119 lines
4.2 KiB
Markdown
119 lines
4.2 KiB
Markdown
# WIP libgccjit codegen backend for rust
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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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The patches in [this repostory](https://github.com/antoyo/libgccjit-patches) need to be applied.
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(Those patches should work when applied on master, but in case it doesn't work, they are known to work when applied on 079c23cfe079f203d5df83fea8e92a60c7d7e878.)
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You can also use my [fork of gcc](https://github.com/antoyo/gcc) which already includes these patches.**
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**Put the path to your custom build of libgccjit in the file `gcc_path`.**
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```bash
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$ git clone https://github.com/antoyo/rustc_codegen_gcc.git
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$ cd rustc_codegen_gcc
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$ ./prepare_build.sh # download and patch sysroot src
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$ ./build.sh --release
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```
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To run the tests:
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```bash
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$ ./prepare.sh # download and patch sysroot src and install hyperfine for benchmarking
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$ ./test.sh --release
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```
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## Usage
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`$cg_gccjit_dir` is the directory you cloned this repo into in the following instructions.
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### Cargo
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```bash
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$ CHANNEL="release" $cg_gccjit_dir/cargo.sh run
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```
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If you compiled cg_gccjit in debug mode (aka you didn't pass `--release` to `./test.sh`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely.
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### Rustc
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> You should prefer using the Cargo method.
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```bash
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$ 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
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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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</dl>
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## Debugging
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Sometimes, libgccjit will crash and output an error like this:
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```
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during RTL pass: expand
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libgccjit.so: error: in expmed_mode_index, at expmed.h:249
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0x7f0da2e61a35 expmed_mode_index
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../../../gcc/gcc/expmed.h:249
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0x7f0da2e61aa4 expmed_op_cost_ptr
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../../../gcc/gcc/expmed.h:271
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0x7f0da2e620dc sdiv_cost_ptr
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../../../gcc/gcc/expmed.h:540
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0x7f0da2e62129 sdiv_cost
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../../../gcc/gcc/expmed.h:558
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0x7f0da2e73c12 expand_divmod(int, tree_code, machine_mode, rtx_def*, rtx_def*, rtx_def*, int)
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../../../gcc/gcc/expmed.c:4335
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0x7f0da2ea1423 expand_expr_real_2(separate_ops*, rtx_def*, machine_mode, expand_modifier)
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../../../gcc/gcc/expr.c:9240
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0x7f0da2cd1a1e expand_gimple_stmt_1
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../../../gcc/gcc/cfgexpand.c:3796
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0x7f0da2cd1c30 expand_gimple_stmt
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../../../gcc/gcc/cfgexpand.c:3857
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0x7f0da2cd90a9 expand_gimple_basic_block
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../../../gcc/gcc/cfgexpand.c:5898
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0x7f0da2cdade8 execute
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../../../gcc/gcc/cfgexpand.c:6582
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```
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To see the code which causes this error, call the following function:
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```c
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gcc_jit_context_dump_to_file(ctxt, "/tmp/output.c", 1 /* update_locations */)
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```
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This will create a C-like file and add the locations into the IR pointing to this C file.
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Then, rerun the program and it will output the location in the second line:
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```
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libgccjit.so: /tmp/something.c:61322:0: error: in expmed_mode_index, at expmed.h:249
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```
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Or add a breakpoint to `add_error` in gdb and print the line number using:
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```
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p loc->m_line
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```
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### How to use a custom-build rustc
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* Build the stage1 compiler (`rustup toolchain link debug-current stage2 build/x86_64-unknown-linux-gnu/stage1`).
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* Clean and rebuild the codegen with `debug-current` in the file `rust-toolchain`.
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* Add `~/.rustup/toolchains/debug-current/lib/rustlib/x86_64-unknown-linux-gnu/lib` to `LD_LIBRARY_PATH`.
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