= 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 link:CONTRIBUTING.md[].

=== Your first compiler modification

1. Create a new git branch to store your changes.
+
----
git checkout -b my-modification
----

2. Consult link: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
----

5. Install in a new opam switch to try things out:
+
----
opam compiler-conf install
----

6. You did it, Well done! Consult link:CONTRIBUTING.md[] to send your contribution upstream.

See our <<Development tips and tricks>> for various helpful details,
for example on how to automatically <<opam compiler script,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:

* http://caml.inria.fr/mantis/view_all_bug_page.php[The OCaml
  bugtracker] contains reported bugs and feature requests. Some
  changes that should be accessible to newcomers are marked with the
  tag
  http://caml.inria.fr/mantis/search.php?project_id=1&sticky_issues=1&sortby=last_updated&dir=DESC&highlight_changed=24&hide_status_id=90&tag_string=junior_job[junior_job].

* The
  https://github.com/ocamllabs/compiler-hacking/wiki/Things-to-work-on[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 link: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 https://github.com/ocaml/ocaml[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 -- link: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 -- link:parsing/[]

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

The logic for Camlp4 and Ppx preprocessing is not in link:parsing/[],
but in link:driver/[], see link:driver/pparse.mli[],
link:driver/pparse.mli[].

==== Typing -- link:typing/[]

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

==== The bytecode compiler -- link:bytecomp/[]

==== The native compiler -- link:middle_end/[] and link:asmcomp/[]

=== Runtime system

=== Libraries

link:stdlib/[]:: The standard library. Each file is largely
independent and should not need further knowledge.

link:otherlibs/[]:: External libraries such as `unix`, `threads`,
`dynlink`, `str` and `bigarray`.

=== Tools

link:lex/[]:: The `ocamllex` lexer generator.

link: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
link:http://gallium.inria.fr/~fpottier/menhir/[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.shared::       common Makefile
  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 link: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 link:parsing/HACKING.adoc[]
  stdlib/::               standard library
  testsuite/::            tests -- see link:testsuite/HACKING.adoc[]
  tools/::                various utilities
  toplevel/::             interactive system
  typing/::               typechecking -- see link:typing/HACKING.adoc[]
  utils/::                utility libraries
  yacc/::                 parser generator

== Development tips and tricks

=== opam compiler script

The separately-distributed script
https://github.com/gasche/opam-compiler-conf[`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 link: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 link: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
link: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 link:INSTALL.adoc#bootstrap[INSTALL.adoc].

=== Continuous integration

==== Github's CI: Travis and AppVeyor

==== INRIA's Continuous Integration (CI)

INRIA provides a Jenkins continuous integration service that OCaml
uses, see link: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
link:https://github.com/ocaml/precheck[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.)