Deno for ARM64

What is this

I put this together because there are no Linux ARM64 binaries for Deno yet. This project compiles ARM binaries and simultaneously releases a combined arm64 & amd64 container image based on Ubuntu.

Install

I have modified the default Deno installation script (https://deno.land/install.sh) to handle Linux ARM64 hosts. You can find it in this gist: deno_install.sh.

This snippet will install deno on any platform. Please read the script source code before running any curl | sh scripts!

curl -s https://gist.githubusercontent.com/LukeChannings/09d53f5c364391042186518c8598b85e/raw/ac8cd8c675b985edd4b3e16df63ffef14d1f0e24/deno_install.sh | sh

Compiling locally

To build a binary locally, using Docker, the following should compile and export a binary into the local directory.

Change the DENO_VERSION as appropriate!

DENO_VERSION=1.24.0

docker build -t deno-build --build-arg DENO_VERSION="${DENO_VERSION}" --progress=plain --file ./Dockerfile.compile .
DENO_BUILD_CONTAINER="$(docker create deno-build)"
docker cp "${DENO_BUILD_CONTAINER}:/deno/target/release/deno" .
docker rm "${CONTAINER_ID}"

How does it work?

Originally this project compiled Deno on GitHub Actions using QEMU to emulate ARM on x86, this would use 8GB+ of RAM and take hours (usually around 3 hours).

In recent Deno releases the memory requirements under QEMU compilation have exceeded the limits of free GitHub Actions and I have resorted to compiling on my ARM-based M1 MacBook Air, which was added as a self-hosted GitHub Actions Runner. This brings compile time for an ARM64 release down to 10-20 minutes.

But I just want a Deno binary for my Raspberry Pi!

You can download the latest Deno binaries from the Releases page. There should be a deno-linux-arm64.zip asset attached to each release.

Once installed, the deno upgrade command will download the latest build from this repository.

FAQ

Why isn't there official support?

The Deno team are waiting for ARM64 GitHub Actions runners:

We use GitHub Actions exclusively. We are working with GitHub on the potential of their ARM64 support.

— @kitsonk

At the moment GitHub Actions Virtual Environments are x86 only — although you can provide your own runners.

The QEMU-based builds take a long time, up to 2 hours just for compiling, and since Deno's CI runs on each push it needs to be fast.

You can follow the issue here.

What about 32-bit ARM?!

Docker's buildx uses QEMU to emulate ARM on x86.

Unfortunately there are bugs related to QEMU and 32-bit ARM that prevent compilation. You can read more here.

As such, compiling for 32-bit ARM needs to be done on a 32-bit ARM computer, and because these systems are typically underpowered, compiling may take a prohibitively long time 😬.

Why can't you just cross-compile?

In order to speed up startup time Deno builds a V8 bytecode snapshot for its JavaScript runtime. These snapshots are architecture-specific, and will cause a crash at runtime if the architecture isn't the same.

For example, when Deno is cross-compiled to ARM64 from an x86_64 architecture, the resulting binary will be ARM64. However, because the build.rs snapshot was generated on an x86 host, the snapshot will be x86 bytecode, and that causes a crash at runtime.

I tried to work around this by patching Deno to disable the compiler snapshot, but there was still a runtime crash with a difficult-to-debug stacktrace, so I think the rabbit hole goes deeper.

I don't think patching Deno is a viable option, since a future change to Deno could cause another kind of incompatibility, and I'd have to maintain a patch. For the time being emulation is a reasonable (if a bit slow) solution to this problem.

Further, Deno depends on rusty_v8, which takes a long time to compile in normal conditions, let alone under emulation.

At time of writing rusty_v8 has problems publishing a pre-compiled ARM64 binary, and so Deno requires being compiled with V8_FROM_SOURCE=1, as well as a number of additional dependencies.

I have forked rusty_v8 for this project and have been able to successfully cross-compile rusty_v8 for ARM64. The standalone binaries can be found here.

The cross compile situation is very complicated. It all revolves around snapshots. When v8 loads Javascript code into memory it essentially compiles it to bytecode(bit of a simplification here) hence v8 being a JIT compiler. On a x86 platform running a native executable this means that the bytecode is x86 bytecode. When we create a snapshot of v8 state it mostly includes this bytecode. If we are cross compiling this snapshot is generated by running a native executable ('build.rs') thus the snapshot generated is native to the compiler host and is not compatible with the platform we are compiling for. If we can either find a way to execute a arm native version of the snapshot generator at compile time or disable snapshots for cross compiles(my previous solution), we should be able to cross compile for arm64.

— @afinch7

Why do you care about building an ARM64 binary?

I'm using Deno for one of my projects — MovieMatch — that has a requirement to provide standalone binaries for popular architectures, as well as a multi-architecture Docker image.

If you're interested in how I build standalone binaries for these architectures, check out the build file here.

How do you run the GitHub actions runner on macOS?

docker run --name github-runner --rm \
  -e REPO_URL="https://github.com/LukeChannings/deno-arm64" \
  -e RUNNER_NAME="deno-arm64-runner" \
  -e RUNNER_TOKEN="${RUNNER_TOKEN}" \
  -e RUNNER_WORKDIR="/tmp/deno-arm64" \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v /tmp/deno-arm64:/tmp/deno-arm64 \
  myoung34/github-runner:latest

If the myoung34/github-runner image is not latest, sometimes the container will restart infinitely. Killing it and pulling the latest fixes the problem.