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Updating clang

We distribute prebuilt packages of LLVM binaries, including clang and lld, that all developers and bots pull at gclient runhooks time. These binaries are just regular LLVM binaries built at a fixed upstream revision. This document describes how to build a package at a newer revision and update Chromium to it. An archive of all packages built so far is at https://is.gd/chromeclang

  1. Check that https://ci.chromium.org/p/chromium/g/chromium.clang/console looks reasonably green. Red bots with seemingly normal test failures are usually ok, that likely means the test is broken with the stable Clang as well.

  2. Sync your Chromium tree to the latest revision to pick up any plugin changes.

  3. Run go/chrome-promote-clang, passing in the revision of the Clang and Rust package version you want to push. For example:

    $ copy_staging_to_prod_and_goma.sh --clang-rev llvmorg-21-init-5118-g52cd27e6-1 --rust-rev f7b43542838f0a4a6cfdb17fbeadf45002042a77-1

    Alternatively, use go/chrome-push-clang-to-goma. This takes a recent dry run CL to update clang, and if the trybots were successful it will copy the binaries from the staging bucket to the production one.

    Writing to the production bucket requires special permissions. Then it will push the packages to RBE. If you do not have the necessary credentials to do the upload, ask [email protected] to find someone who does.

    • Alternatively, to create your own roll CL, you can manually run tools/clang/scripts/upload_revision.py with a recent upstream LLVM commit hash as the argument. After the *_upload_clang and *_upload_rust trybots are successfully finished, run go/chrome-promote-clang with the new Clang and Rust package names.

  4. Run tools/clang/scripts/sync_deps.py to update the deps entries in DEPS.

  5. gclient sync to download those packages.

  6. Run tools/rust/gnrt_stdlib.py to update the GN files for the Rust standard library.

  7. Run an exhaustive set of try jobs to test the new compiler. The CL description created previously by upload_revision.py includes Cq-Include-Trybots: lines for all needed bots, so it's sufficient to just run git cl try (or hit "CQ DRY RUN" on gerrit).

  8. Commit the roll CL from the previous step.

  9. The bots will now pull the prebuilt binary, and RBE will have a matching binary, too.

Performance regressions

After doing a clang roll, you may get a performance bug assigned to you (example). Some performance noise is expected while doing a clang roll.

You can check all performance data for a clang roll via https://chromeperf.appspot.com/group_report?rev=XXXXXX, where XXXXXX is the Chromium revision number, e.g. 778090 for the example bug (look in the first message of the performance bug to find this). Click the checkboxes to display graphs. Hover over points in the graph to see the value and error.

Serious regressions require bisecting upstream commits (TODO: how to repro?). If the regressions look insignificant and there is green as well as red, you can close the bug as "WontFix" with an explanation.

Adding files to the clang package

The clang package is downloaded unconditionally by all bots and devs. It's called "clang" for historical reasons, but nowadays also contains other mission-critical toolchain pieces besides clang.

We try to limit the contents of the clang package. They should meet these criteria:

  • things that are used by most developers use most of the time (e.g. a compiler, a linker, sanitizer runtimes)
  • things needed for doing official builds

Adding a New Package

If you want to make artifacts available that do not meet the criteria for being included in the "clang" package, you can make package.py upload it to a separate zip file and then download it on an opt-in basis by using update.py's --package option. Here is [an example of adding a new package].

To test changes to package.py, change CLANG_SUB_REVISION in update.py to a random number above 2000 and run the *_upload_clang trybots.

Once the change to package.py is in, file a bug under Tools > LLVM requesting that a new package be created ([example bug]).

Once it's been uploaded and rolled, you can download it via:

tools/clang/scripts/update.py --package your-package-name

an example of adding a new package [example bug]: https://crbug.com/335730441

On local patches and cherry-picks

Except for the addition of Clang plugins, which do not affect the compiler output, Chromium's LLVM binaries are vanilla builds of the upstream source code at a specific revision, with no alterations.

We believe this helps when working with upstream: our compiler should behave exactly the same as if someone else built LLVM at the same revision, making collaboration easier.

It also creates an incentive for stabilizing the HEAD revision of LLVM: since we ship a vanilla build of an upstream revision, we have to ensure that a revision can be found which is stable enough to build Chromium and pass all its tests. While allowing local cherry-picks, reverts, or other patches, would probably allow more regular toolchain releases, we believe we can perform toolchain testing and fix issues fast enough that finding a stable revision is possible, and that this is the right trade-off for us and for the LLVM community.

For Rust, since the interface between tip-of-tree rustc and LLVM is less stable, and since landing fixes in Rust is much slower (even after approval, a patch can take more than 24 hours to land), we allow cherry-picks of such fixes in our Rust toolchain build.