Configurations supported by the toolchain team

This document describes considerations to follow when adding a new build config. A build config is something like a new compiler / linker configuration, a new test binary, a new target_os, a new target_cpu, etc.


We update our toolchain (the C/C++/Objective-C compiler clang, the linker lld, and a small assortment of helper binaries) every 2-4 weeks.

This toolchain is used to build Chromium for 7+ platforms (Android, Chromecast, Chrome OS, Fuchsia, iOS, Linux, macOS, Windows) targeting 4+ CPUs (arm, arm64, x86, x64) in 6+ build modes (debug (component non-optimized), release (static build optimized), official build (very optimized and on some platforms LTO+CFI), asan+lsan, msan, tsan), resulting in 130+ different test binaries.

Every toolchain update needs to make sure that none of these combinations break.

To have any chance that this works, we continuously build and run tests in most of these configurations with trunk clang/llvm, to catch regressions and intentinonal changes upstream that cause problems for us.

When we land a toolchain update, we rely on the CQ to make sure all combinations work with the new toolchain. We use all default CQ bots, and a long list of opt-in trybots.

The toolchain team has established contacts to most platform owners in Chromium, so that we can ask for help quickly when needed.

Toolchain guarantees

For configurations that have a bot on the chromium.clang waterfall (which is where all the bots are that test Chromium with trunk clang/llvm) and that are either part of the default CQ or that have an opt-in bot that‘s used on clang rolls, we guarantee that we won’t land a toolchain update that breaks that configuration.

For configurations that don‘t have a clang tip-of-tree (ToT) bot or that aren’t covered on the CQ, we won't revert toolchain updates. We will do our best to fix things quickly (see below for how to file a good bug) and to fix forward to get you unblocked.

Talk to the toolchain team to make sure your new config is supported

If you add a new build config, or a new bot config: You may want to add a chromium.clang ToT bot, and you may want to make sure that there‘s a CQ bot covering your config on clang rolls. (It’s ok if it‘s an opt-in bot, as long as you make sure it’s opted-in for clang rolls. If your opt-in is based on filename patterns, make sure it also fires on changes to tools/clang/scrips/

Do not use -mllvm or -Xclang flags. These are internal flags that aren‘t ready for production use yet. Once they’re ready, they'll become available as regular clang flags.

Follow the style guide. In particular, don't use exceptions.

Talk to us if you‘re adding a new build config or bot config, if you’d like to use an internal flag, if you want to use a flag that's obscure, or if you want general advice on toolchain questions (, or google-internally,

Filing good toolchain bugs

If a toolchain update (“clang roll”) broke you, here's how you can file a bug that we can act on the quickest:

  • File the bug in the Tools>LLVM component.
  • Link to the CL with the toolchain update that broke you.
  • Link to a specific build showing the breakage, ideally the first instance of the breakage.
  • If reproducing your problem requires more than a regular Chromium checkout and replicating what the bot you linked to does, or if you can‘t link to a build: Provide commands on how to reproduce your problem, targeted at someone who knows the chromium build well but doesn’t know your feature / platform at all.
    • Tell us which repo to check out, if needed.
    • Tell us which to use.
    • Tell us which target to build.
    • Tell us how to run your test.

We'll try to be helpful, but see “Toolchain guarantees” above.

Compiler updates can expose latent existing bugs in your code, for example if you have ODR violations, or other undefined behavior. In that case, the fix is to change your code. It can be helpful to make a reduced repro case of the problem before looping us in, so that you can check if your problem is really due to the toolchain update and not due to a bug in your code.