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docs/clang_tool_refactoring.md

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+# Caveats
+  * 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.
+
+# Prerequisites
+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 https://codereview.chromium.org/12746010/. Other useful resources might be the [basic tutorial for Clang's AST matchers](http://clang.llvm.org/docs/LibASTMatchersTutorial.html) or the [AST matcher reference](http://clang.llvm.org/docs/LibASTMatchersReference.html).
+
+Build your tool by running the following command (requires cmake version 2.8.10 or later):
+```
+tools/clang/scripts/update.sh --force-local-build --without-android --with-chrome-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/update.sh --force-local-build --without-android --with-chrome-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 foo.cc
+```
+
+# 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 > out/Debug/compile_commands.json
+```
+
+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/run_tool.py <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/run_tool.py empty_string out/Debug chrome/browser/extensions sync
+```
+
+# Limitations
+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
+`test_tool.py` is the test harness for running tests. To use it, simply run:
+```
+test_tool.py <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 `*-original.cc` in your tool's tests subdirectory. It then runs the tool across those files and compares it to the expected result, stored in `*-expected.cc`