diff options
Diffstat (limited to 'compiler/rustc_codegen_gcc/Readme.md')
| -rw-r--r-- | compiler/rustc_codegen_gcc/Readme.md | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_gcc/Readme.md b/compiler/rustc_codegen_gcc/Readme.md new file mode 100644 index 000000000..fe23a2676 --- /dev/null +++ b/compiler/rustc_codegen_gcc/Readme.md @@ -0,0 +1,147 @@ +# 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. +The patches in [this repository](https://github.com/antoyo/libgccjit-patches) need to be applied. +(Those patches should work when applied on master, but in case it doesn't work, they are known to work when applied on 079c23cfe079f203d5df83fea8e92a60c7d7e878.) +You can also use my [fork of gcc](https://github.com/antoyo/gcc) which already includes these patches.** + +**Put the path to your custom build of libgccjit in the file `gcc_path`.** + +```bash +$ git clone https://github.com/rust-lang/rustc_codegen_gcc.git +$ cd rustc_codegen_gcc +$ git clone https://github.com/llvm/llvm-project llvm --depth 1 --single-branch +$ export RUST_COMPILER_RT_ROOT="$PWD/llvm/compiler-rt" +$ ./prepare_build.sh # download and patch sysroot src +$ ./build.sh --release +``` + +To run the tests: + +```bash +$ ./prepare.sh # download and patch sysroot src and install hyperfine for benchmarking +$ ./test.sh --release +``` + +## Usage + +`$cg_gccjit_dir` is the directory you cloned this repo into in the following instructions. + +### Cargo + +```bash +$ CHANNEL="release" $cg_gccjit_dir/cargo.sh run +``` + +If you compiled cg_gccjit in debug mode (aka you didn't pass `--release` to `./test.sh`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely. + +### Rustc + +> You should prefer using the Cargo method. + +```bash +$ 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.rs +``` + +## Env vars + +<dl> + <dt>CG_GCCJIT_INCR_CACHE_DISABLED</dt> + <dd>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.</dd> + <dt>CG_GCCJIT_DISPLAY_CG_TIME</dt> + <dd>Display the time it took to perform codegen for a crate</dd> +</dl> + +## 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: + +```c +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: + +```c +debug_tree(expr); +``` + +To get the `rustc` command to run in `gdb`, add the `--verbose` flag to `cargo build`. + +### 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-current` in the file `rust-toolchain`. + +### 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](https://book.avr-rust.com/005.1-the-target-specification-json-file.html) (note that the `arch` specified 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`. + * Disable the 128-bit integer types if the target doesn't support them by using `let i128_type = context.new_type::<i64>();` in `context.rs` (same for u128_type). + * 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?). |