Hacking the compiler 🐫

This document is a work-in-progress attempt to provide useful information for people willing to inspect or modify the compiler distribution’s codebase. Feel free to improve it by sending change proposals for it.

If you already have a patch that you would like to contribute to the official distribution, please see CONTRIBUTING.md.

Your first compiler modification

Create a new git branch to store your changes.
```
git checkout -b my-modification
```
Usually, this branch wants to be based on trunk. If your changes must be on a specific release, use its release branch (not the release tag) instead. For example, to make a fix for 4.11.1, base your branch on 4.11 (not on 4.11.1). The configure step for the compiler recognises a development build from the +dev in the version number (see file VERSION), and release tarballs and the tagged Git commits do not have this which causes some important development things to be disabled (ocamltest and converting C compiler warnings to errors).
Consult INSTALL.adoc for build instructions. Here is the gist of it:
```
./configure
make -j 4
```
If you are on a release build and need development options, you can add --enable-ocamltest (to allow running the testsuite) and --enable-warn-error (so you don’t get caught by CI later!).
Try the newly built compiler binaries ocamlc, ocamlopt or their .opt version. To try the toplevel, use:
```
make runtop
```
Hack frenetically and keep rebuilding.
Run the testsuite from time to time.
```
make tests
```
You did it, Well done! Consult CONTRIBUTING.md to send your contribution upstream.

See also our development tips and tricks, for example on how to create an opam switch to test your modified compiler.

What to do

There is always a lot of potential tasks, both for old and newcomers. Here are various potential projects:

The OCaml bugtracker contains reported bugs and feature requests. Some changes that should be accessible to newcomers are marked with the tag newcomer-job.
The OCaml Labs compiler-hacking wiki contains various ideas of changes to propose, some easy, some requiring a fair amount of work.
Documentation improvements are always much appreciated, either in the various .mli files or in the official manual (See manual/README.md). If you invest effort in understanding a part of the codebase, submitting a pull request that adds clarifying comments can be an excellent contribution to help you, next time, and other code readers.
The github project contains a lot of pull requests, many of them being in dire need of a review — we have more people willing to contribute changes than to review someone else’s change. Picking one of them, trying to understand the code (looking at the code around it) and asking questions about what you don’t understand or what feels odd is super-useful. It helps the contribution process, and it is also an excellent way to get to know various parts of the compiler from the angle of a specific aspect or feature.

Again, reviewing small or medium-sized pull requests is accessible to anyone with OCaml programming experience, and helps maintainers and other contributors. If you also submit pull requests yourself, a good discipline is to review at least as many pull requests as you submit.

Structure of the compiler

The compiler codebase can be intimidating at first sight. Here are a few pointers to get started.

Compilation pipeline

The driver — driver/

The driver contains the "main" function of the compilers that drive compilation. It parses the command-line arguments and composes the required compiler passes by calling functions from the various parts of the compiler described below.

Parsing — parsing/

Parses source files and produces an Abstract Syntax Tree (AST) (parsing/parsetree.mli has lot of helpful comments). See parsing/HACKING.adoc.

The logic for Camlp4 and Ppx preprocessing is not in parsing/, but in driver/, see driver/pparse.mli and driver/pparse.ml.

Typing — typing/

Type-checks the AST and produces a typed representation of the program (typing/typedtree.mli has some helpful comments). See typing/HACKING.adoc.

The bytecode compiler — bytecomp/

The native compiler — middle_end/ and asmcomp/

Runtime system

The low-level routines that OCaml programs use during their execution: garbage collection, interaction with the operating system (IO in particular), low-level primitives to manipulate some OCaml data structures, etc. Mostly implemented in C, with some rare bits of assembly code in architecture-specific files. The "includes" corresponding to the .c files are in the runtime/caml subdirectory.

Some files are only used by bytecode programs, some only used by native-compiled programs, but most of the runtime code is common. (See runtime_COMMON_C_SOURCES, runtime_BYTECODE_ONLY_C_SOURCES, and runtime_NATIVE_ONLY_C_SOURCES in Makefile for the list of common, bytecode-only, and native-only source files.)

