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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 Mac and Linux for building Chrome, and it is currently useful for its warning and error messages on Android and 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.

If you use gyp: GYP_DEFINES=clang=1 build/gyp_chromium

If you use gn, run gn args and add is_clang = true to your args.gn file.

Build: ninja -C out/Debug 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, use this to build with gcc if you don't want to build with clang:

GYP_DEFINES=clang=0 build/gyp_chromium

Mailing List

http://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)

To run other plugins, add these to your GYP_DEFINES:

  • clang_load: Absolute path to a dynamic library containing your plugin
  • clang_add_plugin: tells clang to run a specific PluginASTAction

So for example, you could use the plugin in this directory with:

  • GYP_DEFINES='clang=1 clang_load=/path/to/libFindBadConstructs.so clang_add_plugin=find-bad-constructs' gclient runhooks

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: http://build.chromium.org/p/chromium.fyi/console?category=win%20clang

python tools\clang\scripts\update.py
set GYP_DEFINES=clang=1
python build\gyp_chromium

# or, if you use gn, run `gn args` and add `is_clang = true` to your args.gn
ninja -C out\Debug 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:

  • Debug info is very limited.
  • To get colored diagnostics, you need to be running ansicon.

Using a custom clang binary

If you want to try building Chromium with your own clang binary that you've already built, set make_clang_dir to the directory containing bin/clang (i.e. the directory you ran cmake in, or your Release+Asserts folder if you use the configure/make build). You also need to disable chromium's clang plugin by setting clang_use_chrome_plugins=0, as it likely won't load in your custom clang binary.

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):

GYP_DEFINES="clang=1 fastbuild=1 component=shared_library \
clang_use_chrome_plugins=0 make_clang_dir=$HOME/src/llvm-build" \
build/gyp_chromium

You can then run head out/Release/build.ninja and check that the first to lines set cc and cxx to your clang binary. If things look good, run ninja -C out/Release 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. Or you could set werror= in the line above to disable warnings as errors (but this only works on Linux).

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. LLD does not yet support generating PDB files, which makes it hard to debug Chrome while using LLD.

If you use gyp, you can enable it with GYP_DEFINES=lld=1. If you use gn, set use_lld = true in args.gn. Currently this configuration is only supported on Windows.