llvm-project/llvm/utils/gn/README.rst

=====================
Building LLVM with GN
=====================

.. contents::
   :local:

.. _Introduction:

Introduction
============

*Warning* The GN build is experimental and best-effort. It might not work,
and if you use it you're expected to feel comfortable to unbreak it if
necessary. LLVM's official build system is CMake, if in doubt use that.
If you add files, you're expected to update the CMake build but you don't need
to update GN build files. Reviewers should not ask authors to update GN build
files. Keeping the GN build files up-to-date is on the people who use the GN
build.

*Another Warning* Right now, we're in the process of getting the GN build
checked in. As of this writing, it's not yet functional at all. Check back
in a few weeks!

`GN <https://gn.googlesource.com/gn/>`_ is another metabuild system. It always
creates ninja files, but it can create some IDE projects (MSVC, Xcode, ...)
which then shell out to ninja for the actual build.

Its main features are that GN is very fast (it currently produces ninja files
for LLVM's build in 35ms on the author's laptop, compared to 66s for CMake) --
a 2000x difference), and since it's so fast it doesn't aggressively cache,
making it possible to switch e.g. between release and debug builds in one build
directory.

It is arguable easier to configure than the CMake build, and has native support
for building with multiple toolchains in one build directory. The build
description is declarative-ish, allowing GN to print it in a json format that
can fairly easily be converted to other metabuild system inputs.

The main motivation behind the GN build is that some people find it more
convenient for day-to-day hacking on LLVM than CMake. Distribution, building
just parts of LLVM, and embedding the LLVM GN build from other builds are a
non-goal for the GN build.

This is a `good overview of GN <https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit#slide=id.g119d702868_0_12>`_.

.. _Quick start:

Quick start
===========

*Warning* Right now, we're in the process of getting the GN build checked in.
As of this writing, it's not yet functional at all.

GN only works in the monorepo layout.

#. Obtain a `gn binary <https://gn.googlesource.com/gn/#getting-started>`_.

#. In the root of the monorepo, run
   `gn gen --dotfile=$PWD/llvm/utils/gn/.gn --root=. out/gn` (`out/gn` is the
   build directory, it can have any name, and you can have as many as you want,
   each with different build settings).

#. Run e.g. `ninja -C out/gn check-lld` to build all prerequisites for and
   run the LLD tests.

By default, you get a release build with assertions enabled that targets
the host arch. You can set various build options by editing `out/gn/args.gn`,
for example putting `is_debug = true` in there gives you a debug build. Run
`gn args --list out/gn` to see a list of all possible options. After touching
`out/gn/args.gn`, just run ninja, it will re-invoke gn before starting the
build.

GN has extensive built-in help; try e.g. `gn help gen` to see the help
for the `gen` command. The full GN reference is also `available online
<https://gn.googlesource.com/gn/+/master/docs/reference.md>`_.

GN has an autoformatter: `git ls-files '*.gn' '*.gni' | xargs -n 1 gn format`
after making GN build changes is your friend.

To not put `BUILD.gn` into the main tree, they are all below `utils/gn/tree`.
For example, the build file for `llvm/lib/Support` is in
`utils/gn/tree/llvm/lib/Support`.

.. _Philosophy:

Philosophy
==========

GN believes in using GN arguments to configure the build explicitly, instead
of implicitly figuring out what to do based on what's available on the current
system.

configure is used for three classes of feature checks:

- compiler checks. In GN, these could use exec_script to identify the host
  compiler at GN time. For now the build has explicit toggles for compiler
  features. (Maybe there could be a script that writes args.gn based on the
  host compiler).  It's possible we'll use exec_script() for this going forward,
  but we'd have one exec_script call to identify compiler id and version,
  and then base GN arg default values of compiler id and version instead of
  doing one exec_script per feature check.
  (In theory, the config approach means a new os / compiler just needs to tweak
  the checks and not the code, but in practice a) new os's / compilers are rare
  b) they will require code changes anyhow, so the configure tradeoff seems
  not worth it.)

- library checks. For e.g. like zlib, GN thinks it's better to say "we require
  zlib, else we error at build time" than silently omitting features. People
  who really don't want to install zlib can explicitly set the GN arg to turn
  off zlib.

- header checks (does system header X exist). These are generally not needed
  (just keying this off the host OS works fine), but if they should become
  necessary in the future, they should be done at build time and the few
  targets that need to know if header X exists then depend on that build-time
  check while everything else can build parallel with it.

- LLVM-specific build toggles (assertions on/off, debug on/off, targets to
  build, ...). These map cleanly to GN args (which then get copied into
  config.h in a build step).

For the last two points, it would be nice if LLVM didn't have a single
`config.h` header, but one header per toggle. That way, when e.g.
`llvm_enable_terminfo` is toggled, only the 3 files caring about that setting
would need to be rebuilt, instead of everything including `config.h`.

GN doesn't believe in users setting arbitrary cflags from an environment
variable, it wants the build to be controlled by .gn files.