Rust on Dreamcast

Ferris holding his Dreamcast controller

WIP: This article is currently under construction. The repos linked to below are not yet live.

Rust is a systems programming language rising in popularity which emphasizes memory safety and performance. Due to its operating at a low level, it is an ideal candidate for running on the Dreamcast. Doing so presents a bit of a challenge, however, as the official Rust compiler is based on the LLVM toolchain infrastructure, which does not support the Dreamcast CPU's SuperH architecture. Dreamcast programming is instead typically done with GCC, the GNU Compiler Collection. There are currently two viable solutions to this challenge:

• rustc_codegen_gcc: A libgccjit-based codegen backend for rustc (preferred method)
• gccrs: a Rust frontend for GCC

Neither solution is complete at this time, and both are under active development. Using either of them to target the Dreamcast should be considered experimental. rustc_codegen_gcc is quite further along, however, and is quite usable with some patience with its current limitations and rapid change. On the other hand, while gccrs can compile for Dreamcast, it is in a very early stage, with much of the language unimplemented and no libcore support. Below we will focus on using rustc_codegen_gcc. For more information on using gccrs, see the gccrs page.

Using rustc_codegen_gcc to develop on Dreamcast

With rustc_codegen_gcc, we can interface the standard rustc compiler frontend with libgccjit, a GCC code-generation API. With the help of the Rust-for-Dreamcast repo and the kos-rs crate containing KallistiOS bindings, we can set up rustc_codegen_gcc to compile Rust programs with core and alloc support (but not the entirety of std). Rust-for-Dreamcast includes wrapper scripts to invoke the rustc and cargo tools in a familiar way. The familiar borrow checker still works, and one can import and use no_std crates. Despite this support, rustc_codegen_gcc is still in active development, so if using such a setup, expect that things may change rapidly over time. We will need to use some patches and workarounds to make this solution work. See the rustc_codegen_gcc progress reports for more information on the project's progress.

What Works

• libcore -- the core components of the language for running on bare metal (basics like integers, floats, enums, bools, chars, tuples, arrays, slices, closures, iterators, etc.)
• liballoc -- the core components of the language that require a heap, including collections (Vec, String, Box, etc.)
• linking to KallistiOS -- KallistiOS and kos-ports can be used if one manually manages interoperating with C via unsafe
• including no_std crates with the cargo build system

Future Goals

• libc support -- Adding KallistiOS support to Rust's libc crate
• libstd support -- built-in language support for I/O, networking, threads, time and date, HashMap/HashSet, unwinding on panic, etc.
• KallistiOS bindings -- properly idiomatic Rust support for KallistiOS
• Inclusion as a tier 3 target officially
• Expansion of cargo-dc to support more dcdev-specific functionallity like generating Dreamcast disc images

Prerequisites

We will build rustc_codegen_gcc support for the Dreamcast in the instructions below. Before we begin, though:

• You must already have a KallistiOS development environment set up. This means you have created a cross-compiling toolchain for SH4, you have set up your KallistiOS environ.sh file, and you have built KallistiOS with it. Ideally, you will already have some familiarity with KallistiOS. See Getting Started with Dreamcast development for more information, as well as the KallistiOS Doxygen.
  • For the purposes of this guide, we will assume you are using the standard paths for Dreamcast development tools; i.e. your environment is set up in /opt/toolchains/dc. Some included scripts and examples may assume this.
  • Your KallistiOS installation will need its KOS_SH4_PRECISION setting set to -m4-single. At this time, rustc_codegen_gcc support will not compile with KallistiOS's default -m4-single-only setting. This setting can be changed in KallistiOS's environ.sh, but changing the setting may require you to rebuild your toolchain if you have not built it with m4-single support (which is off by default, but can be enabled in the config.mk file). Once you modify the setting in your environ.sh and re-source the environ.sh, you'll need to rebuild KallistiOS with a make clean and make for the changes to take effect. kos-ports being used will also need rebuilding with -m4-single. Keep in mind, however, that because KallistiOS doesn't officially support -m4-single yet, some things may be broken, especially libraries in kos-ports that haven't been heavily tested with this setting.
• You must already have a relatively up-to-date Rust installation, either using your operating system's package manager or rustup. Ideally, you will already have some familiarity with Rust's tools.

If you run into any errors or other challenges while following this tutorial, or simply need clarification on any of the steps, feel free to ask for assistance on the message board and we would be happy to aid you and update the guide for the benefit of future readers and others in the community.

Building a cross-compiling libgccjit.so for rustc_codegen_gcc

Before we can use rustc_codegen_gcc, we must compile libgccjit.so, the libgccjit library, for your system. This entails building a unique copy of the SH4 toolchain in its own directory under /opt/toolchains/dc/rust, using a forked version of GCC with enhancements made to libgccjit. The forked version is based on the latest GCC 14.0.1 development branch.

• NOTE: This forked version of GCC 14.0.1 with libgccjit changes is actively developed alongside rustc_codegen_gcc itself, so if you update your rustc_codegen_gcc installation, you may also need to rebuild libgccjit to pull down changes rustc_codegen_gcc depends upon.

