# Clang Tool Refactoring [TOC] ## Introduction Clang tools can help with global refactorings of Chromium code. Clang tools can take advantage of clang's AST to perform refactorings that would be impossible with a traditional find-and-replace regexp: * Constructing `scoped_ptr` from `NULL`: * Implicit conversions of `scoped_refptr` to `T*`: * Rename everything in Blink to follow Chromium style: ## Caveats An invocation of the clang tool runs on one build config. Code that only compiles on one platform or code that is guarded by a set of compile-time flags can be problematic. Performing a global refactoring typically requires running the tool once in each build config with code that needs to be updated. Other minor issues: * Requires a git checkout. * Requires [some hacks to run on Windows](https://codereview.chromium.org/718873004). ## Prerequisites A Chromium checkout created with `fetch` should have everything needed. For convenience, add `third_party/llvm-build/Release+Asserts/bin` to `$PATH`. ## Writing the tool LLVM uses C++11 and CMake. Source code for Chromium clang tools lives in [//tools/clang](https://chromium.googlesource.com/chromium/src/tools/clang/+/master). It is generally easiest to use one of the already-written tools as the base for writing a new tool. Chromium clang tools generally follow this pattern: 1. Instantiate a [`clang::ast_matchers::MatchFinder`](http://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder.html). 2. Call `addMatcher()` to register [`clang::ast_matchers::MatchFinder::MatchCallback`](http://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder_1_1MatchCallback.html) actions to execute when [matching](http://clang.llvm.org/docs/LibASTMatchersReference.html) the AST. 3. Create a new `clang::tooling::FrontendActionFactory` from the `MatchFinder`. 4. Run the action across the specified files with [`clang::tooling::ClangTool::run`](http://clang.llvm.org/doxygen/classclang_1_1tooling_1_1ClangTool.html#acec91f63b45ac7ee2d6c94cb9c10dab3). 5. Serialize generated [`clang::tooling::Replacement`](http://clang.llvm.org/doxygen/classclang_1_1tooling_1_1Replacement.html)s to `stdout`. Other useful references when writing the tool: * [Clang doxygen reference](http://clang.llvm.org/doxygen/index.html) * [Tutorial for building tools using LibTooling and LibASTMatchers](http://clang.llvm.org/docs/LibASTMatchersTutorial.html) ### Edit serialization format ``` ==== BEGIN EDITS ==== r:::path/to/file1:::offset1:::length1:::replacement text r:::path/to/file2:::offset2:::length2:::replacement text ... ==== END EDITS ==== ``` The header and footer are required. Each line between the header and footer represents one edit. Fields are separated by `:::`, and the first field must be `r` (for replacement). In the future, this may be extended to handle header insertion/removal. A deletion is an edit with no replacement text. The edits are applied by [`run_tool.py`](#Running), which understands certain conventions: * The tool should munge newlines in replacement text to `\0`. The script knows to translate `\0` back to newlines when applying edits. * When removing an element from a 'list' (e.g. function parameters, initializers), the tool should emit a deletion for just the element. The script understands how to extend the deletion to remove commas, etc. as needed. TODO: Document more about `SourceLocation` and how spelling loc differs from expansion loc, etc. ### Why not RefactoringTool? While clang has a [`clang::tooling::RefactoringTool`](http://clang.llvm.org/doxygen/classclang_1_1tooling_1_1RefactoringTool.html) to automatically apply the generated replacements and save the results, it doesn't work well for Chromium: * Clang tools run actions serially, so runtime scales poorly to tens of thousands of files. * A parsing error in any file (quite common in NaCl source) prevents any of the generated replacements from being applied. ## Building Synopsis: ```shell tools/clang/scripts/update.py --bootstrap --force-local-build --without-android \ --tools blink_gc_plugin plugins rewrite_to_chrome_style ``` Running this command builds the [Oilpan plugin](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/blink_gc_plugin/), the [Chrome style plugin](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/plugins/), and the [Blink to Chrome style rewriter](https://chromium.googlesource.com/chromium/src/+/master/tools/clang/rewrite_to_chrome_style/). Additional arguments to `--tools` should be the name of subdirectories in [//tools/clang](https://chromium.googlesource.com/chromium/src/+/master/tools/clang). Generally, `--tools` should always include `blink_gc_plugin` and `plugins`: otherwise, Chromium won't build. It is important to use --bootstrap as there appear to be [bugs](https://crbug.com/580745) in the clang library this script produces if you build it with gcc, which is the default. ## Running First, build all chromium targets to avoid failures due to missing dependecies that are generated as part of the build: ```shell ninja -C out/Debug ``` Then run the actual tool: ```shell tools/clang/scripts/run_tool.py \ --generate-compdb out/Debug ... ``` `--generate-compdb` can be omitted if the compile DB was already generated and the list of build flags and source files has not changed since generation. ``, ``, etc are optional arguments to filter the files to run the tool across. This is helpful when sharding global refactorings into smaller chunks. For example, the following command will run the `empty_string` tool across just the files in `//base`: ```shell tools/clang/scripts/run_tool.py empty_string \ --generated-compdb \ out/Debug base ``` ## Debugging Dumping the AST for a file: ```shell clang++ -cc1 -ast-dump foo.cc ``` Using `clang-query` to dynamically test matchers (requires checking out and building [clang-tools-extras](https://github.com/llvm-mirror/clang-tools-extra)): ```shell clang-query -p path/to/compdb base/memory/ref_counted.cc ``` `printf` debugging: ```c++ clang::Decl* decl = result.Nodes.getNodeAs("decl"); decl->dumpColor(); clang::Stmt* stmt = result.Nodes.getNodeAs("stmt"); stmt->dumpColor(); ``` By default, the script hides the output of the tool. The easiest way to change that is to `return 1` from the `main()` function of the clang tool. ## Testing Synposis: ```shell test_tool.py ``` The name of the tool binary and the subdirectory for the tool in `//tools/clang` must match. The test runner finds all files that match the pattern `//tools/clang//tests/*-original.cc`, runs the tool across those files, and compared it to the `*-expected.cc` version. If there is a mismatch, the result is saved in `*-actual.cc`.