the server to attach to the tmux session we’d started
  • Get the state of the tmux windows in the tmux session and show them as new tabs or windows in iTerm2

The tmux session we had created on the server has just one window. So if we right-click on iTerm2 and click on New Window … > Swift dev, two new windows shall open:

  1. First, a window that runs ssh and then the tmux attach command. This window shall have no command prompt.
  2. Next, a window that shows the tmux session’s tmux window, with a command prompt.

With the second window selected, iTerm2’s New Window and New Tab commands will show a prompt asking if we want to create a new tmux tab or window. If we pick that option, iTerm2 will create a new tmux window in our tmux session and will open that window in a new tab / window. When closing such a tab / window, iTerm2 will ask us if we want to kill that window in the tmux session, or do we want to just hide it. I prefer killing a tmux window when the corresponsing tab is closed.

If you’re done working on the server, you can close the first window, which shall close the ssh connection and will then close all the tmux windows. This can also happen when the ssh connection is broken due to inactivity or a broken network connection.

To get back to working on the server, you can right-click on iTerm2 and click on New Window … > Swift dev again, and all our tmux windows will be back.

Configuring iTerm2’s tmux integration

This is how I like my iTerm2 configured for working on Swift:

  • Make each tab correspond to a tmux window without showing the prompts mentioned earlier

    Go to iTerm2 > Preferences > Advanced, search for “tmux”.

    • Under Tmux Integration, turn on Suppress alert asking what kind of tab to use in tmux integration and Suppress alert asking what kind of window to open in tmux integration.

    • Under Warnings, turn on Suppress kill/hide dialog when closing a tmux tab and Suppress kill/hide dialog when closing a tmux window.

    With that, while working on a tmux window, ⌘-T and ⌘-N open new tmux windows, and closing a tab with a tmux window kills the tmux window.

  • Use different-sized windows

    Previously, tmux required that all tmux windows in a session be resized to the size of the smallest client viewport. So when we have tmux windows from one tmux session spread across multiple iTerm2 windows, resizing one of those iTerm2 windows will resize the others as well.

    If you have tmux version 2.9 or later (to know the tmux version, run tmux -V on the server), we can fix this:

    • Go to iTerm2 > Preferences > Advanced. Under Experimental Features, turn on Allow variable window sizes in tmux integration.

    • Go to iTerm2 > Preferences > General and select the tmux tab. For Open tmux windows as:, select “Native windows”.

    I learnt about this option from here.

  • Account for many tmux windows in a tmux session

    When connecting to a tmux session, if you have too many tmux windows (10 tmux windows by default), iTerm2 will show the “tmux dashboard” instead of opening the windows directly. We can then open the unopened tmux windows in tabs or windows from the dashboard.

    I find this limit of 10 to be too low, so I set it to 20. This can be configured in the tmux dashboard itself, by going to Shell > tmux > Dashboard in the iTerm2 menu.

  • Color scheme

    By default, the tmux windows are created with a profile called “tmux”. I changed the color scheme for this profile by doing this:

    1. Go to iTerm2 > Preferences > Profiles
    2. Select tmux on the left, select the Colors tab on the right
    3. Select a preset in Color Presets (I recommend Solarized Dark)
  • Always show tab bar

    Even if there’s just one tab in a window, I like the tab bar to show up so that I can easily rearrange tmux windows into iTerm2 windows as I want. To do that:

    1. Go to iTerm2 > Preferences > Appearance
    2. Go to the Tabs tab
    3. Turn on Show tab bar even when there is only one tab

If you want to poke around for more tmux-specific configuration options, you can try these:

  • See the iTerm2 wiki page on configuring iTerm2’s tmux integration
  • Go to iTerm2 > Preferences > General and select the tmux tab
  • Go to iTerm2 > Preferences > Advanced and search for “tmux”

Build Swift

Get the dependencies

On the server, install all the required dependancies to build Swift using the apt-get command from the Swift repository.

Get the code

Before building Swift, we need to clone Swift repository, and a bunch of other repositories as well. We create a directory under which all these repositories will be placed:

$ mkdir ~/swift-source

And then clone all the required repositories under that directory:

$ cd ~/swift-source
$ git clone https://github.com/apple/swift.git
$ ./swift/utils/update-checkout --clone

Build it

The Swift project’s build system uses CMake to create Ninja build files, and Ninja to use those build files to compile and link code. The Swift repository includes a build script called build-script that builds CMake and Ninja from source, and then uses that CMake and Ninja to build llvm, Swift and the Swift standard library.

By default, build-script will spawn as many parallel jobs as there are CPUs in the machine, which is good while compiling. However, linking uses a lot of memory, so we don’t want to be doing multiple link jobs at a time.

There's no way I know of to tell the build system to not use parallel jobs only while linking, so the easy way out is to ask the build script to run all jobs sequentially (`-j1`). This will however increase the initial build time quite a bit.

Update: To tell the build system not to use parallel jobs while linking, we can set the relevant CMake properties using the --llvm-cmake-options and --swift-cmake-options arguments.

