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# WIP libgccjit codegen backend for rust
[](https://web.libera.chat/#rustc_codegen_gcc)
[](https://matrix.to/#/#rustc_codegen_gcc:matrix.org)
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.
|
