HACKING.adoc

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

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

   ./configure
   make world.opt

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. Install in a new opam switch to try things out:

   opam compiler-conf install

7. You did it, Well done! Consult CONTRIBUTING.md to send your contribution upstream.

See our Development tips and tricks for various helpful details, for example on how to automatically create an opam switch from a compiler branch.

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 junior_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, driver/pparse.mli.

Typing — typing/

Type-checks the AST and produces a typed representation of the program (parsing/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 ./ocamldoc/stdlib_html/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

Changes

what’s new with each release

configure

configure script

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.nt

Windows Makefile (deprecated)

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

asmcomp/

native-code compiler and linker

asmrun/

native-code runtime library

boot/

bootstrap compiler

bytecomp/

bytecode compiler and linker

byterun/

bytecode interpreter and runtime system

compilerlibs/

the OCaml compiler as a library

config/

configuration files

debugger/

source-level replay debugger

driver/

driver code for the compilers

emacs/

editing mode and debugger interface for GNU Emacs

experimental/

experiments not built by default

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

otherlibs/

several additional libraries

parsing/

syntax analysis — see parsing/HACKING.adoc

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

opam compiler script

The separately-distributed script opam-compiler-conf can be used to easily build opam switches out of a git branch of the compiler distribution. This lets you easily install and test opam packages from an under-modification compiler version.

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

Bootstrapping

The OCaml compiler is bootstrapped. This means that previously-compiled bytecode versions of the compiler, dependency generator 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 INSTALL.adoc.

Continuous integration

Github’s CI: Travis and AppVeyor

The script that is run on Travis continuous integration servers is .travis-ci.sh; its configuration can be found as a Travis configuration file in .travis.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 .travis.yml:

CI_KIND=build XARCH=x64 bash -ex .travis-ci.sh

The scripts support two other kinds of tests (values of the CI_KIND variable) which both inspect the patch submitted as part of a pull request. tests checks that the testsuite has been modified (hopefully, improved) by the patch, and changes checks that the Changes file has been modified (hopefully to add a new entry).

These tests rely on the $TRAVIS_COMMIT_RANGE variable which you can set explicitly to reproduce them locally.

The changes check 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.

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, enable email notifications, and manually restart builds. If you would like to do this but have trouble doing it, please contact Damien Doligez or Gabriel Scherer.

Running INRIA’s CI on a github Pull Request (PR)

If you have suspicions that a PR may fail on exotic architectures (it touches 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 by pushing to a precheck branch of the main repository.

This is done by pushing to a specific github repository that the CI watches, namely ocaml/precheck. You thus need to have write/push/commit access to this repository to perform this operation.

Just checkout the commit/branch you want to test, then run

git push --force git@github.com:ocaml/precheck.git HEAD:trunk

(This is the syntax to push the current HEAD state into the trunk reference on the specified remote.)