Contributing to rustc_codegen_gcc

Welcome to the rustc_codegen_gcc project! This guide will help you get started as a contributor. The project aims to provide a GCC codegen backend for rustc, allowing Rust compilation on platforms unsupported by LLVM and potentially improving runtime performance through GCC's optimizations.

Getting Started

Setting Up Your Development Environment

For detailed setup instructions including dependencies, build steps, and initial testing, please refer to our README. The README contains the most up-to-date information on:

• Required dependencies and system packages
• Repository setup and configuration
• Build process
• Basic test verification

Once you've completed the setup process outlined in the README, you can proceed with the contributor-specific information below.

Communication Channels

• Matrix: Join our Matrix channel
• IRC: Join us on IRC
• GitHub Issues: For bug reports and feature discussions

We encourage new contributors to join our communication channels and introduce themselves. Feel free to ask questions about where to start or discuss potential contributions.

Understanding Core Concepts

Sysroot & compilation flags

What is the sysroot?

The sysroot is the directory that stores the compiled standard library (core, alloc, std, test, …) and compiler built-ins. Rustup ships these libraries pre-compiled with LLVM.

rustc_codegen_gcc replaces LLVM with the GCC backend.

The freshly compiled sysroot ends up in build/build_sysroot/....

A rebuild of sysroot is needed when

• the backend changes in a way that affects code generation, or
• the user switches toolchains / updates submodules.

Both backend and sysroot can be built using different profiles. That is exactly what the --sysroot, --release-sysroot and --release flag supported by the build system script y.sh take care of.

Typical flag combinations

Command	Backend Profile	Sysroot Profile	Usage Scenario
./y.sh build	dev*	n/a	Build backend in dev mode with optimized dependencies without rebuilding sysroot
./y.sh build --release	release (optimized)	n/a	Build backend in release mode with optimized dependencies without rebuilding sysroot
./y.sh build --release --sysroot	release (optimized)	dev	Build backend in release mode with optimized dependencies and sysroot in dev mode (unoptimized)
./y.sh build --sysroot	dev*	dev	Build backend in dev mode with optimized dependencies and sysroot in dev mode (unoptimized)
./y.sh build --release-sysroot --sysroot	dev*	release (optimized)	Build backend in dev mode and sysroot in release mode, both with optimized dependencies

* In dev mode, dependencies are compiled with optimizations, while the code of the backend itself is not.

Note: --release-sysroot must be used together with --sysroot.

Common Development Tasks

Running Specific Tests

To run specific tests, use appropriate flags such as:

• ./y.sh test --test-libcore
• ./y.sh test --std-tests
• ./y.sh test --cargo-tests -- <name of test>

Additionally, you can run the tests of libgccjit:

# libgccjit tests
cd gcc-build/gcc
make check-jit
# For a specific test:
make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-asm.cc"

Debugging Tools

The project provides several environment variables for debugging:

• CG_GCCJIT_DUMP_GIMPLE: Dumps the GIMPLE IR
• CG_RUSTFLAGS: Additional Rust flags
• CG_GCCJIT_DUMP_MODULE: Dumps a specific module
• CG_GCCJIT_DUMP_TO_FILE: Creates C-like representation

Full list of debugging options can be found in the README.

Making Contributions

Finding Issues to Work On

1. Look for issues labeled with good first issue or help wanted
2. Check the progress report for larger initiatives
3. Consider improving documentation or investigating failing tests (except failing-ui-tests12.txt)

Pull Request Process

1. Fork the repository and create a new branch
2. Make your changes with clear commit messages
3. Add tests for new functionality
4. Update documentation as needed
5. Submit a PR with a description of your changes

Code Style Guidelines

• Follow Rust standard coding conventions
• Ensure your code passes rustfmt and clippy
• Add comments explaining complex logic, especially in GCC interface code

Additional Resources

• Rustc Dev Guide
• GCC Internals Documentation
• Project-specific documentation in the doc/ directory:
  • Common errors
  • Debugging
  • Debugging libgccjit
  • Git subtree sync
  • List of useful commands
  • Send a patch to GCC

Getting Help

If you're stuck or unsure about anything:

1. Check the existing documentation in the doc/ directory
2. Ask in the IRC or Matrix channels
3. Open a GitHub issue for technical problems
4. Comment on the issue you're working on if you need guidance

Remember that all contributions, including documentation improvements, bug reports, and feature requests, are valuable to the project.