148 lines
6.0 KiB
Markdown
148 lines
6.0 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 repository](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/rust-lang/rustc_codegen_gcc.git
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$ cd rustc_codegen_gcc
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$ git clone https://github.com/llvm/llvm-project llvm --depth 1 --single-branch
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$ export RUST_COMPILER_RT_ROOT="$PWD/llvm/compiler-rt"
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$ ./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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p loc->m_filename->m_buffer
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```
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To print a debug representation of a tree:
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```c
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debug_tree(expr);
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```
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To get the `rustc` command to run in `gdb`, add the `--verbose` flag to `cargo build`.
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### How to use a custom-build rustc
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* Build the stage2 compiler (`rustup toolchain link debug-current build/x86_64-unknown-linux-gnu/stage2`).
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* Clean and rebuild the codegen with `debug-current` in the file `rust-toolchain`.
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### How to build a cross-compiling libgccjit
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#### Building libgccjit
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* Follow these instructions: https://preshing.com/20141119/how-to-build-a-gcc-cross-compiler/ with the following changes:
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* 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`.
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* Some shells, like fish, don't define the environment variable `$MACHTYPE`.
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* 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).
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#### Configuring rustc_codegen_gcc
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* Set `TARGET_TRIPLE="m68k-unknown-linux-gnu"` in config.sh.
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* 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).
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* Set `linker='-Clinker=m68k-linux-gcc'`.
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* Set the path to the cross-compiling libgccjit in `gcc_path`.
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* 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).
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* Comment the line: `context.add_command_line_option("-masm=intel");` in src/base.rs.
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* (might not be necessary) Disable the compilation of libstd.so (and possibly libcore.so?).
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