rust/compiler/rustc_codegen_gcc

WIP libgccjit codegen backend for rust

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.

Getting Started

Note: This requires a patched libgccjit in order to work. You need to use my fork of gcc which already includes these patches. The default configuration (see below in the Quick start section) will download a libgccjit built in the CI that already contains these patches, so you don't need to build this fork yourself if you use the default configuration.

Dependencies

• rustup: follow instructions on the official website
• consider to install DejaGnu which is necessary for running the libgccjit test suite. website
• additional packages: flex, libmpfr-dev, libgmp-dev, libmpc3, libmpc-dev

Quick start

1. Clone and configure the repository:

   git clone https://github.com/rust-lang/rustc_codegen_gcc
   cd rustc_codegen_gcc
   cp config.example.toml config.toml
   

2. Build and test:

   ./y.sh prepare  # downloads and patches sysroot
   ./y.sh build --sysroot --release
   
   # Verify setup with a simple test
   ./y.sh cargo build --manifest-path tests/hello-world/Cargo.toml
   
   # Run full test suite (expect ~100 failing UI tests)
   ./y.sh test --release
   

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, so don't hesitate to take a look there if you encounter an issue):

$ git clone https://github.com/rust-lang/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:

--enable-languages=jit,c++

Then to run libgccjit tests:

$ 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:

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

and to comment the download-gccjit setting:

gcc-path = "[MY PATH]"
# download-gccjit = true

Then you can run commands like this:

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

To run the tests:

$ ./y.sh test --release

Usage

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

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

To check if all is working correctly, run:

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

Cargo

$ 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:

$ ./y.sh rustc my_crate.rs

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

$ 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

Environment variables

• 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 folder:

• Common errors
• Debugging GCC LTO
• Debugging libgccjit
• Git subtree sync
• List of useful commands
• Send a patch to GCC

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.