See runtime/HACKING.adoc.

Libraries

stdlib/: The standard library. Each file is largely independent and should not need further knowledge.
otherlibs/: External libraries such as unix, threads, dynlink and str.

Instructions for building the full reference manual are provided in manual/README.md. However, if you only modify the documentation comments in .mli files in the compiler codebase, you can observe the result by running

make html_doc

and then opening ./api_docgen/ocamldoc/build/html/libref/index.html in a web browser. The documentation is located in ./api_docgen/odoc/build/html/libref/index.html when --with-odoc is passed to the configure script.

Tools

lex/: The ocamllex lexer generator.
yacc/: The ocamlyacc parser generator. We do not recommend using it for user projects in need of a parser generator. Please consider using and contributing to menhir instead, which has tons of extra features, lets you write more readable grammars, and has excellent documentation.

Complete file listing

BOOTSTRAP.adoc: instructions for bootstrapping
Changes: what’s new with each release
CONTRIBUTING.md: how to contribute to OCaml
HACKING.adoc: this file
INSTALL.adoc: instructions for installation
LICENSE: license and copyright notice
Makefile: main Makefile
Makefile.common: common Makefile definitions
README.adoc: general information on the compiler distribution
README.win32.adoc: general information on the Windows ports of OCaml
VERSION: version string. Run tools/autogen after changing.
asmcomp/: native-code compiler and linker
boot/: bootstrap compiler
build-aux/: autotools support scripts
bytecomp/: bytecode compiler and linker
compilerlibs/: the OCaml compiler as a library
configure: configure script
configure.ac: autoconf input file
debugger/: source-level replay debugger
driver/: driver code for the compilers
flexdll/: git submodule — see README.win32.adoc
lex/: lexer generator
man/: man pages
manual/: system to generate the manual
middle_end/: the flambda optimisation phase
ocamldoc/: documentation generator
ocamltest/: test driver
otherlibs/: several additional libraries
parsing/: syntax analysis — see parsing/HACKING.adoc
release-info/: documentation and tools to prepare releases
runtime/: bytecode interpreter and runtime systems
stdlib/: standard library
testsuite/: tests — see testsuite/HACKING.adoc
tools/: various utilities
toplevel/: interactive system
typing/: typechecking — see typing/HACKING.adoc
utils/: utility libraries
winpthreads/: winpthreads submodule — see further
yacc/: parser generator

Development tips and tricks

Keep merge commits when merging and cherry-picking Github PRs

Having the Github PR number show up in the git log is very useful for later triaging. We recently disabled the "Rebase and merge" button, precisely because it does not produce a merge commit.

When you cherry-pick a PR in another branch, please cherry-pick this merge-style commit rather than individual commits, whenever possible. (Picking a merge commit typically requires the -m 1 option.) You should also use the -x option to include the hash of the original commit in the commit message.

git cherry-pick -x -m 1 <merge-commit-hash>

Testing with `opam`

If you are working on a development version of the compiler, you can create an opam switch from it by running the following from the development repository:

opam switch create . --empty
opam install .

If you want to test someone else’s development version from a public git repository, you can build a switch directly (without cloning their work locally) by pinning:

opam switch create my-switch-name --empty
opam pin add ocaml-variants git+https://$REPO#branch

Incremental builds with `opam`

This section documents some tips to speed up your workflow when you need to alternate between testing your branch and patching the compiler. We’ll assume that you’re currently in a clone of the compiler’s source code.

Initial setup

For the rest of the section to work, you’ll need your compiler to be configured in the same way as opam would have configured it. The simplest way is to run the normal commands for the switch initialization, with the extra --inplace-build flag:

opam switch create . --empty
opam install . --inplace-build

However, if you need specific configuration options, you can also configure it manually, as long as you make sure that the configuration prefix is the one where opam would install the compiler. You will then need to install the compiler, either from the working directory (that you must build yourself) or using the regular sandboxed builds.

