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+# GitHub Copilot Instructions for Reactant.jl
+
+## Project Overview
+
+Reactant.jl is a Julia package that compiles Julia functions into MLIR (Multi-Level Intermediate Representation) and runs advanced optimizations, including automatic differentiation with EnzymeMLIR, to create executables for CPU/GPU/TPU via XLA. It operates as a tracing system for high-performance computing.
+
+## Language and Code Style
+
+### Julia Code
+- **Language**: Julia 1.10+
+- **Code Style**: Use [Blue style](https://github.com/JuliaDiff/BlueStyle) as enforced by JuliaFormatter
+- **Formatter Config**: See `.JuliaFormatter.toml` with `style = "blue"` and `always_use_return = true`
+- **Type Stability**: Maintain type stability for performance-critical code
+- **Broadcasting**: Prefer Julia's dot notation for array operations (e.g., `sin.(x) .+ y`)
+- **Naming Conventions**: Use descriptive names; types are CamelCase, functions and variables are snake_case
+
+### C++ Code
+- **Style**: LLVM style as specified in `.clang-format`
+- **Location**: C++ code lives in `deps/ReactantExtra/`
+- **Build System**: Bazel is used for building C++ components
+
+## Project Structure
+
+```
+├── src/              # Main Julia source code
+│   ├── Reactant.jl   # Main module file
+│   ├── TracedRArray.jl, ConcreteRArray.jl  # Core array types
+│   ├── Compiler.jl   # Compilation infrastructure
+│   ├── Enzyme.jl     # Automatic differentiation integration
+│   ├── mlir/         # MLIR bindings and utilities
+│   └── xla/          # XLA integration
+├── ext/              # Package extensions (conditional loading)
+├── lib/              # Sub-packages
+│   └── ReactantCore/ # Core functionality
+├── deps/             # Build dependencies and C++ code
+│   └── ReactantExtra/ # C++ API and Bazel build files
+├── test/             # Test suite
+│   ├── runtests.jl   # Main test runner
+│   ├── integration/  # Integration tests
+│   └── nn/           # Neural network tests
+└── docs/             # Documentation
+```
+
+## Core Concepts
+
+### Array Types
+- **ConcreteRArray**: Underlying buffer for device data (CPU/GPU/TPU)
+- **TracedRArray**: Traced version used during compilation (no access to actual values)
+- Conversion: Use `Reactant.to_rarray()` to convert Julia arrays to RArrays
+
+### Compilation
+- Use `@compile` macro to compile functions
+- Compiled functions capture control flow at compile time
+- Only ConcreteRArray updates are captured in compiled code
+
+### Backends
+- Default backend can be set with `Reactant.set_default_backend("cpu"/"gpu"/"tpu")`
+- Supports CPU, GPU (CUDA), and TPU via XLA
+
+## Testing
+
+### Test Structure
+- **Framework**: Uses `SafeTestsets` and Julia's built-in `Test`
+- **Test Groups**: Tests are organized into three groups:
+  - `core`: Basic functionality, tracing, compilation, autodiff
+  - `neural_networks`: NNlib, Flux, LuxLib, Lux integration
+  - `integration`: CUDA, KernelAbstractions, FFT, MPI, etc.
+- **Backend Testing**: Tests can run with different backends (CPU/GPU)
+- **Runtime Testing**: Tests run with both "pjrt" and "ifrt" runtimes
+
+### Running Tests
+```bash
+# Run all tests
+julia --project=. test/runtests.jl
+
+# Run specific test group
+REACTANT_TEST_GROUP=core julia --project=. test/runtests.jl
+
+# Run with GPU backend
+REACTANT_BACKEND_GROUP=gpu julia --project=. test/runtests.jl
+```
+
+### Writing Tests
+- Use `@safetestset` for isolated test environments
+- Follow existing patterns in test files
+- Test both forward and reverse-mode automatic differentiation where applicable
+- Include edge cases and type stability checks
+
+## CI/CD
+
+### Workflows
+- **CI**: Main test suite runs on Julia 1.10, 1.11 on multiple platforms (Ubuntu, macOS, Windows, ARM, TPU)
+- **Format Check**: Enforces Julia code style via JuliaFormatter
+- **Format Check (C++)**: Enforces LLVM style for C++ code via clang-format
+- **Format Check (Bazel)**: Enforces Bazel file formatting with buildifier
+- **Documentation**: Builds with Documenter.jl and DocumenterVitepress
+- **Benchmarks**: Performance tracking on push to main
+
+### Continuous Integration
+- Tests run on push to `main` and `release-*` branches
+- PRs trigger CI on relevant file changes
+- Concurrency controls prevent redundant builds
+
+## Dependencies and Extensions
+
+### Core Dependencies
+- **EnzymeCore/Enzyme**: Automatic differentiation
+- **LLVM.jl**: LLVM integration
+- **Functors.jl**: Recursive structure traversal
+- **Adapt.jl**: Array type adaptation
+
+### Package Extensions
+Reactant uses Julia's package extensions for optional integrations:
+- CUDA, KernelAbstractions for GPU computing
+- NNlib for neural network primitives
+- Zygote for alternative AD (Julia < 1.12)
+- MPI for distributed computing
+- AbstractFFTs, SpecialFunctions, etc.
