# Clang Chromium ships a prebuilt [clang](http://clang.llvm.org) binary. It's just upstream clang built at a known-good revision that we bump every two weeks or so. This is the only supported compiler for building Chromium. [TOC] ## Using gcc on Linux `is_clang = false` will make the build use system gcc on Linux. There are no bots that test this and there is no guarantee it will work, but we accept patches for this configuration. ## Mailing List https://groups.google.com/a/chromium.org/group/clang/topics ## Using plugins The [chromium style plugin](https://dev.chromium.org/developers/coding-style/chromium-style-checker-errors) 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/build.py --without-android --without-fuchsia` to build the plugin. 1. Run `ninja -C third_party/llvm-build/Release+Asserts/` to build incrementally. 1. 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) 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. 1. Wait for a compiler roll to bring in the flag. 1. Start passing the new flag in `GN` and verify the new behavior. 1. Enable the new plugin behavior unconditionally and update the plugin to ignore the flag. 1. Wait for another compiler roll. 1. Stop passing the flag from `GN`. 1. Remove the flag completely. ## Using the clang static analyzer See [clang_static_analyzer.md](clang_static_analyzer.md). ## Windows clang is the default compiler on Windows. It uses MSVC's SDK, so you still need to have Visual Studio with C++ support installed. ## 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 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 ``` On Windows, for `clang_base_path` use something like this instead: ``` clang_base_path = "c:/src/llvm-build" ``` You can then look in `out/gn/toolchain.ninja` and check that the `rule cc` and `rule cxx` commands run your clang binary. If things look good, run `ninja -C out/gn` to build. Chromium tries to be buildable with its currently pinned clang, and with clang trunk. Set `llvm_force_head_revision = true` in your args.gn if the clang you're trying to build with is closer to clang trunk than to Chromium's pinned clang (which `tools/clang/scripts/update.py --print-revision` prints). ## Related documents * [Toolchain support](toolchain_support.md) gives an overview of clang rolls, and documents when to revert clang rolls and how to file good toolchain bugs. * [Updating clang](updating_clang.md) documents the mechanics of updating clang, and which files are included in the default clang package. * [Clang Sheriffing](clang_sheriffing.md) contains instructions for how to debug compiler bugs, for clang sheriffs. * [Clang Tool Refactoring](clang_tool_refactoring.md) has notes on how to build and run refactoring tools based on clang's libraries.