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https://github.com/rust-lang/rustc_codegen_gcc

libgccjit AOT codegen for rustc
https://github.com/rust-lang/rustc_codegen_gcc

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libgccjit AOT codegen for rustc

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# WIP libgccjit codegen backend for rust



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This is a GCC codegen for rustc, which means it can be loaded by the existing rustc frontend, but benefits from GCC: more architectures are supported and GCC's optimizations are used.



**Despite its name, libgccjit can be used for ahead-of-time compilation, as is used here.**



## Motivation



The primary goal of this project is to be able to compile Rust code on platforms unsupported by LLVM.

A secondary goal is to check if using the gcc backend will provide any run-time speed improvement for the programs compiled using rustc.



### Dependencies



**rustup:** Follow the instructions on the official [website](https://www.rust-lang.org/tools/install)



**DejaGnu:** Consider to install DejaGnu which is necessary for running the libgccjit test suite. [website](https://www.gnu.org/software/dejagnu/#downloading)



## Building



**This requires a patched libgccjit in order to work.

You need to use my [fork of gcc](https://github.com/antoyo/gcc) which already includes these patches.**



```bash

$ cp config.example.toml config.toml

```



If don't need to test GCC patches you wrote in our GCC fork, then the default configuration should

be all you need. You can update the `rustc_codegen_gcc` without worrying about GCC.



### Building with your own GCC version



If you wrote a patch for GCC and want to test it without this backend, you will need

to do a few more things.



To build it (most of these instructions come from [here](https://gcc.gnu.org/onlinedocs/jit/internals/index.html), so don't hesitate to take a look there if you encounter an issue):



```bash

$ git clone https://github.com/antoyo/gcc

$ sudo apt install flex libmpfr-dev libgmp-dev libmpc3 libmpc-dev

$ mkdir gcc-build gcc-install

$ cd gcc-build

$ ../gcc/configure \

    --enable-host-shared \

    --enable-languages=jit \

    --enable-checking=release \ # it enables extra checks which allow to find bugs

    --disable-bootstrap \

    --disable-multilib \

    --prefix=$(pwd)/../gcc-install

$ make -j4 # You can replace `4` with another number depending on how many cores you have.

```



If you want to run libgccjit tests, you will need to also enable the C++ language in the `configure`:



```bash

--enable-languages=jit,c++

```



Then to run libgccjit tests:



```bash

$ cd gcc # from the `gcc-build` folder

$ make check-jit

# To run one specific test:

$ make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-asm.cc"

```



**Put the path to your custom build of libgccjit in the file `config.toml`.**



You now need to set the `gcc-path` value in `config.toml` with the result of this command:



```bash

$ dirname $(readlink -f `find . -name libgccjit.so`)

```



and to comment the `download-gccjit` setting:



```toml

gcc-path = "[MY PATH]"

# download-gccjit = true

```



Then you can run commands like this:



```bash

$ ./y.sh prepare # download and patch sysroot src and install hyperfine for benchmarking

$ ./y.sh build --sysroot --release

```



To run the tests:



```bash

$ ./y.sh test --release

```



## Usage



You have to run these commands, in the corresponding order:



```bash

$ ./y.sh prepare

$ ./y.sh build --sysroot

```

To check if all is  working correctly, run:



 ```bash

$ ./y.sh cargo build --manifest-path tests/hello-world/Cargo.toml

```



### Cargo



```bash

$ CHANNEL="release" $CG_GCCJIT_DIR/y.sh cargo run

```



If you compiled cg_gccjit in debug mode (aka you didn't pass `--release` to `./y.sh test`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely.



### LTO



To use LTO, you need to set the variable `EMBED_LTO_BITCODE=1` in addition to setting `lto = "fat"` in the `Cargo.toml`.



Failing to set `EMBED_LTO_BITCODE` will give you the following error:



```

error: failed to copy bitcode to object file: No such file or directory (os error 2)

```



### Rustc



If you want to run `rustc` directly, you can do so with:



```bash

$ ./y.sh rustc my_crate.rs

```



You can do the same manually (although we don't recommend it):



```bash

$ LIBRARY_PATH="[gcc-path value]" LD_LIBRARY_PATH="[gcc-path value]" rustc +$(cat $CG_GCCJIT_DIR/rust-toolchain | grep 'channel' | cut -d '=' -f 2 | sed 's/"//g' | sed 's/ //g') -Cpanic=abort -Zcodegen-backend=$CG_GCCJIT_DIR/target/release/librustc_codegen_gcc.so --sysroot $CG_GCCJIT_DIR/build_sysroot/sysroot my_crate.rs

```



## Env vars



 * _**CG_GCCJIT_DUMP_ALL_MODULES**_: Enables dumping of all compilation modules. When set to "1", a dump is created for each module during compilation and stored in `/tmp/reproducers/`.

 * _**CG_GCCJIT_DUMP_MODULE**_: Enables dumping of a specific module. When set with the module name, e.g., `CG_GCCJIT_DUMP_MODULE=module_name`, a dump of that specific module is created in `/tmp/reproducers/`.

 * _**CG_RUSTFLAGS**_: Send additional flags to rustc. Can be used to build the sysroot without unwinding by setting `CG_RUSTFLAGS=-Cpanic=abort`.

 * _**CG_GCCJIT_DUMP_TO_FILE**_: Dump a C-like representation to /tmp/gccjit_dumps and enable debug info in order to debug this C-like representation.

 * _**CG_GCCJIT_DUMP_RTL**_: Dumps RTL (Register Transfer Language) for virtual registers.

 * _**CG_GCCJIT_DUMP_RTL_ALL**_: Dumps all RTL passes.

 * _**CG_GCCJIT_DUMP_TREE_ALL**_: Dumps all tree (GIMPLE) passes.

 * _**CG_GCCJIT_DUMP_IPA_ALL**_: Dumps all Interprocedural Analysis (IPA) passes.

 * _**CG_GCCJIT_DUMP_CODE**_: Dumps the final generated code.

 * _**CG_GCCJIT_DUMP_GIMPLE**_: Dumps the initial GIMPLE representation.

 * _**CG_GCCJIT_DUMP_EVERYTHING**_: Enables dumping of all intermediate representations and passes.

 * _**CG_GCCJIT_KEEP_INTERMEDIATES**_: Keeps intermediate files generated during the compilation process.

 * _**CG_GCCJIT_VERBOSE**_: Enables verbose output from the GCC driver.



## Extra documentation



More specific documentation is available in the [`doc`](./doc) folder:



 * [Common errors](./doc/errors.md)

 * [Debugging GCC LTO](./doc/debugging-gcc-lto.md)

 * [Debugging libgccjit](./doc/debugging-libgccjit.md)

 * [Git subtree sync](./doc/subtree.md)

 * [List of useful commands](./doc/tips.md)

 * [Send a patch to GCC](./doc/sending-gcc-patch.md)



## Licensing



While this crate is licensed under a dual Apache/MIT license, it links to `libgccjit` which is under the GPLv3+ and thus, the resulting toolchain (rustc + GCC codegen) will need to be released under the GPL license.



However, programs compiled with `rustc_codegen_gcc` do not need to be released under a GPL license.