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 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. Let's add them to our environment's PATH now:

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

You may also want to add the above line to your shell's startup file or else you'll need to run it 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

Copy the architecture configuration file, sh-elf.json, to its place:

cp /opt/toolchains/dc/rust/misc/sh-elf.json /opt/toolchains/dc/rust/rustc_codegen_gcc/sh-elf.json

• patch out duplicate symbols in src/context.rs
• add SH4 precision mode to src/base.rs
• patch build_system/src/config.rs to alter default linker call
• patch build_sysroot/Cargo.toml to remove std and test from being pulled in
• y.sh clean
• y.sh prepare --cross
• y.sh 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 Rust compiler frontend for GCC. This essentially means implementing a 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, nor is borrow checking implemented. It is possible to use this compiler by compiling either GCC 14.0.1-dev or GCCRS git repo.