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158 lines
5.6 KiB
Markdown
158 lines
5.6 KiB
Markdown
# WIP libgccjit codegen backend for rust
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[](https://web.libera.chat/#rustc_codegen_gcc)
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[](https://matrix.to/#/#rustc_codegen_gcc:matrix.org)
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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.
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**Despite its name, libgccjit can be used for ahead-of-time compilation, as is used here.**
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## Motivation
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The primary goal of this project is to be able to compile Rust code on platforms unsupported by LLVM.
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A secondary goal is to check if using the gcc backend will provide any run-time speed improvement for the programs compiled using rustc.
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## Building
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**This requires a patched libgccjit in order to work.
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You need to use my [fork of gcc](https://github.com/antoyo/gcc) which already includes these patches.**
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```bash
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$ cp config.example.toml config.toml
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```
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If don't need to test GCC patches you wrote in our GCC fork, then the default configuration should
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be all you need. You can update the `rustc_codegen_gcc` without worrying about GCC.
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### Building with your own GCC version
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If you wrote a patch for GCC and want to test it without this backend, you will need
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to do a few more things.
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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):
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```bash
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$ git clone https://github.com/antoyo/gcc
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$ sudo apt install flex libmpfr-dev libgmp-dev libmpc3 libmpc-dev
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$ mkdir gcc-build gcc-install
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$ cd gcc-build
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$ ../gcc/configure \
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--enable-host-shared \
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--enable-languages=jit \
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--enable-checking=release \ # it enables extra checks which allow to find bugs
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--disable-bootstrap \
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--disable-multilib \
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--prefix=$(pwd)/../gcc-install
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$ make -j4 # You can replace `4` with another number depending on how many cores you have.
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```
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If you want to run libgccjit tests, you will need to also enable the C++ language in the `configure`:
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```bash
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--enable-languages=jit,c++
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```
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Then to run libgccjit tests:
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```bash
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$ cd gcc # from the `gcc-build` folder
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$ make check-jit
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# To run one specific test:
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$ make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-asm.cc"
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```
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**Put the path to your custom build of libgccjit in the file `config.toml`.**
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You now need to set the `gcc-path` value in `config.toml` with the result of this command:
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```bash
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$ dirname $(readlink -f `find . -name libgccjit.so`)
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```
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and to comment the `download-gccjit` setting:
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```toml
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gcc-path = "[MY PATH]"
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# download-gccjit = true
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```
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Then you can run commands like this:
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```bash
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$ ./y.sh prepare # download and patch sysroot src and install hyperfine for benchmarking
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$ ./y.sh build --release
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```
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To run the tests:
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```bash
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$ ./y.sh test --release
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```
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## Usage
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`$CG_GCCJIT_DIR` is the directory you cloned this repo into in the following instructions:
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```bash
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export CG_GCCJIT_DIR=[the full path to rustc_codegen_gcc]
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```
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### Cargo
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```bash
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$ CHANNEL="release" $CG_GCCJIT_DIR/y.sh cargo run
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```
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If you compiled cg_gccjit in debug mode (aka you didn't pass `--release` to `./y.sh test`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely.
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### LTO
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To use LTO, you need to set the variable `FAT_LTO=1` and `EMBED_LTO_BITCODE=1` in addition to setting `lto = "fat"` in the `Cargo.toml`.
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Don't set `FAT_LTO` when compiling the sysroot, though: only set `EMBED_LTO_BITCODE=1`.
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Failing to set `EMBED_LTO_BITCODE` will give you the following error:
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```
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error: failed to copy bitcode to object file: No such file or directory (os error 2)
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```
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### Rustc
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> You should prefer using the Cargo method.
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```bash
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$ 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
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```
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## Env vars
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<dl>
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<dt>CG_GCCJIT_INCR_CACHE_DISABLED</dt>
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<dd>Don't cache object files in the incremental cache. Useful during development of cg_gccjit
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to make it possible to use incremental mode for all analyses performed by rustc without caching
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object files when their content should have been changed by a change to cg_gccjit.</dd>
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<dt>CG_GCCJIT_DISPLAY_CG_TIME</dt>
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<dd>Display the time it took to perform codegen for a crate</dd>
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<dt>CG_RUSTFLAGS</dt>
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<dd>Send additional flags to rustc. Can be used to build the sysroot without unwinding by setting `CG_RUSTFLAGS=-Cpanic=abort`.</dd>
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<dt>CG_GCCJIT_DUMP_TO_FILE</dt>
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<dd>Dump a C-like representation to /tmp/gccjit_dumps and enable debug info in order to debug this C-like representation.</dd>
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</dl>
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## Extra documentation
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More specific documentation is available in the [`doc`](./doc) folder:
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* [Common errors](./doc/errors.md)
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* [Debugging GCC LTO](./doc/debugging-gcc-lto.md)
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* [Debugging libgccjit](./doc/debugging-libgccjit.md)
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* [Git subtree sync](./doc/subtree.md)
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* [List of useful commands](./doc/tips.md)
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* [Send a patch to GCC](./doc/sending-gcc-patch.md)
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## Licensing
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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.
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However, programs compiled with `rustc_codegen_gcc` do not need to be released under a GPL license.
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