+
+### Adding Dependencies
+- Update `Project.toml` [deps] for required dependencies
+- Add to [weakdeps] and create extension in `ext/` for optional dependencies
+- Specify version bounds in [compat] section
+
+## Development Workflow
+
+### Setting Up Development Environment
+```bash
+# Clone the repository
+git clone https://github.com/EnzymeAD/Reactant.jl.git
+cd Reactant.jl
+
+# Install dependencies
+julia --project=. -e 'using Pkg; Pkg.instantiate()'
+
+# Build (if needed)
+julia --project=. deps/build_local.jl
+```
+
+### Code Formatting
+```bash
+# Format Julia code
+julia --project=. -e 'using JuliaFormatter; format(".")'
+
+# Format C++ code (requires clang-format)
+find deps/ReactantExtra -name "*.cpp" -o -name "*.h" | xargs clang-format -i
+
+# Format Bazel files (requires buildifier)
+buildifier -r deps/ReactantExtra/
+```
+
+### Documentation
+- Documentation is built with Documenter.jl and DocumenterVitepress
+- Docstrings should follow Julia conventions
+- Run `julia --project=docs docs/make.jl` to build locally
+
+## Best Practices
+
+### Performance
+- Minimize allocations in hot loops
+- Use `@inbounds` carefully when bounds are guaranteed
+- Prefer type-stable code; avoid type unions and `Any`
+- Use `@compile` to generate optimized executables for production code
+
+### Automatic Differentiation
+- Test both forward and reverse mode when adding new operations
+- Ensure proper handling of mutation and aliasing
+- Use EnzymeCore for defining custom derivatives when needed
+
+### MLIR/XLA Integration
+- MLIR operations are in `src/mlir/` and follow Dialect conventions
+- XLA integration code is in `src/xla/`
+- C++ API changes require updating Bazel build files and Julia bindings
+
+### Error Handling
+- Provide informative error messages
+- Use Julia's exception system appropriately
+- Document expected failure modes in docstrings
+
+### Compatibility
+- Maintain compatibility with Julia 1.10+
+- Follow [ColPrac](https://github.com/SciML/ColPrac) guidelines
+- Keep dependencies up-to-date via Dependabot
+
+## Common Tasks
+
+### Adding a New Operation
+1. Implement the Julia function in appropriate file in `src/`
+2. Add tracing support if needed in `src/Tracing.jl`
+3. Add tests in `test/ops.jl` or appropriate test file
+4. Add documentation if it's a public API
+5. Format code with JuliaFormatter
+
+### Adding a New Test
+1. Create test file in `test/` (or appropriate subdirectory)
+2. Add to `test/runtests.jl` with `@safetestset`
+3. Group appropriately (core/integration/neural_networks)
+4. Run tests locally before submitting PR
+
+### Updating MLIR Bindings
+1. Modify `deps/ReactantExtra/API.cpp` if needed
+2. Update Bazel BUILD files
+3. Run regeneration workflow or script
+4. Test thoroughly with existing test suite
+
+## Resources
+
+- [Documentation](https://enzymead.github.io/Reactant.jl/dev)
+- [Issue Tracker](https://github.com/EnzymeAD/Reactant.jl/issues)
+- [Contributing Guide](https://github.com/SciML/ColPrac)
+- [Enzyme Documentation](https://enzyme.mit.edu/)
+- [XLA Documentation](https://www.tensorflow.org/xla)
+
+## Questions?
+
+For questions or clarifications, open an issue or discussion on GitHub. The maintainers actively monitor the repository and are happy to help!