Clang Tool Refactoring


  • The current workflow requires git.
  • This doesn‘t work on Windows... yet. I’m hoping to have a proof-of-concept working on Windows as well in a month several centuries from now.


Everything needed should be in a default Chromium checkout using gclient. third_party/llvm-build/Release+Asserts/bin should be in your $PATH.

Writing the Tool

An example clang tool is being implemented in Other useful resources might be the basic tutorial for Clang's AST matchers or the AST matcher reference.

Build your tool by running the following command (requires cmake version 2.8.10 or later):

tools/clang/scripts/ --force-local-build --without-android \
--tools <tools>

<tools> is a semicolon delimited list of subdirectories in tools/clang to build. The resulting binary will end up in third_party/llvm-build/Release+Asserts/bin. For example, to build the Chrome plugin and the empty_string tool, run the following:

tools/clang/scripts/ --force-local-build --without-android \
--tools plugins empty_string

When writing AST matchers, the following can be helpful to see what clang thinks the AST is:

clang++ -cc1 -ast-dump

Running the tool

First, you'll need to generate the compilation database with the following command:

cd $HOME/src/chrome/src
ninja -C out/Debug -t compdb cc cxx objc objcxx > \

This will dump the command lines used to build the C/C++ modules in all of Chromium into the resulting file. Then run the following command to run your tool across all Chromium code:

# Make sure all chromium targets are built to avoid missing generated
# dependencies
ninja -C out/Debug
tools/clang/scripts/ <toolname> \
<path/to/directory/with/compile_commands.json> <path 1> <path 2> ...

<path 1>, <path 2>, etc are optional arguments you use to filter the files that will be rewritten. For example, if you only want to run the empty-string tool on files in chrome/browser/extensions and sync, you'd do something like:

tools/clang/scripts/ empty_string out/Debug \
chrome/browser/extensions sync


Since the compile database is generated by ninja, that means that files that aren‘t compiled on that platform won’t be processed. That means if you want to apply a change across all Chromium platforms, you'll have to run the tool once on each platform.

Testing is the test harness for running tests. To use it, simply run: <tool name>

Note that name of the built tool and the subdirectory it lives in at tools/clang must match. What the test harness does is find all files that match the pattern * in your tool's tests subdirectory. It then runs the tool across those files and compares it to the expected result, stored in *