# Example with regular opam build
opam switch create . --empty
opam install .
./configure --prefix=$(opam var prefix) # put extra configuration args here

# Example with installation from the current directory
opam switch create . --empty
./configure --prefix=$(opam var prefix) # put extra configuration args here
make -j
opam install . --assume-built

Basic workflow

We will assume that the workflow alternates between work on the compiler and external (opam-related) commands. As an example, debugging an issue in the compiler can be done by a first step that triggers the issue (by installing a given opam package), then adding some logging to the compiler, re-trigger the issue, and based on the logs either add more logging, or try a patch, and so on.

The part of this workflow that we’re going to optimize is when we switch from working on the compiler to using the compiler. The basic way to do this is to run opam install . again, but this will recompile the compiler from scratch and also trigger a recompilation of all the packages in the switch.

Using `opam-custom-install`

The opam-custom-install plugin allows you to install a package using a custom command instead of the package-supplied one. It can be installed following instructions here.

In our case, we need to build the compiler, and when we’ve built everything that we need then we run opam custom-install ocaml-variants — make install. This will make opam remove the previously installed version of the compiler (if any), then install the new one in its stead.

# reinstall the compiler, and rebuild all opam packages
opam custom-install ocaml-variants -- make install

Since most opam packages depend on the compiler, this will trigger a reinstallation of all the packages in the switch. If you want to avoid that (for instance, your patch only adds some logging so you expect the core libraries and all the already compiled packages to be identical), you can use the additional --no-recompilations flag. There are no checks that it’s safe to do so, so if your patch ends up changing even slightly one of the core libraries' files, you will likely get inconsistent assumptions errors later.

# reinstall the compiler, leaving the opam packages untouched -- unsafe!
opam custom-install --no-recompilations ocaml-variants -- make install

Note about the first installation: When you start from an empty switch, and install a compiler (in our case, the ocaml-variants package provided by the compiler’s opam file), then a number of additional packages are installed to ensure that the switch will work correctly. Mainly, the ocaml package needs to be installed, and while it’s done automatically when using regular opam commands, the custom-install plugin will not force installation of dependencies. Moreover, if you try to fix the problem by manually installing the ocaml package, opam will try to recompile ocaml-variants, using the default instructions. You can get around this by running opam reinstall --forget-pending just after the opam custom-install command and just before the opam install ocaml command. Full example:

opam switch create . --empty
./configure --prefix=$(opam var prefix) --disable-ocamldoc --disable-ocamltest
make world && make opt
opam custom-install ocaml-variants -- make install
opam reinstall --forget-pending --yes
opam install ocaml
# You now have a working switch, in which you can start installing packages

One advantage of this plugin over a plain make install is that it correctly tracks the files associated with the compiler, so if your make install command only installs the bytecode versions of the tools, then with opam-custom-install you will end up in a state where only the bytecode tools are installed, whereas with a raw make install you will have stale native binaries remaining in your switch. Since it’s significantly faster to build the bytecode version of the tools, and many opam packages will pick the native version of the compilers if present and the bytecode version otherwise, you can build your initial switch with the native versions (to get quickly to a state where a bug appears), then clean your working directory and start building bytecode tools only for the actual debugging phase.

Without `opam-custom-install`

You can achieve some improvements using built-in opam commands.

Using opam install . --assume-built will simply remove the package for the compiler, then run the installation instructions (make install) in the working directory, tracking the installed files correctly. The main difference with the opam-custom-install version is that there’s no way to prevent this command from triggering a full recompilation of your switch.

You can also run make install manually, which will not trigger a recompilation, but will not remove the previous version either and can mess with `opam’s tracking of installed files.

Useful Makefile targets and options

Besides the targets listed in INSTALL.adoc for build and installation, the following targets may be of use:

make runtop: builds and runs the ocaml toplevel of the distribution (optionally uses rlwrap for readline+history support) (use make runtop-with-otherlibs if you need Unix or other otherlibs/ libraries)
make natruntop: builds and runs the native ocaml toplevel (experimental)
make partialclean: Clean the OCaml files but keep the compiled C files.
make depend: Regenerate the .depend file. Should be used each time new dependencies are added between files.
make -C testsuite parallel: see testsuite/HACKING.adoc

You can use make foo V=1 to build the target foo and show full commands instead of abbreviated names like OCAMLC, etc. This can be useful to know the flags to use to manually rebuild a file.