We will first clone the rust-for-dreamcast repository, which contains various supporting files needed to create Rust support for Dreamcast. Using git, clone the rust-for-dreamcast repository to /opt/toolchains/dc/rust:

git clone https://github.com/darcagn/rust-for-dreamcast /opt/toolchains/dc/rust

Enter your KallistiOS installation's dc-chain directory:

cd /opt/toolchains/dc/kos/utils/dc-chain

Clear out any existing build files:

make clean-keep-archives

Copy the necessary toolchain patches to your dc-chain setup:

cp /opt/toolchains/dc/rust/toolchain/*.diff patches/

Copy the rustc_codegen_gcc configuration file into place:

cp /opt/toolchains/dc/rust/toolchain/config.mk.rustc.sample config.mk

Make any desired changes to the configuration (e.g., change makejobs=-j2 to the number of CPU threads you'd like to use during compilation), and then compile the SH4 toolchain:

make build-sh4

When this command is completed successfully, a libgccjit.so will be installed to /opt/toolchains/dc/rust/sh-elf/lib/libgccjit.so.

Building rustc_codegen_gcc

Clone the rustc_codegen_gcc to your rust directory:

git clone https://github.com/rust-lang/rustc_codegen_gcc.git /opt/toolchains/dc/rust/rustc_codegen_gcc

rustc_codegen_gcc needs a config.toml file that specifies the location of libgccjit.so. Let's write the the gcc-path to the location of our libgccjit.so library file in this file:

echo 'gcc-path = "/opt/toolchains/dc/rust/sh-elf/lib"' > /opt/toolchains/dc/rust/rustc_codegen_gcc/config.toml

The rust-for-dreamcast repository contains scripts and wrappers to assist you in building rustc_codegen_gcc and using it in conjunction with cargo and rustc. We'll need to add the path to those scripts to our PATH environment variable:

export PATH="/opt/toolchains/dc/rust/bin:$PATH"

You may also want to add the above lines to your shell's startup file or else you'll need to run them every time you open a new shell.

Now we can use the included scripts to set up rustc_codegen_gcc. Various patches need to be applied to rustc_codegen_gcc for it to compile properly for our target platform. Let's apply them:

rcg-dc patch

Now we can prepare and build rustc_codegen_gcc!

rcg-dc prepare
rcg-dc build

Using Rust for Dreamcast

If all went well, rustc_codegen_gcc will have built successfully.

You can now use the scripts included in the Rust for Dreamcast repo:

• rcg-dc script can be used to rebuild the rustc_codegen_gcc code after updating or editing it
• rustc-dc script can be used to compile Rust modules
• cargo-dc script can be used to build Rust crates

Examples are included with the Rust for Dreamcast repo to help you get started:

• cargo-hello demonstrates how to create a simple "Hello, world!" application with KallistiOS using cargo
• cargo-cube demonstrates a Rust project using KallistiOS with GLdc
• cargo-addlib demonstrates how to create a Rust library that can be included with a KallistiOS project
• rustc-hello demonstrates how to compile and include a Rust module into a standard KallistiOS Makefile-based project

These examples rely on the kos-rs crate being present on your computer locally, so let's pull it down now:

git clone https://github.com/darcagn/kos-rs /opt/toolchains/dc/rust/kos-rs

Creating a new Rust project with Cargo

• This will follow the cargo-hello example

Creating a Rust project using kos-ports libraries

• This will follow the cargo-cube example

Creating a Rust library for Dreamcast

• This will follow the cargo-addlib example

Compiling individual modules into object files with rustc

• This will follow the rustc-hello example

To incorporate Rust source files into a standard KallistiOS Makefile-based project, you can use the rustc-dc wrapper. If we assume the Rust module file is named example.rs, you'll need to add example.o as an object file in your Makefile's OBJS = declaration. Additionally, you'll need to add the following lines so that make knows how to compile Rust modules into .o object files:

%.o: %.rs
	rustc-dc $< -o $@

Alternatively, you can add those lines to your KallistiOS Makefile.rules file to avoid having to place it in every project's Makefile.

An example "Hello, world!" program built in this style which also demonstrates basic C interoperation is included with the Rust-for-Dreamcast repository, located at examples/rustc-hello.

Creating a new project using Cargo

cargo-dc simplifies invoking cargo and creating Dreamcast crates. When using cargo in this setup, we will need to compile our program and all crate code into a static library .a file, and link it with a KallistiOS trampoline function to start the Rust code. Your Rust code will start with the function you specify as rust_main(). Once you cargo-dc build your Dreamcast code into a .a file, use cargo-dc link to automatically link it with this KallistiOS trampoline function and generate an ELF file. Instructions to do this follow.

First, let's clone the kos-rs repo:

git clone https://github.com/darcagn/kos-rs /opt/toolchains/dc/rust/kos-rs

Create a new crate using cargo-dc:

cargo-dc new example --lib

Change the crate to a static library in Cargo.toml by changing the crate-type as follows:

crate-type = ["staticlib"]

Add the kos-rs crate to your Cargo.toml file:

[dependencies]
kos = { package = "kos-rs",  path = "/opt/toolchains/dc/rust/kos-rs" }

Add the following function to your crate's src/lib.rs file:

#[no_mangle]
pub extern "C" fn rust_main(_argc: i32, _argv: *const u8) -> i32 {
    [...]
    return 0;
}

The rust_main() function will serve as the entry point to your Rust code.

An example "Hello, world!" style program built using kos-rs and cargo-dc is included with the Rust-for-Dreamcast repository, located at examples/cargo-hello. Type cargo-dc build to build the project, then cargo-dc link to link against KallistiOS and generate a cargo-hello.elf. Make sure you have your KallistiOS environ.sh sourced in your terminal before running the link command.

Integrating a Cargo project with a KallistiOS project

We can also build a crate based on kos-rs and integrate the Rust code with other C code and KOS libraries. An example rotating 3D cube program built using kos-rs and cargo-dc combined with a Makefile-based KallistiOS project is included with the Rust-for-Dreamcast repository, located at examples/rust_cube. Type cargo-dc build to build the project, then invoke make to build the KallistiOS project and link the Rust code within it.