Rust on Dreamcast

Preliminary support exists for developing for the Dreamcast using the Rust programming language. This is a bit of a challenge, however, as the official Rust compiler is based on the LLVM toolchain infrastructure, which does not support the Dreamcast's CPU SuperH architecture. Dreamcast programming is instead done with GCC, the GNU Compiler Collection. There exists two solutions to this problem:

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

rustc_codegen_gcc

GCC includes a component called libgccjit which provides an API for an embeddable just-in-time code generator using GCC, useful for creating programs like interpreters. However, this component can also be used to generate code ahead of time as well. rustc_codegen_gcc is a project which interfaces the official Rust compiler with the libgccjit API to generate machine code from Rust using the GCC backend. With this, we can compile Rust programs for Dreamcast using familiar compiler tools such as rustc and cargo! The familiar borrow checker still works, and one can write #![no_std] crates with full libcore support. An experimental crate binding with KallistiOS provides liballoc functionality such as a heap and familiar collections like Vec, String, etc. as well.

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 and you have built KallistiOS with it. See Getting Started with Dreamcast development for more information.
  • 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.
  • Your KallistiOS installation will need its floating point precision setting set to m4-single. This setting is available in the environ.sh, but it may require you to rebuild your main toolchain if you have not built it with m4-single support.
• You must already have a relatively up-to-date Rust installation, either using your operating system's package manager or rustup.

Building a cross-compiling libgccjit.so for rustc_codegen_gcc

First, we must compile libgccjit.so, the cross-compiling shared library, for your system. This entails building another copy of the SH4 toolchain once more in its own directory under /opt/toolchains/dc/rust, using a forked version of GCC with enhancements to libgccjit.

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_codegen_gcc.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

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 both the path to those scripts and the path to our newly built toolchain our PATH environment variable. Additionally, we need to set an environment variable for the installation path of rustc_codegen_gcc. Let's add both:

export PATH="/opt/toolchains/dc/rust/bin:/opt/toolchains/dc/rust/sh-elf/bin:$PATH"
export CG_GCCJIT_DIR="/opt/toolchains/dc/rust/rustc_codegen_gcc"

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.

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

Set the gcc_path file:

echo /opt/toolchains/dc/rust/sh-elf/lib > /opt/toolchains/dc/rust/rustc_codegen_gcc/gcc_path

Various patches need to be applied to rustc_codegen_gcc for it to compile properly for our target platform. Let's apply them:

patch -p1 < ../patches/*.diff

Now let's build rustc_codegen_gcc!

rust-dc clean
rust-dc prepare
rust-dc build

Creating a new project using Cargo

• create an empty staticlib crate with target in .cargo/config
• add kos-rs crate from /opt/toolchains/dc/rust/kos-rs path

gccrs

gccrs implements a new Rust compiler frontend for GCC. This essentially means creating a separate new Rust compiler from the ground up using the GCC toolchain infrastructure. This project is in early stages and is targeting the Rust 1.49 revision from December 2020. As of this writing (February 2024), it is not yet able to compile Rust's libcore, so many basic language features are unimplemented or not functional. Additionally, Rust standard tooling like cargo is not available. Borrow checking is not implemented, but the project plans to later use the next-generation Rust borrow checker Polonius from the official Rust project.

It is possible to use this compiler by building the GCC 14.0.1-dev toolchain or the gccrs latest toolchain. GCC 14.0.1-dev will get you the latest code upstreamed by the gccrs team into the main development branch of GCC, while the gccrs git repo will get you the absolute latest bleeding edge updates to gccrs. The GCC 14.0.1-dev configuration file is available within the official KallistiOS repo's dc-chain script, while the latest gccrs configuration is available within the Rust for Dreamcast repository. Brief instructions follow for setting up the latest gccrs toolchain. See Getting Started with Dreamcast development for more detailed information on how to set up and run dc-chain.

Building a gccrs-enabled toolchain

Follow the Getting Started with Dreamcast development guide for creating a Dreamcast toolchain until you arrive at the instructions for setting up the dc-chain configuration file. At this point, you should have a shell open to /opt/toolchains/dc/kos/utils/dc-chain.

Clone the Rust for Dreamcast repository:

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

Copy the GCC patch in place:

cp rust/toolchain/gcc-rs-kos.diff patches/

Copy the dc-chain configuration file into place:

cp rust/toolchain/config.mk.gccrs.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). Note that to avoid conflicting with an existing stable toolchain at the default path (i.e. /opt/toolchains/dc/sh-elf), we will be installing to /opt/toolchains/dc/gccrs/sh-elf instead. To begin compilation and installation, run:

make build-sh4

After building everything, you can clean up the extraneous files in your dc-chain directory by entering:

make clean

Setting up Makefiles to compile Rust modules

As mentioned before, cargo is not available to use with gccrs, so for our example, we will place our .rs modules within a typical KallistiOS Makefile project. If we assume the module file is named example.rs, you'll need to add example.rox 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 rox object files:

%.rox: %.rs
	kos-cc -frust-incomplete-and-experimental-compiler-do-not-use $(CFLAGS) -c $< -o $@

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

In your example.rs file, your main function will need to be declared like so:.

#[no_mangle]
pub extern fn main() -> i32 {
    [...]
}

Make sure before you compile your code that you set export KOS_CC_BASE="/opt/toolchains/dc/gccrs/sh-elf" in your KallistiOS environ.sh file or make will not find your gccrs compiler executable.