Clang Tidy

Introduction

clang-tidy is a clang-based C++ “linter” tool. Its purpose is to provide an extensible framework for diagnosing and fixing typical programming errors, like style violations, interface misuse, or bugs that can be deduced via static analysis.

Where is it?

clang-tidy is available in two places in Chromium:

  • In Chromium checkouts
  • In code review on Gerrit

Clang-tidy automatically runs on any CL that Chromium committers upload to Gerrit, and will leave code review comments there. This is the recommended way of using clang-tidy.

Enabled checks

Chromium globally enables a subset of all of clang-tidy's checks (see ${chromium}/src/.clang-tidy). We want these checks to cover as much as we reasonably can, but we also strive to strike a reasonable balance between signal and noise on code reviews. Hence, a large number of clang-tidy checks are disabled.

Adding a new check

New checks require review from cxx@chromium.org. If you propose a check and it gets approved, you may turn it on, though please note that this is only provisional approval: we get signal from users clicking “Not Useful” on comments. If feedback is overwhelmingly “users don't find this useful,” the check may be removed.

Traditionally, petitions to add checks include an evaluation of the check under review. Crucially, this includes two things:

  • a count of how many times this check fires across Chromium
  • a random sample (>30) of places where the check fires across Chromium

It‘s expected that the person proposing the check has manually surveyed every clang-tidy diagnostic in the sample, noting any bugs, odd behaviors, or interesting patterns they’ve noticed. If clang-tidy emits FixIts, these are expected to be considered by the evaluation, too.

An example of a previous proposal email thread is here.

Evaluating: running clang-tidy across Chromium

Running clang-tidy requires some setup. First, you'll need to sync clang-tidy, which requires adding checkout_clang_tidy to your .gclient file:

solutions = [
  {
    'custom_vars': {
      'checkout_clang_tidy': True,
    },
  }
]

Your next run of gclient runhooks should cause clang-tidy to be synced.

To run clang-tidy across all of Chromium, you‘ll need a checkout of Chromium’s build/ repository. Once you have that and a Chromium out/ dir with an args.gn, running clang-tidy across all of Chromium is a single command:

$ cd ${chromium}/src
$ ${chromium_build}/recipes/recipe_modules/tricium_clang_tidy/resources/tricium_clang_tidy_script.py \
    --base_path $PWD \
    --out_dir out/Linux \
    --findings_file all_findings.json \
    --clang_tidy_binary $PWD/third_party/llvm-build/Release+Asserts/bin/clang-tidy \
    --all

To only run clang-tidy against certain files, replace the --all parameter with the individual file paths.

All clang-tidy checks are run on Linux builds of Chromium, so please set up your args.gn to build Linux.

all_findings.json is where all of clang-tidy's findings will be dumped. The format of this file is detailed in tricium_clang_tidy_script.py.

Note that the above command will use Chromium‘s top-level .clang-tidy file (or .clang-tidy files scattered throughout third_party/, depending on the files we lint. In order to test a new check, you’ll have to add it to Chromium's top-level .clang-tidy file.

Ignoring a check

If a check is invalid on a particular piece of code, clang-tidy supports // NOLINT and // NOLINTNEXTLINE for ignoring all lint checks in the current and next lines, respectively. To suppress a specific lint, you can put it in parenthesis, e.g., // NOLINTNEXTLINE(modernize-use-nullptr). For more, please see the documentation.

Please note that adding comments that exist only to silence clang-tidy is actively discouraged. These comments clutter code, can easily get out-of-date, and don‘t provide much value to readers. Moreover, clang-tidy only complains on Gerrit when lines are touched, and making Chromium clang-tidy clean is an explicit non-goal; making code less readable in order to silence a rarely-surfaced complaint isn’t a good trade-off.

If clang-tidy emits a diagnostic that's incorrect due to a subtlety in the code, adding an explanantion of what the code is doing with a trailing NOLINT may be fine. Put differently, the comment should be able to stand on its own even if we removed the NOLINT. The fact that the comment also silences clang-tidy is a convenient side-effect.

