| Swift | LLVM* | LLDB |
|---|---|---|
| master | stable | stable |
master is the place for active development on Swift. If you‘re just working on Swift, that’s where you'll spend most of your time.
LLVM repos automatically merge changes from the latest release branch (see below) into stable. This generally means stable is just an alias for the latest release branch. If you want to do Swift-related development on LLVM projects, see “The Upstream Branches” below.
LLDB is a bit more complicated. Because it‘s an LLVM project originally, it follows the LLVM branch names. However, it also contains extra support for Swift, and so it depends on being in sync with Swift as well. Work on LLDB is usually paired with work on Swift, so following Swift’s branch is the way to go.
To switch from one set of branches to another, you can use utils/update-checkout in the Swift repository with the --scheme option. You can use any of the branch names as the argument to --scheme: in this case, either master or stable.
* “LLVM” refers to several repositories from the LLVM project that do not depend on Swift: LLVM, Clang, and compiler-rt. LLDB is different because it imports some of Swift's own headers and thus depends on Swift.
| Swift | LLVM* | LLDB |
|---|---|---|
| swift-x.y-branch | swift-x.y-branch | swift-x.y-branch |
At some point before a release, a release branch will be created in every repository with a name like swift-4.0-branch. (The actual number is chosen by Apple.) After the branch has been created, commits must make it to this branch to make it into the release. In some cases, the release manager for the branch will decide to merge in all additional changes from master; otherwise, cherry-picking changes and making a new pull request is the way to go. If there are any “patch” releases (e.g. Swift 4.0.1), they will also come from this branch.
Note that these branches come not from the “development” branches (above), but the “upstream” branches (below). This is because they need to contain the latest changes not just from Swift, but from the LLVM projects (LLVM, Clang, compiler-rt, and LLDB) as well. For some releases, the release branch for the LLVM projects will be timed to coincide with the corresponding llvm.org release branch.
| Swift | LLVM* | LLDB |
|---|---|---|
| master-next | upstream-with-swift | upstream-with-swift |
upstream-with-swift is a branch for LLVM that includes all changes necessary to support Swift. Changes from llvm.org‘s master branch are automatically merged in. Why isn’t this just stable? Well, because LLVM changes very rapidly, and that wouldn't be very stable. However, we do want to make sure the Swift stuff keeps working.
If you are making changes to LLVM to support Swift, you'll probably need to work on them in stable to test them against Swift itself, but they should be committed to upstream-with-swift, and cherry-picked to the current release branch (swift-x.y-branch) if needed. Remember, the release branches are automerged into stable on a regular basis.
(If you‘re making changes to LLVM or LLDB that *aren’t* about Swift, they should generally be made on llvm.org instead, then cherry-picked to the active release branch or stable.)
master-next is an effort to keep Swift building with the latest LLVM changes. Ideally when LLVM changes, no Swift updates are needed, but that isn‘t always the case. In these situations, any adjustments can go into Swift’s master-next branch. Changes from Swift's master are automatically merged into master-next as well.
LLDB‘s upstream-with-swift has both aspects: changes are automatically merged in from stable and from llvm.org’s master branch. Again, ideally there are no changes necessary here, but in practice LLDB may need updates to continue building against LLVM‘s upstream-with-swift and Swift’s master-next.
swift/utils/update-checkout --scheme [branch]
You can use any of the branch names as the argument to --scheme, such as master or stable. See update-checkout --help for more options.
Swift: new commits go to master
LLVM/Clang/compiler-rt: new commits go to upstream-with-swift
LLDB: new commits go to stable
...then cherry-pick to the release branch (swift-x.y-branch) if necessary, following the appropriate release process. (Usually this means filling out a standard template, finding someone to review your code if that hasn‘t already happened, and getting approval from that repo’s release manager.)
Some branches are automerged into other branches, to keep them in sync. This is just a process that runs git merge at a regular interval. These are run by Apple and are either on a “frequent” (sub-hourly) or nightly schedule.
master is automerged into master-nextswift-x.y-branch (the latest release branch) is automerged into stablemaster is automerged into upstream-with-swiftswift-x.y-branch, if they are in syncswift-x.y-branch (the latest release branch) is automerged into stablestable is automerged into upstream-with-swiftmaster is also automerged into upstream-with-swiftswift-x.y-branch, if they are in sync