Clang Sheriffing

Chromium bundles its own pre-built version of Clang. This is done so that Chromium developers have access to the latest and greatest developer tools provided by Clang and LLVM (ASan, CFI, coverage, etc). In order to update the compiler (roll clang), it has to be tested so that we can be confident that it works in the configurations that Chromium cares about.

Fortunately, Chromium happens to be a pretty decent stress test for a C++ compiler, so we maintain a waterfall of builders that continuously build fresh versions of Clang and use them to build and test Chromium. “Clang sheriffing” is the process of monitoring that waterfall, determining if any compile or test failures are due to an upstream compiler change, filing bugs upstream, and often reverting bad changes in LLVM. This document describes some of the processes and techniques for doing that.

Is it the compiler?

Chromium does not always build and pass tests in all configurations that everyone cares about. Some configurations simply take too long to build (ThinLTO) or be tested (dbg) on the CQ before committing. And, some tests are flaky. So, our console is often filled with red boxes, and the boxes don't always need to be green to roll clang.

Oftentimes, if a bot is red with a test failure, it‘s not a bug in the compiler. To check this, the easiest and best thing to do is to try to find a corresponding builder that doesn’t use ToT clang. For standard configurations, start on the waterfall that corresponds to the OS of the red bot, and search from there. If the failing bot is Google Chrome branded, go to the (Google internal) official builder list and start searching from there.

If you are feeling charitable, you can try to see when the test failure was introduced by looking at the history in the bot. One way to do this is to add ?numbuilds=200 to the builder URL to see more history. If that isn‘t enough history, you can manually binary search build numbers by editing the URL until you find where the regression was introduced. If it’s immediately clear what CL introduced the regression (i.e. caused tests to fail reliably in the official build configuration), you can simply load the change in gerrit and revert it, linking to the first failing build that implicates the change being reverted.

If the failure looks like a compiler bug, these are the common failures we see and what to do about them:

  1. compiler crash
  2. compiler warning change
  3. compiler error
  4. miscompile
  5. linker errors

Compiler crash

This is probably the most common bug. The standard procedure is to do these things:

  1. Use got_clang_revision property from first red and last green build to find upstream regression range

  2. File a crbug documenting the crash. Include the range, and any other bots displaying the same symptoms.

  3. Collect the crash reproduction files and reproduce the crash locally. When clang crashes, it prints something like this:

    Preprocessed source(s) and associated run script(s) are located at:
    clang: note: diagnostic msg: C:\src\tmp\t-8f292b.cpp
    clang: note: diagnostic msg: C:\src\tmp\

    If you re-run the shell script, it should reproduce the crash.

  4. Identify the revision that introduced the crash. First, look at the commit messages in the LLVM revision range to see if one modifies the code near the point of the crash. If so, try reverting it locally, rebuild, and run the reproducer to see if the crash goes away.

    If that doesn't work, use git bisect. Use this as a template for the bisect run script:

    cd $(dirname $0)  # get into llvm build dir
    ninja -j900 clang || exit 125 # skip revisions that don't compile
    ./   # exit 0 if good, 0 if bad
  5. Reply to the commit on llvm-commits or on the code review on Phabricator letting the author know that you believe they introduced a regression, and that you will revert the patch and provide a reduced reproducer.

  6. Revert the upstream LLVM change once you are confident that you have the right culprit. Tell the patch author that you will provide a reproduction.

  7. Start a reduction using CReduce. Follow the docs there for writing an interestingness test and use it to reduce the input to something that can be provided upstream. Send the reduced reproducer to the patch author.

Compiler warning change

New Clang versions often find new bad code patterns to warn on. Chromium builds with -Werror, so improvements to warnings often turn into build failures in Chromium. Once you understand the code pattern Clang is complaining about, file a bug to do either fix or silence the new warning.

If this is a completely new warning, disable it by adding -Wno-NEW-WARNING to this list of disabled warnings if llvm_force_head_revision is true. Here is an example. This will keep the ToT bots green while you decide what to do.

Sometimes, behavior changes and a pre-existing warning changes to warn on new code. In this case, fixing Chromium may be the easiest and quickest fix. If there are many sites, you may consider changing clang to put the new diagnostic into a new warning group so you can handle it as a new warning as described above.

If the warning is high value, then eventually our team or other contributors will end up fixing the crbug and there is nothing more to do. If the warning seems low value, pass that feedback along to the author of the new warning upstream. It‘s unlikely that it should be on by default or enabled by -Wall if users don’t find it valuable. If the warning is particularly noisy and can't be easily disabled without disabling other high value warnings, you should consider reverting the change upstream and asking for more discussion.

Compiler error

This rarely happens, but sometimes clang becomes more strict and no longer accepts code that it previously did. The standard procedure for a new warning may apply, but it's more likely that the upstream Clang change should be reverted, if the C++ code in question in Chromium looks valid.


Miscompiles tend to result in crashes, so if you see a test with the CRASHED status, this is probably what you want to do.

  1. Bisect object files to find the object with the code that changed.
  2. Debug it with a traditional debugger

Linker error

TODO: Describe object file bisection, identify obj with symbol that no longer has the section.