Chromium Docs

Clangd

Introduction

clangd is a clang-based language server. It brings IDE features (e.g. diagnostics, code completion, code navigations) to your editor.

Quick Start

  • Get clangd
  • Make sure generated ninja files are up-to-date
  • Optional: build chrome normally to get generated headers
  • Generate compilation database (note: it's not regenerated automatically):
tools/clang/scripts/generate_compdb.py -p out/Default > compile_commands.json
Note: If you‘re using a different build directory, you’ll need to replace out/Default in this and other commands with your build directory.
  • Indexing is enabled by default (since clangd 9), note that this might consume lots of CPU and RAM. There's also a remote-index service to have an instant project-wide index without consuming local resources (requires clangd 12+ built with remote index support).
  • Use clangd in your favourite editor

Getting clangd

For the best results, you should use a clangd that exactly matches the version of Clang used by Chromium. This avoids problems like mismatched versions of compiler diagnostics.

The easiest way to do this is to set the checkout_clangd var in .gclient:

solutions = [
  {
    "url": "https://chromium.googlesource.com/chromium/src.git",
    "managed": False,
    "name": "src",
    "custom_deps": {},
    "custom_vars": {
      "checkout_clangd": True,
    },
  },
]

After this, gclient will keep the binary at third_party/llvm-build/Release+Asserts/bin/clangd in sync with the version of Clang used by Chromium.

Alternatively, you may use the build_clang_tools_extra.py script to build clangd from source:

tools/clang/scripts/build_clang_tools_extra.py --fetch out/Default clangd

The resulting binary will be at out/Default/tools/clang/third_party/llvm/build/bin/clangd.

Once you have an appropriate clangd binary, you must configure your editor to use it, either by placing it first on your PATH, or through editor-specific configuration.

Note: The clangd provided by Chromium does not support optional features like remote indexing (see https://crbug.com/1358258), such that clangd --version will not mention grpc, and you will see “Unknown Index key External” warnings in the clangd log.

If you want those features, you'll need to use a different build of clangd, such as the clangd/clangd releases on GitHub.

Setting Up

  1. Make sure generated ninja files are up-to-date.
gn gen out/Default
  1. Generate the compilation database, clangd needs it to know how to build a source file.
tools/clang/scripts/generate_compdb.py -p out/Default > compile_commands.json

Note: the compilation database is not regenerated automatically. You need to regenerate it manually whenever build rules change, e.g., when you have new files checked in or when you sync to head.

If using Windows PowerShell, use the following command instead to set the output's encoding to UTF-8 (otherwise Clangd will hit “YAML:1:4: error: Got empty plain scalar” while parsing it).

tools/clang/scripts/generate_compdb.py -p out/Default | out-file -encoding utf8 compile_commands.json
  1. Optional: build chrome normally. This ensures generated headers exist and are up-to-date. clangd will still work without this step, but it may give errors or inaccurate results for files which depend on generated headers.
ninja -C out/Default chrome

  1. Optional: configure clangd to use remote-index service for an instant project-wide index and reduced local CPU and RAM usage. See instructions.

  2. Use clangd in your favourite editor, see detailed instructions.

    • Optional: You may want to add --header-insertion=never to the clangd flags, so that your editor doesn‘t automatically add incorrect #include lines. The feature doesn’t correctly handle some common Chromium headers like base/functional/callback_forward.h.

Background Indexing

clangd builds an incremental index of your project (all files listed in the compilation database). The index improves code navigation features (go-to-definition, find-references) and code completion.

  • clangd only uses idle cores to build the index, you can limit the total amount of cores by passing the -j=<number> flag;
  • the index is saved to the .cache/clangd/index in the project root; index shards for common headers e.g. STL will be stored in $HOME/.cache/clangd/index;
  • background indexing can be disabled by the --background-index=false flag; Note that, disabling background-index will limit clangd’s knowledge about your codebase to files you are currently editing.

Note: the first index time may take hours (for reference, it took 2~3 hours on a 48-core, 64GB machine). A full index of Chromium (including v8, blink) takes ~550 MB disk space and ~2.7 GB memory in clangd.

Note: Remote-index service replaces background-index with some downsides like being ~a day old (Clangd will still know about your changes in the current editing session) and not covering all configurations (not available for mac&windows specific code or non-main branches).

Questions

If you have any questions, reach out to clangd/clangd or clangd-dev@lists.llvm.org.