For example:

Not OK; comment exists just to silence clang-tidy:

// NOLINTNEXTLINE
for (int i = 0; i < arr.size(); i++) {
  // ...
}

Not OK; comment exists just to verbosely silence clang-tidy:

// Clang-tidy doesn't get that we can't range-for-ize this loop. NOLINTNEXTLINE
for (int i = 0; i < arr.size(); i++) {
  // ...
}

Not OK; it‘s obvious that this loop modifies arr, so the comment doesn’t actually clarify anything:

// It'd be invalid to make this into a range-for loop, since the body might add
// elements to `arr`. NOLINTNEXTLINE
for (int i = 0; i < arr.size(); i++) {
  if (i % 4) {
    arr.push_back(4);
    arr.push_back(2);
  }
}

OK; comment calls out a non-obvious property of this loop's body. As an afterthought, it silences clang-tidy:

// It'd be invalid to make this into a range-for loop, since the call to `foo`
// here might add elements to `arr`. NOLINTNEXTLINE
for (int i = 0; i < arr.size(); i++) {
  foo();
  bar();
}

In the end, as always, what is and isn't obvious at some point is highly context-dependent. Please use your best judgement.

But I want to run it locally

If you want to sync the officially-supported clang-tidy to your workstation, add the following to your .gclient file:

solutions = [
  {
    'custom_vars': {
      'checkout_clang_tidy': True,
    },
  },
]

If you already have solutions and custom_vars, just add checkout_clang_tidy to the existing custom_vars map.

Once the above update has been made, run gclient runhooks, and clang-tidy should appear at src/third_party/llvm-build/Release+Asserts/bin/clang-tidy if your Chromium tree is sufficiently up-to-date.

Running clang-tidy locally

Note that the local flows with clang-tidy are experimental, and require an LLVM checkout. Tricium is happy to run on WIP CLs, and we strongly encourage its use.

That said, assuming you have the LLVM sources available, you'll need to bring your own clang-apply-replacements binary if you want to use the -fix option noted below.

Note: If you‘re on a system that offers a clang tools through its package manager (e.g., on Debian/Ubuntu, sudo apt-get install clang-tidy clang-tools), you might not need an LLVM checkout to make the required binaries and scripts (clang-tidy, run-clang-tidy and clang-apply-replacements) available in your $PATH. However, the system packaged binaries might be several versions behind Chromium’s toolchain, so not all flags are guaranteed to work. If this is a problem, consider building clang-tidy from the same revision the current toolchain is using, rather than filing a bug against the toolchain component. This can be done as follows:

tools/clang/scripts/build_clang_tools_extra.py \
    --fetch out/Release clang-tidy clang-apply-replacements

Running clang-tidy is then (hopefully) simple.

  1. Build chrome normally.
ninja -C out/Release chrome
  1. Enter the build directory
cd out/Release
  1. Export Chrome's compile command database
gn gen . --export-compile-commands
  1. Run clang-tidy.
<PATH_TO_LLVM_SRC>/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py \
    -p . \# Set the root project directory, where compile_commands.json is.
    # Set the clang-tidy binary path, if it's not in your $PATH.
    -clang-tidy-binary <PATH_TO_LLVM_BUILD>/bin/clang-tidy \
    # Set the clang-apply-replacements binary path, if it's not in your $PATH
    # and you are using the `fix` behavior of clang-tidy.
    -clang-apply-replacements-binary \
        <PATH_TO_LLVM_BUILD>/bin/clang-apply-replacements \
    # The checks to employ in the build. Use `-*,...` to omit default checks.
    -checks=<CHECKS> \
    -header-filter=<FILTER> \# Optional, limit results to only certain files.
    -fix \# Optional, used if you want to have clang-tidy auto-fix errors.
    'chrome/browser/.*' # A regex of the files you want to check.

Copy-Paste Friendly (though you'll still need to stub in the variables):
<PATH_TO_LLVM_SRC>/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py \
    -p . \
    -clang-tidy-binary <PATH_TO_LLVM_BUILD>/bin/clang-tidy \
    -clang-apply-replacements-binary \
        <PATH_TO_LLVM_BUILD>/bin/clang-apply-replacements \
    -checks=<CHECKS> \
    -header-filter=<FILTER> \
    -fix \
    'chrome/browser/.*'

Note that the source file regex must match how the build specified the file. This means that on Windows, you must use (escaped) backslashes even from a bash shell.

Questions

Questions about the local flow? Reach out to rdevlin.cronin@chromium.org, thakis@chromium.org, or gbiv@chromium.org.

Questions about the Gerrit flow? Email tricium-dev@google.com or infra-dev+tricium@chromium.org, or file a bug against Infra>LUCI>BuildService>PreSubmit>Tricium. Please CC gbiv@chromium.org on any of these.

Discoveries? Update the doc!