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.
Building
This requires a patched libgccjit in order to work. You need to use my fork of gcc which already includes these patches.
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/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:
--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 gcc_path.
$ dirname $(readlink -f `find . -name libgccjit.so`) > gcc_pathYou also need to set RUST_COMPILER_RT_ROOT:
$ git clone https://github.com/llvm/llvm-project llvm --depth 1 --single-branch
$ export RUST_COMPILER_RT_ROOT="$PWD/llvm/compiler-rt"Then you can run commands like this:
$ ./prepare.sh # download and patch sysroot src and install hyperfine for benchmarking
$ LIBRARY_PATH=$(cat gcc_path) LD_LIBRARY_PATH=$(cat gcc_path) ./build.sh --releaseTo run the tests:
$ ./test.sh --releaseUsage
$cg_gccjit_dir is the directory you cloned this repo into in the following instructions.
Cargo
$ CHANNEL="release" $cg_gccjit_dir/cargo.sh runIf you compiled cg_gccjit in debug mode (aka you didn't pass --release to ./test.sh) you should use CHANNEL="debug" instead or omit CHANNEL="release" completely.
Rustc
You should prefer using the Cargo method.
$ rustc +$(cat $cg_gccjit_dir/rust-toolchain) -Cpanic=abort -Zcodegen-backend=$cg_gccjit_dir/target/release/librustc_codegen_gcc.so --sysroot $cg_gccjit_dir/build_sysroot/sysroot my_crate.rsEnv vars
- CG_GCCJIT_INCR_CACHE_DISABLED
- Don't cache object files in the incremental cache. Useful during development of cg_gccjit to make it possible to use incremental mode for all analyses performed by rustc without caching object files when their content should have been changed by a change to cg_gccjit.
- CG_GCCJIT_DISPLAY_CG_TIME
- Display the time it took to perform codegen for a crate
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.
Debugging
Sometimes, libgccjit will crash and output an error like this:
during RTL pass: expand
libgccjit.so: error: in expmed_mode_index, at expmed.h:249
0x7f0da2e61a35 expmed_mode_index
../../../gcc/gcc/expmed.h:249
0x7f0da2e61aa4 expmed_op_cost_ptr
../../../gcc/gcc/expmed.h:271
0x7f0da2e620dc sdiv_cost_ptr
../../../gcc/gcc/expmed.h:540
0x7f0da2e62129 sdiv_cost
../../../gcc/gcc/expmed.h:558
0x7f0da2e73c12 expand_divmod(int, tree_code, machine_mode, rtx_def*, rtx_def*, rtx_def*, int)
../../../gcc/gcc/expmed.c:4335
0x7f0da2ea1423 expand_expr_real_2(separate_ops*, rtx_def*, machine_mode, expand_modifier)
../../../gcc/gcc/expr.c:9240
0x7f0da2cd1a1e expand_gimple_stmt_1
../../../gcc/gcc/cfgexpand.c:3796
0x7f0da2cd1c30 expand_gimple_stmt
../../../gcc/gcc/cfgexpand.c:3857
0x7f0da2cd90a9 expand_gimple_basic_block
../../../gcc/gcc/cfgexpand.c:5898
0x7f0da2cdade8 execute
../../../gcc/gcc/cfgexpand.c:6582
To see the code which causes this error, call the following function:
gcc_jit_context_dump_to_file(ctxt, "/tmp/output.c", 1 /* update_locations */)This will create a C-like file and add the locations into the IR pointing to this C file. Then, rerun the program and it will output the location in the second line:
libgccjit.so: /tmp/something.c:61322:0: error: in expmed_mode_index, at expmed.h:249
Or add a breakpoint to add_error in gdb and print the line number using:
p loc->m_line
p loc->m_filename->m_buffer
To print a debug representation of a tree:
debug_tree(expr);(defined in print-tree.h)
To print a debug reprensentation of a gimple struct:
debug_gimple_stmt(gimple_struct)To get the rustc command to run in gdb, add the --verbose flag to cargo build.
To have the correct file paths in gdb instead of /usr/src/debug/gcc/libstdc++-v3/libsupc++/eh_personality.cc:
Maybe by calling the following at the beginning of gdb:
set substitute-path /usr/src/debug/gcc /path/to/gcc-repo/gcc
TODO(antoyo): but that's not what I remember I was doing.
How to use a custom-build rustc
- Build the stage2 compiler (
rustup toolchain link debug-current build/x86_64-unknown-linux-gnu/stage2). - Clean and rebuild the codegen with
debug-currentin the filerust-toolchain.
How to install a forked git-subtree
Using git-subtree with rustc requires a patched git to make it work.
The PR that is needed is here.
Use the following instructions to install it:
git clone [email protected]:tqc/git.git
cd git
git checkout tqc/subtree
make
make install
cd contrib/subtree
make
cp git-subtree ~/binThen, do a sync with this command:
PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
cd ../rustc_codegen_gcc
git checkout master
git pull
git checkout sync_branch_name
git merge masterTODO: write a script that does the above.
How to use mem-trace
rustc needs to be built without jemalloc so that mem-trace can overload malloc since jemalloc is linked statically, so a LD_PRELOAD-ed library won't a chance to intercept the calls to malloc.
How to generate GIMPLE
If you need to check what gccjit is generating (GIMPLE), then take a look at how to generate it in gimple.md.
How to build a cross-compiling libgccjit
Building libgccjit
- Follow these instructions: https://preshing.com/20141119/how-to-build-a-gcc-cross-compiler/ with the following changes:
- Configure gcc with
../gcc/configure --enable-host-shared --disable-multilib --enable-languages=c,jit,c++ --disable-bootstrap --enable-checking=release --prefix=/opt/m68k-gcc/ --target=m68k-linux --without-headers. - Some shells, like fish, don't define the environment variable
$MACHTYPE. - Add
CFLAGS="-Wno-error=attributes -g -O2"at the end of the configure command for building glibc (CFLAGS="-Wno-error=attributes -Wno-error=array-parameter -Wno-error=stringop-overflow -Wno-error=array-bounds -g -O2"for glibc 2.31, which is useful for Debian).
Configuring rustc_codegen_gcc
- Set
TARGET_TRIPLE="m68k-unknown-linux-gnu"in config.sh. - Since rustc doesn't support this architecture yet, set it back to
TARGET_TRIPLE="mips-unknown-linux-gnu"(or another target having the same attributes). Alternatively, create a target specification file (note that thearchspecified in this file must be supported by the rust compiler). - Set
linker='-Clinker=m68k-linux-gcc'. - Set the path to the cross-compiling libgccjit in
gcc_path. - Comment the line:
context.add_command_line_option("-masm=intel");in src/base.rs. - (might not be necessary) Disable the compilation of libstd.so (and possibly libcore.so?).