My first patch to libc++

I listened to an older cppcast episode and it mentioned that the C++17 mathematical special functions are not yet included in llvm’s standard library libc++. How hard could it be?

Follow me down the rabbit hole how I implemented std::hermite for libc++.

Some important pages for reference:

• Documentation, building instructions, testing instructions, etc. can be found at https://libcxx.llvm.org/.
• Development and issue tracking happens at https://github.com/llvm/llvm-project.
• The merge request: https://github.com/llvm/llvm-project/pull/89982.

Getting the development environment up and running:

1. Fork the project at github, e.g. https://github.com/PaulXiCao/llvm-project/
2. Checkout

   git clone git@github.com:PaulXiCao/llvm-project.git
   cd llvm-project
   

3. Create runtimes/CMakeUserPresets.json:

   {
    "version": 3,
    "configurePresets": [
        {
            "name": "gcc",
            "description": "gcc, mold, ccache",
            "hidden": false,
            "generator": "Ninja",
            "cacheVariables": {
                "CMAKE_CXX_COMPILER_LAUNCHER": "ccache",
                "CMAKE_EXE_LINKER_FLAGS": "-fuse-ld=mold",
                "CMAKE_SHARED_LINKER_FLAGS": "-fuse-ld=mold",
                "LLVM_ENABLE_RUNTIMES": "libcxx;libcxxabi;libunwind"
            }
        },
        {
            "name": "systemClang",
            "description": "system-wide clang, mold, ccache",
            "inherits": "gcc",
            "cacheVariables": {
                "CMAKE_C_COMPILER": "clang",
                "CMAKE_CXX_COMPILER": "clang++"
            }
        }
    ]
   }
   

4. Initiate CMake: $ cmake -B build -S runtimes --preset systemClang
5. Build: $ ninja -C build cxx-test-depends
6. Run tests: $ libcxx/utils/libcxx-lit build/ -sv libcxx/test/std/numerics/c.math/cmath.pass.cpp

Figuring out the build process (especially the test program) took quite some time.

Then came the review process. It span across 4 months and roughly 150 comments and 80 commits. Summary of feedback

• Use unconditional includes. E.g. not

  #if _LIBCPP_STD_VER >= 17
  #  include <type_traits>
  #endif 
  

• Use granularized headers.
• Hide functions in headers from ABI. E.g. _LIBCPP_HIDE_FROM_ABI _Real __hermite(..) {..}
• Qualify all function calls. E.g. std::__hermite(); instead of __hermite();
• Test files should specify their valid standards. E.g. // UNSUPPORTED: c++03, c++11, c++14
• (Add a status page for mathematical special function implementation.)
• Refactor tests to fit surrounding code.
• Add functions to header synopsis for documentation purposes.
• And some more discussions (e.g. NaN handling, accuracy counted in ULPs)..

This was quite some work but I learned a lot! The reviewers were more than welcoming. I enjoyed it quite a lot :)