Jake Goulding

Rust on an Arduino Uno

We have an Arduino Uno that’s been sitting around gathering dust for a little while, so I decided to see how Rust worked on it.

A bit of searching led to a fork of Rust with AVR support, AVR-Rust. This is built on top of a fork of LLVM with AVR support, AVR-LLVM. Both of these projects are led by Dylan McKay.

The current documentation for AVR-Rust is a bit lacking, and it was forked from a development version of Rust 1.4. The current development version is Rust 1.7, making the fork about 4.5 months old. However, the changes to LLVM are in the process of being merged into upstream, laying the groundwork for merging the changes into Rust as well.

Let’s start out by doing the bare minimum and try to get a version of rustc that can target the AVR chip:

git clone https://github.com/avr-rust/rust.git
mkdir build && cd build

You’ll note that there’s nothing AVR specific here. Every Rust compiler is actually a cross-compiler, a compiler that executes on one architecture but produces code for another architecture. Because this fork of Rust has support files for AVR, it will be able to produce the correct executable code.

Unfortunately, I couldn’t get a basic file to compile out of the box.

So I did what any sane person would do – I started changing code without knowing exactly what the failure was or what the code I was changing did.

First I tried updating the branch of LLVM that AVR-Rust uses. There are two branches in the repository – avr-support is more actively updated and avr-rust-support lags behind.

Merging avr-support into avr-rust-support went smoothly, but the Rust LLVM driver code needed to be updated to handle the newer LLVM version. I grabbed the diff from the main Rust repository and applied that. This seemed to work, but then I got a segfault from the stage 1 Rust compiler, deep in the internals of LLVM.

make: *** [x86_64-apple-darwin/stage1/lib/rustlib/x86_64-apple-darwin/lib/stamp.term] Segmentation fault: 11

So I continued changing more stuff!

I merged current Rust into the AVR fork of Rust and resolved the merge conflicts as best I could figure out. After fixing a few new errors and some poor merge conflicts, I was on my way. Until I hit the segfault again.

That means it’s actually time to try to figure out where the segfault was coming from. I configured another build with some debug information:

./configure --enable-debug --disable-docs --enable-llvm-assertions --enable-debug-assertions

And built. This takes a long time, as nothing gets optimized. And then it turns out that doing this also hides the segfault. Ugh.

However, I do get to a new error:

ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Fortunately, I know where to tweak that in the source. The downside is I’ll need to wait for another long build cycle…

Continue on to part 2.