Cross-compiling OCaml with GitHub Actions

Ecosystem
1

For those that are interested in cross-compiling, there is:

  • a GitHub Actions workflow with an example workflow run that creates an OCaml cross-compiler from:
    1. macOS x86_64 host into macOS arm64 target executables (to make universal binaries, for example, on non-Apple Silicon hardware)
    2. Linux x86_64 host into Android arm64 (v8)
    3. Linux x86_64 host into Android arm32 (v7a)
    4. Linux x86_64 host into Android x86_64
  • internally there are some patches to the OCaml 4.12.1 source code to create a cross-compiling OCaml compiler
  • a document I wrote to help me understand what was happening: Compiling OCaml in Depth — Diskuv OCaml documentation

For example, the GitHub Actions workflow contains a test that illustrates how to use it. When the following is run on 64-bit AMD/Intel Ubuntu Linux:

$ dist/android_arm32v7a-on-linux_x86/opt/mlcross/android_arm32v7a/bin/ocamlopt.opt \
  hello_world.ml -o hello_world.opt.exe

the hello_world.opt.exe should run on Android arm32 (v7a):

$ file hello_world.opt.exe
hello_world.opt.exe: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /system/bin/linker, with debug_info, not stripped

This is just a preview and I’m not generating binary packages at the moment. But if it is useful enough to you that you will test it out (ex. copy/paste the GitHub workflow, or do it manually on your mac or Ubuntu machine), please tell me if you have any problems. Be aware it will likely be months before I package it up in a simpler form because it will take time to upstream patches into the OCaml compiler trunk.

This is a continuation of the closed topic Cross-compiling implementations / how they work . Thanks EduardoRFS, Antonio Nuno Monteiro and Romain Beauxis for showing me the technique!

7 Likes
2

Thanks! This is an excellent summary.

Just a small remark: the assembly bits in runtime/ (amd64.S, riscv.S, etc) are not used in the bytecode runtime (ocamlrun). They are only used with the native-code compiler, they are linked into the native-code version of the runtime system (libasmrun).

Cheers,
Nicolas

2 Likes
3

Thanks for that feedback! I’ve updated the doc.

4

Today all the actions in your repo are failing, saying something like
/home/runner/work/_temp/fcadd2f3-cd46-4ae5-b328-5ff9a6ffb097.sh: line 1: vendor/dkml-compiler/src/r-c-ocaml-1-setup.sh: No such file or directory

Is it normal?

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awesome. thanks for doing this. I’m new to OCaml and would love to run compiled apps on android at some point.

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As mentioned in the original post, that was a preview for a few months. Jan 2022 was long ago, and I didn’t get any feedback that anybody was using it.

Regardless, I rolled up the cross-compiling functionality into the dkml-base-compiler package. But a huge dose of caution … cross-compiling is not officially supported by OCaml! And unlike when I originally wrote the post, I now actively use my non-free DkSDK CMake product because, among other things, it wraps dkml-base-compiler cross-compiling into a “normal” IDE push-button experience (in fact, as we speak I’m helping some high schoolers use Android Studio to cross-compile OCaml into their Android Java app). So if you use dkml-base-compiler for its cross-compiling features, make sure you have intermediate-advanced comfort with the OCaml compiler, and be comfortable with getting your own support.

Here is what you’ll need if you (or anyone searching in the future) are that intermediate-advanced compiler user and want to get dkml-base-compiler working in your environment:

All the best!

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