Additionally, there are some developer specific targets in Makefile.dev. These targets are automatically available when working in a Git clone of the repository, but are not available from a tarball.

Automatic configure options

If you have options to configure which you always (or at least frequently) use, it’s possible to store them in Git, and configure will automatically add them. For example, you may wish to avoid building the debug runtime by default while developing, in which case you can issue git config --global ocaml.configure '--disable-debug-runtime'. The configure script will alert you that it has picked up this option and added it before any options you specified for configure.

Options are added before those passed on the command line, so it’s possible to override them, for example ./configure --enable-debug-runtime will build the debug runtime, since the enable flag appears after the disable flag. You can also use the full power of Git’s config command and have options specific to particular clone or worktree.

Speeding up configure

configure includes the standard -C option which caches various test results in the file config.cache and can use those results to avoid running tests in subsequent invocations. This mechanism works fine, except that it is easy to clean the cache by mistake (e.g. with git clean -dfX). The cache is also host-specific which means the file has to be deleted if you run configure with a new --host value (this is quite common on Windows, where configure is also quite slow to run).

You can elect to have host-specific cache files by issuing git config --global ocaml.configure-cache .. The configure script will now automatically create ocaml-host.cache (e.g. ocaml-x86_64-pc-windows.cache, or ocaml-default.cache). If you work with multiple worktrees, you can share these cache files by issuing git config --global ocaml.configure-cache ... The directory is interpreted relative to the configure script.

Bootstrapping

The OCaml compiler is bootstrapped. This means that previously-compiled bytecode versions of the compiler and lexer are included in the repository under the boot/ directory. These bytecode images are used once the bytecode runtime (which is written in C) has been built to compile the standard library and then to build a fresh compiler. Details can be found in BOOTSTRAP.adoc.

Speeding up builds

Once you’ve built a natively-compiled ocamlc.opt, you can use it to speed up future builds by copying it to boot:

cp ocamlc.opt boot/

If boot/ocamlc changes (e.g. because you ran make bootstrap), then the build will revert to the slower bytecode-compiled ocamlc until you do the above step again.

Using merlin

During the development of the compiler, the internal format of compiled object files evolves, and quickly becomes incompatible with the format of the last OCaml release. In particular, even an up-to-date merlin will be unable to use them during most of the development cycle: opening a compiler source file with merlin gives a frustrating error message.

To use merlin on the compiler, you want to build the compiler with an older version of itself. One easy way to do this is to use the experimental build rules for Dune, which are distributed with the compiler (with no guarantees that the build will work all the time). Assuming you already have a recent OCaml version installed with merlin and dune, you can just run the following from the compiler sources:

./configure # if not already done
make clean && dune build @libs

which will do a bytecode build of all the distribution (without linking the executables), using your OCaml compiler.

Merlin will be looking at the artefacts generated by dune (in _build), rather than trying to open the incompatible artefacts produced by a Makefile build. In particular, you need to repeat the dune build every time you change the interface of some compilation unit, so that merlin is aware of the new interface.

You only need to run configure once, but you will need to run make clean every time you want to run dune after you built something with make; otherwise dune will complain that build artefacts are present among the sources.

Finally, there will be times where the compiler simply cannot be built with an older version of itself. One example of this is when a new primitive is added to the runtime, and then used in the standard library straight away, since the rest of the compiler requires the stdlib library to build, nothing can be build. In such situations, you will have to either live without merlin, or develop on an older branch of the compiler, for example the maintenance branch of the last released version. Developing a patch from a release branch can later introduce a substantial amount of extra work, when you rebase to the current development version. But it also makes it a lot easier to test the impact of your work on third-party code, by installing a local opam switch: opam packages tend to be compatible with released versions of the compiler, whereas most packages are incompatible with the in-progress development version.

