| # Clang and C++ Modules in Chromium |
| |
| ## Overview |
| |
| Modules in C++ have the potential to significantly speed up building Chromium. |
| With textual inclusion, many headers are re-parsed over |
| [35000 times](https://commondatastorage.googleapis.com/chromium-browser-clang/include-analysis.html) |
| and with modules could be reduced to the order of 10 times. |
| |
| Clang supports two types of modules: |
| [Clang header modules](https://clang.llvm.org/docs/Modules.html) and |
| [C++20 modules](https://clang.llvm.org/docs/StandardCPlusPlusModules.html) (also |
| called Clang Modules and Standard C++ Modules). We're currently exploring Clang |
| header modules because: |
| 1. Other projects (e.g. internally at Google) have had success deploying them to |
| their code bases with large performance wins. |
| 2. They can be experimented with without rewrites to the code base or changes to |
| the build system. |
| |
| We're currently enabling modules for libc++ and they can be enabled with the GN |
| arg `use_clang_modules`. |
| |
| ## Current limitations |
| |
| ### Configurations |
| |
| Clang modules don't play nice with code with RTTI / exceptions depending on |
| code without, and vice versa. Work is ongoing to fix this, but for now, it |
| remains a problem ([crbug.com/403415459](https://crbug.com/403415459)). |
