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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

  1. 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).

  2. Consult INSTALL.adoc for build instructions. Here is the gist of it:

    ./configure
make

    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!).

  3. Try the newly built compiler binaries ocamlc, ocamlopt or their .opt version. To try the toplevel, use:

    make runtop

  4. Hack frenetically and keep rebuilding.

  5. Run the testsuite from time to time.

    make tests

  6. 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

Libraries

stdlib/

The standard library. Each file is largely independent and should not need further knowledge.

otherlibs/

External libraries such as unix, threads, dynlink, str and bigarray.

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/html/libref/index.html in a web browser.

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

Makefile.tools

used by manual/ and testsuite/ Makefiles

README.adoc

general information on the compiler distribution

README.win32.adoc

general information on the Windows ports of OCaml

VERSION

version string. Run make configure 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

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
# Replace $VERSION by the trunk version
opam pin add ocaml-variants.$VERSION+branch git+https://$REPO#branch

Useful Makefile targets

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)

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

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, and generate a .merlin file.

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.

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.

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.

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:

XARCH=x64 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 2015. 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 [email protected].

To be notified by email of build failures, you can subscribe to the [email protected] 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:

  1. Make sure you have an account on Inria’s CI as described before.

  2. Make sure you have been added to the ocaml project.

  3. 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.

  4. Make your branch publicly available. For instance, you can fork OCaml’s GitHub repository and then push "mybranch" to your fork.

  5. Visit https://ci.inria.fr/ocaml/job/precheck and log in. Click on "Build with parameters".

  6. Fill in the REPO_URL and BRANCH fields as appropriate and run the build.

  7. 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 [email protected] mailing list by visiting its web page.

Happy Hacking!