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Clang

Clang is the main supported compiler when building Chromium on all platforms.

Known clang bugs and feature requests.

[TOC]

Building with clang

This happens by default, with clang binaries being fetched by gclient during the gclient runhooks phase. To fetch them manually, or build a local custom clang, use

tools/clang/scripts/update.py

Run gn args and make sure there is no is_clang = false in your args.gn file.

Build: ninja -C out/gn chrome

Reverting to gcc on Linux or MSVC on Windows

There are no bots that test this but is_clang = false will revert to gcc on Linux and to Visual Studio on Windows. There is no guarantee it will work.

Mailing List

https://groups.google.com/a/chromium.org/group/clang/topics

Using plugins

The chromium style plugin is used by default when clang is used.

If you're working on the plugin, you can build it locally like so:

  1. Run ./tools/clang/scripts/update.py --force-local-build --without-android to build the plugin.
  2. Run ninja -C third_party/llvm-build/Release+Asserts/ to build incrementally.
  3. Build with clang like described above, but, if you use goma, disable it.

To test the FindBadConstructs plugin, run:

(cd tools/clang/plugins/tests && \
 ./test.py ../../../../third_party/llvm-build/Release+Asserts/bin/clang \
           ../../../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so)

Since the plugin is rolled with clang changes, behavior changes to the plugin should be guarded by flags to make it easy to roll clang. A general outline:

  1. Implement new plugin behavior behind a flag.
  2. Wait for a compiler roll to bring in the flag.
  3. Start passing the new flag in GN and verify the new behavior.
  4. Enable the new plugin behavior unconditionally and update the plugin to ignore the flag.
  5. Wait for another compiler roll.
  6. Stop passing the flag from GN.
  7. Remove the flag completely.

Using the clang static analyzer

See clang_static_analyzer.md.

Windows

Since October 2017, clang is the default compiler on Windows. It uses MSVC's linker and SDK, so you still need to have Visual Studio with C++ support installed.

To use MSVC's compiler (if it still works), use is_clang = false.

Current brokenness:

  • To get colored diagnostics, you need to be running ansicon.

Using a custom clang binary

Set clang_base_path in your args.gn to the llvm build directory containing bin/clang (i.e. the directory you ran cmake). This [must][1] be an absolute path. You also need to disable chromium's clang plugin.

Here's an example that also disables debug info and enables the component build (both not strictly necessary, but they will speed up your build):

clang_base_path = getenv("HOME") + "/src/llvm-build"
clang_use_chrome_plugins = false
is_debug = false
symbol_level = 1
is_component_build = true

You can then run head out/gn/toolchain.ninja and check that the first to lines set cc and cxx to your clang binary. If things look good, run ninja -C out/gn to build.

If your clang revision is very different from the one currently used in chromium

  • Check tools/clang/scripts/update.py to find chromium's clang revision
  • You might have to tweak warning flags.

Using LLD

Experimental!

LLD is a relatively new linker from LLVM. The current focus is on Windows and Linux support, where it can link Chrome approximately twice as fast as gold and MSVC's link.exe as of this writing.

Set use_lld = true in args.gn.