139 lines · plain
1=====================2Building LLVM with GN3=====================4 5.. contents::6 :local:7 8.. _Introduction:9 10Introduction11============12 13*Warning* The GN build is experimental and best-effort. It might not work,14and if you use it you're expected to feel comfortable to unbreak it if15necessary. LLVM's official build system is CMake, if in doubt use that.16If you add files, you're expected to update the CMake build but you don't need17to update GN build files. Reviewers should not ask authors to update GN build18files. Keeping the GN build files up-to-date is on the people who use the GN19build.20 21`GN <https://gn.googlesource.com/gn/>`_ is a metabuild system. It always22creates ninja files, but it can create some IDE projects (MSVC, Xcode, ...)23which then shell out to ninja for the actual build.24 25The main motivation behind the GN build is that some people find it more26convenient for day-to-day hacking on LLVM than CMake. Distribution, building27just parts of LLVM, and embedding the LLVM GN build from other builds are28non-goals for the GN build.29 30This is a `good overview of GN <https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit#slide=id.g119d702868_0_12>`_.31 32.. _Quick start:33 34Quick start35===========36 37#. ``git clone https://github.com/llvm/llvm-project.git; cd llvm-project`` if38 you don't have a checkout yet.39 40#. ``llvm/utils/gn/get.py`` to download a prebuilt gn binary if you're on a41 64-bit X86 system running Linux, macOS, or Windows. `Build gn yourself42 <https://gn.googlesource.com/gn/#getting-started>`_ if you're on a different43 platform or don't want to trust prebuilt binaries.44 45#. ``llvm/utils/gn/gn.py gen out/gn`` to run GN and create build files.46 ``out/gn`` is the build directory, it can have any name, and you can have as47 many as you want, each with different build settings. (The ``gn.py`` script48 adds ``--dotfile=llvm/utils/gn/.gn --root=.`` and just runs regular ``gn``;49 you can manually pass these parameters and not use the wrapper if you50 prefer.)51 52#. ``echo out >> .git/info/exclude`` to tell git to ignore files below ``out``.53 54#. ``ninja -C out/gn check-lld`` to build all prerequisites for and run the LLD55 tests.56 57By default, you get a release build with assertions enabled that targets58the host arch. You can set build options by editing ``out/gn/args.gn``, for59example putting ``is_debug = true`` in there gives you a debug build. Run60``llvm/utils/gn/gn.py args --list out/gn`` to see a list of all possible61options. After touching ``out/gn/args.gn`` just run ninja: it will re-invoke gn62before starting the build.63 64GN has extensive built-in help; try e.g. ``llvm/utils/gn/gn.py help gen`` to see65the help for the ``gen`` command. The full GN reference is also `available66online <https://gn.googlesource.com/gn/+/refs/heads/main/docs/reference.md>`_.67 68GN has an autoformatter:69``git ls-files '*.gn' '*.gni' | xargs llvm/utils/gn/gn.py format``70after making GN build changes is your friend.71 72To not put ``BUILD.gn`` files into the main tree, they are all below73``utils/gn/secondary``. For example, the build file for ``llvm/lib/Support``74is in ``utils/gn/secondary/llvm/lib/Support``.75 76.. _Syncing GN files from CMake files:77 78Syncing GN files from CMake files79=================================80 81Sometimes after pulling in the latest changes, the GN build doesn't work.82Most of the time this is due to someone adding a file to CMakeLists.txt file.83Run ``llvm/utils/gn/build/sync_source_lists_from_cmake.py`` to print a report84of which files need to be added to or removed from ``BUILD.gn`` files to85match the corresponding ``CMakeLists.txt``. You have to manually read the output86of the script and implement its suggestions.87 88If new ``CMakeLists.txt`` files have been added, you have to manually create89a new corresponding ``BUILD.gn`` file below ``llvm/utils/gn/secondary/``.90 91If the dependencies in a ``CMakeLists.txt`` file have been changed, you have to92manually analyze and fix.93 94.. _Philosophy:95 96Philosophy97==========98 99GN believes in using GN arguments to configure the build explicitly, instead100of implicitly figuring out what to do based on what's available on the current101system.102 103configure is used for three classes of feature checks:104 105- compiler checks. In GN, these could use exec_script to identify the host106 compiler at GN time. For now the build has explicit toggles for compiler107 features. (Maybe there could be a script that writes args.gn based on the108 host compiler). It's possible we'll use exec_script() for this going forward,109 but we'd have one exec_script call to identify compiler id and version,110 and then base GN arg default values of compiler id and version instead of111 doing one exec_script per feature check.112 (In theory, the config approach means a new os / compiler just needs to tweak113 the checks and not the code, but in practice a) new os's / compilers are rare114 b) they will require code changes anyhow, so the configure tradeoff seems115 not worth it.)116 117- library checks. For e.g. like zlib, GN thinks it's better to say "we require118 zlib, else we error at build time" than silently omitting features. People119 who really don't want to install zlib can explicitly set the GN arg to turn120 off zlib.121 122- header checks (does system header X exist). These are generally not needed123 (just keying this off the host OS works fine), but if they should become124 necessary in the future, they should be done at build time and the few125 targets that need to know if header X exists then depend on that build-time126 check while everything else can build parallel with it.127 128- LLVM-specific build toggles (assertions on/off, debug on/off, targets to129 build, ...). These map cleanly to GN args (which then get copied into130 config.h in a build step).131 132For the last two points, it would be nice if LLVM didn't have a single133``config.h`` header, but one header per toggle. That way, when e.g.134``llvm_enable_zlib`` is toggled, only the 3 files caring about that setting135would need to be rebuilt, instead of everything including ``config.h``.136 137GN doesn't believe in users setting arbitrary cflags from an environment138variable, it wants the build to be controlled by .gn files.139