License

The OCaml distribution in this repository, which includes the OCaml compiler and runtime, the OCaml standard library, and some extra tools, is licensed under the LGPL version 2.1, with a linking exception. See LICENSE for the full license text. Using the SPDX standard for license identifiers, this is the license "LGPL-2.1 WITH OCaml-LGPL-linking-exception". Let us include here a few explanations about the intent of this licensing choice — note that only the license text has legal value.

The broad intent of the LGPL is that if you modify the OCaml distribution, and distribute your modified version to your users, then you have to provide the source code of your version — the LGPL, like the GPL and unlike the BSD and MIT licenses, is "hereditary"; modified versions of LGPL programs must remain free software under the same license. (For more general discussion of the LGPL license, see the FAQ maintained by the GNU project.)

On the other hand, if your code is not derived from the OCaml distribution, it only uses it, then you can use the licensing terms of your choice. We use an explicit "linking exception" precisely to make it clear that it is fine to link statically or dynamically with parts of the OCaml distribution, typically the OCaml runtime and the standard library. Linking exceptions are a common practice for language compilers and runtime libraries, for example the GCC project uses such an exception. See the Wikipedia article on linking exceptions for more details.

Finally: If you have your own fork of the OCaml distribution, it must be distributed to your own users under the same license (LGPL + exception), and your users will also benefit from the linking exception.

Continuous integration

check-typo

The tools/check-typo script enforces various typographical rules in the OCaml compiler codebase.

Running ./tools/check-typo from the repository root will check all source files. This can be fairly slow (2 minutes for example). Use ./tools/check-typo <path> to run it on some file or directory (recursively) only.

Running ./tools/check-typo-since trunk checks all files that changed in the commits since trunk — this work with any git reference. It runs much faster than a full ./tools/check-typo, typically instantly.

You can also setup a git commit-hook to automatically run check-typo on the changes you commit, by copying the file tools/pre-commit-githook to .git/hooks/pre-commit. If changes in a commit alter the configure script, the hook also checks that the committed configure script is up-to-date.

Some files need special rules to opt out of check-typo checks; this is specified in the .gitattributes file at the root of the repository, using typo.foo attributes.

Regenerating the `configure` script

The configure script may be regenerated by running tools/autogen, with GNU Autoconf installed.

If you’re using a version of GNU Autoconf that differs from our CI, then the generated configure script might differ too. The latest Ubuntu LTS packages may be used as a reasonable base to generate this file. Using a Docker image with GNU Autoconf installed, and git rebase --exec, one can properly regenerate the configure script for each commit in a branch.

docker build -t ocaml-make-configure:latest - <<'EOF'
# syntax=docker/dockerfile:1
FROM ubuntu:latest
RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
    --mount=type=cache,target=/var/lib/apt,sharing=locked \
    apt update && apt-get --no-install-recommends install -y autoconf make
WORKDIR /mnt
ENTRYPOINT ["tools/autogen"]
EOF
git rebase trunk --exec "docker run --rm -it -v $PWD:/mnt ocaml-make-configure:latest"

GitHub’s Continuous Integration: GitHub Actions and AppVeyor

The scripts that are run on GitHub Actions are described in .github/workflows/build.yml.

For example, if you want to reproduce the default build on your machine, you can use the configuration values and run command taken from tools/ci/actions/runner.sh:

bash -ex tools/ci/actions/runner.sh configure

The .github/workflows/hygiene.yml script supports other kinds of tests which inspect the patch submitted as part of a pull request. These tests rely on ancillary data generated by GitHub Actions which you have to set explicitly to reproduce them locally.

Changes updated checks that the Changes file has been modified (hopefully to add a new entry). It can be disabled by including "(no change entry needed)" in one of your commit messages — but in general all patches submitted should come with a Changes entry; see the guidelines in CONTRIBUTING.md.

