rust/Readme.md

# Cranelift codegen backend for rust

The goal of this project is to create an alternative codegen backend for the rust compiler based on [Cranelift](https://github.com/bytecodealliance/wasmtime/blob/main/cranelift).
This has the potential to improve compilation times in debug mode.
If your project doesn't use any of the things listed under "Not yet supported", it should work fine.
If not please open an issue.

## Building and testing

```bash
$ git clone https://github.com/rust-lang/rustc_codegen_cranelift
$ cd rustc_codegen_cranelift
$ ./y.sh prepare
$ ./y.sh build
```

To run the test suite replace the last command with:

```bash
$ ./test.sh
```

For more docs on how to build and test see [build_system/usage.txt](build_system/usage.txt) or the help message of `./y.sh`.

## Precompiled builds

Alternatively you can download a pre built version from the [releases] page.
Extract the `dist` directory in the archive anywhere you want.
If you want to use `cargo clif build` instead of having to specify the full path to the `cargo-clif` executable, you can add the `bin` subdirectory of the extracted `dist` directory to your `PATH`.
(tutorial [for Windows](https://stackoverflow.com/a/44272417), and [for Linux/MacOS](https://unix.stackexchange.com/questions/26047/how-to-correctly-add-a-path-to-path/26059#26059)).

[releases]: https://github.com/rust-lang/rustc_codegen_cranelift/releases/tag/dev

## Usage

rustc_codegen_cranelift can be used as a near-drop-in replacement for `cargo build` or `cargo run` for existing projects.

Assuming `$cg_clif_dir` is the directory you cloned this repo into and you followed the instructions (`y.sh prepare` and `y.sh build` or `test.sh`).

In the directory with your project (where you can do the usual `cargo build`), run:

```bash
$ $cg_clif_dir/dist/cargo-clif build
```

This will build your project with rustc_codegen_cranelift instead of the usual LLVM backend.

For additional ways to use rustc_codegen_cranelift like the JIT mode see [usage.md](docs/usage.md).

## Building and testing with changes in rustc code

This is useful when changing code in `rustc_codegen_cranelift` as part of changing [main Rust repository](https://github.com/rust-lang/rust/).
This can happen, for example, when you are implementing a new compiler intrinsic.

Instruction below uses `$RustCheckoutDir` as substitute for any folder where you cloned Rust repository.

You need to do this steps to successfully compile and use the cranelift backend with your changes in rustc code:

1. `cd $RustCheckoutDir`
2. Run `python x.py setup` and choose option for compiler (`b`).
3. Build compiler and necessary tools: `python x.py build --stage=2 compiler library/std src/tools/rustdoc src/tools/rustfmt`
   * (Optional) You can also build cargo by adding `src/tools/cargo` to previous command.
4. Copy cargo from a nightly toolchain: `cp $(rustup +nightly which cargo) ./build/host/stage2/bin/cargo`. Note that you would need to do this every time you rebuilt `rust` repository.
5. Link your new `rustc` to toolchain: `rustup toolchain link stage2 ./build/host/stage2/`.
6. (Windows only) compile the build system: `rustc +stage2 -O build_system/main.rs -o y.exe`.
7. You need to prefix every `./y.sh` (or `y` if you built `build_system/main.rs` as `y`) command by `rustup run stage2` to make cg_clif use your local changes in rustc.
  * `rustup run stage2 ./y.sh prepare`
  * `rustup run stage2 ./y.sh build`
  * (Optional) run tests: `rustup run stage2 ./y.sh test`
8. Now you can use your cg_clif build to compile other Rust programs, e.g. you can open any Rust crate and run commands like `$RustCheckoutDir/compiler/rustc_codegen_cranelift/dist/cargo-clif build --release`.

## Configuration

See the documentation on the `BackendConfig` struct in [config.rs](src/config.rs) for all
configuration options.

## Not yet supported

* Inline assembly ([no cranelift support](https://github.com/bytecodealliance/wasmtime/issues/1041))
    * On UNIX there is support for invoking an external assembler for `global_asm!` and `asm!`.
* SIMD ([tracked here](https://github.com/rust-lang/rustc_codegen_cranelift/issues/171), `std::simd` fully works, `std::arch` is partially supported)
* Unwinding on panics ([no cranelift support](https://github.com/bytecodealliance/wasmtime/issues/1677), `-Cpanic=abort` is enabled by default)

## License

Licensed under either of

  * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
    http://www.apache.org/licenses/LICENSE-2.0)
  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
    http://opensource.org/licenses/MIT)

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you shall be dual licensed as above, without any
additional terms or conditions.