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Clang

Clang is a compiler with many desirable features (outlined on their website).

Chrome can be built with Clang. It is now the default compiler on Android, Mac and Linux for building Chrome, and it is currently useful for its warning and error messages on Windows.

See the open bugs.

[TOC]

Build instructions

Get clang (happens automatically during gclient runhooks on Mac and Linux):

tools/clang/scripts/update.py

Only needs to be run once per checkout, and clang will be automatically updated by gclient runhooks.

Regenerate the ninja build files with Clang enabled. Again, on Linux and Mac, Clang is the default compiler.

Run gn args and add is_clang = true to your args.gn file.

Build: ninja -C out/gn chrome

Reverting to gcc on linux

We don't have bots that test this, but building with gcc4.8+ should still work on Linux. If your system gcc is new enough, run gn args and add is_clang = false.

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)

Using the clang static analyzer

See clang_static_analyzer.md.

Windows

clang can be used as compiler on Windows. Clang uses Visual Studio's linker and SDK, so you still need to have Visual Studio installed.

Things should compile, and all tests should pass. You can check these bots for how things are currently looking: https://build.chromium.org/p/chromium.fyi/console?category=win%20clang

python tools\clang\scripts\update.py
# run `gn args` and add `is_clang = true` to your args.gn, then...
ninja -C out\gn chrome

The update.py script only needs to be run once per checkout. Clang will be kept up to date by gclient runhooks.

Current brokenness:

  • To get colored diagnostics, you need to be running ansicon.
  • Debug info does now work, but support for it is new. If you see something not working right, please file a bug and mark it as blocking the clang/win debug info tracking bug.

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
is_clang = true  # Implicitly set on Mac, Linux, iOS; needed on Win and Android.

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.