The Windows ports take a long time to test - INRIA’s precheck service is the best to use when all 6 Windows ports need testing for a branch, but the AppVeyor scripts also support the other ports. The matrix is controlled by the following environment variables, which should be set in appveyor.yml:

PORT - this must be set on each job. Either mingw, msvc or cygwin followed by 32 or 64.
BOOTSTRAP_FLEXDLL - must be set on each job. Either true or false. At present, must be false for Cygwin builds. Controls whether flexlink is bootstrapped as part of the test or installed from a binary archive.
FORCE_CYGWIN_UPGRADE. Default: 0. Set to 1 to force an upgrade of Cygwin packages as part of the build. Normally a full upgrade is only triggered if the packages installed require it.
BUILD_MODE. Default: world.opt. Either world.opt, steps, or C. Controls whether the build uses the world.opt target or the classic world, opt, opt.opt targets. The C build is a fast test used to build just enough of the tree to cover the C sources (it’s used to test old MSVC compilers).
SDK. Defaults to Visual Studio 2022. Specifies the exact command to run to set-up the Microsoft build environment.
CYGWIN_DIST. Default: 64. Either 64 or 32, selects 32-bit or 64-bit Cygwin as the build environment.

INRIA’s Continuous Integration (CI)

INRIA provides a Jenkins continuous integration service that OCaml uses, see https://ci.inria.fr/ocaml/. It provides a wider architecture support (MSVC and MinGW, a zsystems s390x machine, and various MacOS versions) than the Travis/AppVeyor testing on github, but only runs on commits to the trunk or release branches, not on every PR.

You do not need to be an INRIA employee to open an account on this jenkins service; anyone can create an account there to access build logs and manually restart builds. If you would like to do this but have trouble doing it, please email ocaml-ci-admin@inria.fr.

To be notified by email of build failures, you can subscribe to the ocaml-ci-notifications@inria.fr mailing list by visiting its web page.

Running INRIA’s CI on a publicly available git branch

If you have suspicions that your changes may fail on exotic architectures (they touch the build system or the backend code generator, for example) and would like to get wider testing than github’s CI provides, it is possible to manually start INRIA’s CI on arbitrary git branches even before opening a pull request as follows:

Make sure you have an account on Inria’s CI as described before.
Make sure you have been added to the ocaml project.
Prepare a branch with the code you’d like to test, say "mybranch". It is probably a good idea to make sure your branch is based on the latest trunk.
Make your branch publicly available. For instance, you can fork OCaml’s GitHub repository and then push "mybranch" to your fork.
Visit https://ci.inria.fr/ocaml/job/precheck and log in. Click on "Build with parameters".
Fill in the REPO_URL and BRANCH fields as appropriate and run the build.
You should receive a bunch of e-mails with the build logs for each slave and each tested configuration (with and without flambda) attached.

Changing what the CI does

INRIA’s CI "main" and "precheck" jobs run the script tools/ci-build. In particular, when running the CI on a publicly available branch via the "precheck" job as explained in the previous section, you can edit this script to change what the CI will test.

For instance, parallel builds are only tested for the "trunk" branch. In order to use "precheck" to test parallel build on a custom branch, add this at the beginning of tools/ci-build:

OCAML_JOBS=10

The `caml-commits` mailing list

If you would like to receive email notifications of all commits made to the main git repository, you can subscribe to the caml-commits@inria.fr mailing list by visiting its web page.

The `winpthreads` library for the MSVC port

The winpthreads library is used to emulate pthread for the MSVC port. Upstream bundles it along with all the MinGW libraries so our winpthreads submodule points to git subtree repository rather than upstream directly.

To recreate the winpthreads repository from upstream, you can do:

git clone -o upstream https://git.code.sf.net/p/mingw-w64/mingw-w64 winpthreads
cd winpthreads
git checkout upstream/master
git branch -D master
git subtree -P mingw-w64-libraries/winpthreads split -b master

As subtree splitting is deterministic, repeating these operations later will allow to update master, for instance by:

git fetch upstream
git checkout upstream/master
git subtree -P mingw-w64-libraries/winpthreads split -b tmp
git checkout master
git merge --ff-only tmp
git branch -d tmp

and then go on updating the winpthreads submodule in the ocaml repository.

Happy Hacking!

Files

HACKING.adoc

Latest commit

History