To be able to work on compiler features, we want to build the compiler in debug mode. If you work on other parts of Swift (say the Swift standard library), or on other aspects of the compiler (like performance), the options would be different.

To build only Swift in debug mode and everything else in release mode, we say:

$ cd ~/swift-source
$ ./swift/utils/build-script --release --debug-swift --llvm-cmake-options==-DLLVM_PARALLEL_LINK_JOBS=1 --swift-cmake-options=-DSWIFT_PARALLEL_LINK_JOBS=1

This build can take about 3 hours, and will write the build results to build/Ninja-ReleaseAssert+swift-DebugAssert.

It helps to set up SWIFT_BUILD_DIR and PATH environment variables like this:

$ cat >> ~/.bash_profile
export SWIFT_BUILD_DIR=~/swift-source/build/Ninja-ReleaseAssert+swift-DebugAssert/swift-linux-x86_64
export PATH=${PATH}:${SWIFT_BUILD_DIR}/bin # For swift, swiftc, etc.
export PATH=${PATH}:~/swift-source/llvm-project/utils/lit # For lit.py
$ source ~/.bash_profile

Having swift and swiftc in our PATH would help us run the built Swift from any directory. Having lit.py in our PATH would help us run the Swift testsuite from any directory.

In case you want to build everything with debug information (and your server has 8 GB RAM or more), you can pass --release-debuginfo instead of --release to build-script. This build will take longer to complete, and the build results will be placed in a different directory.

Set up the development environment


Setup username and email for creating commit messages.

git config --global user.name “Roopesh Chander”
git config --global user.email roop@roopc.net

Setup the editor for editing commit messages and interactive rebase. I use vim.

git config --global core.editor vim


The Swift compiler’s C++ code uses two spaces for indentation, so we can add these to our ~/.vimrc:

set expandtab
set tabstop=2
set shiftwidth=2

For help in editing Swift, SIL and gyb files, we can use the vim support files from the Swift repository itself by:

  • adding this line to ~/.vimrc:

    set runtimepath+=~/swift-source/swift/utils/vim
  • making the filetype detection files accessible under ~/.vim/ftdetect:

    $ mkdir -p ~/.vim/ftdetect
    $ cd ~/.vim/ftdetect
    $ ln -s ~/swift-source/swift/utils/vim/ftdetect/*.vim .


We just need to install lldb:

$ sudo apt-get install lldb

Set up our fork of Swift

Contributions to the Swift compiler are done through GitHub pull requests. To create pull requests, we should have a fork of the Swift repository under our GitHub account. To fork Swift, go to Swift’s GitHub page on a web browser and click on the “Fork” button.

We already have a clone of the main Swift repository. We want to be able to push from there to our fork on GitHub. We don’t want to be specifying our GitHub password every time we push, so we’d like to authenticate to GitHub automatically using SSH keys. However, if we make our private keys available on the server, either by copying our private keys or by using SSH forwarding, it means that in case the server is compromised, that key can be used to control everything in our GitHub account. So, in the interest of security, we’re going to create a separate keypair that we shall use to push to just one repository – our fork of Swift. Since this key controls just this one repository, we can even afford to make the key not password-protected.

  1. On the server, create a keypair:

    $ ssh-keygen

    and just press enter for the prompts (no password required). This shall create ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub.

  2. Start the SSH agent and add the private key to it:

    $ eval `ssh-agent -s`
    $ ssh-add ~/.ssh/id_rsa
  3. Copy the public key:

    $ cat ~/.ssh/id_rsa.pub # Copy the output
  4. Add this public key as a Deploy key in GitHub:

    • Go to your fork’s GitHub page on a web browser (the URL should be like https://github.com/<your-github-username>/swift) and go to Settings > Deploy keys > Add deploy key
    • Under Title, specify a title so that we can identify the deploy key’s origin in the future
    • Under Key, paste the public key we copied in the previous step
    • Tick the checkbox to Allow write access
    • Click on “Add key”
  5. Back on the server, go to our clone of the Swift repository and add our fork as another remote called fork:

    $ cd ~/swift-source/swift
    $ git remote add fork git@github.com:<your-GitHub-username>/swift.git

At this point, we have a working setup. If you’re new to Swift compiler development, now would be a good time to start looking at starter bugs. In the next post, we’ll see how we can work on the Swift compiler with a setup like this.

If the server reboots, we’ll lose the tmux sessions. In that case, we have to start a fresh tmux session. We should also restart the SSH agent and ssh-add the private key for pushing to our fork.

Keep in mind that the server is a recurring cost. So if we know we’re going to not work on the compiler for a few weeks, it can be a good idea to delete the server and spawn a new one when we’re ready. Before deleting, we should take a look at all local branches and the stash, push anything worth saving to our fork, and file-transfer any uncommitted unit testcases, so that we can get back to work with minimum disruption.

If you have questions, corrections, or feedback, please get in touch.

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