user0/rust

- Readme.md: add missing "you" ("If don't" → "If you don't")
- Readme.md: fix wrong preposition ("without this backend" → "with this backend")
- Readme.md: fix double space
- doc/errors.md: fix transposed letters ("libgccijt" → "libgccjit")
- doc/debugging.md: remove extra word ("Run do the command" → "Run the command")
- doc/debugging.md: fix past participle ("cannot be ran" → "cannot be run")
- doc/tips.md: add missing verb ("won't a chance" → "won't have a chance")
- doc/gimple.md: fix preposition ("interested into" → "interested in")

2026-02-11 17:38:07 -05:00

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Tips

The following shows how to do different random small things we encountered and thought could be useful.

How to send arguments to the GCC linker

CG_RUSTFLAGS="-Clink-args=-save-temps -v" ../y.sh cargo build

How to send arguments to GCC

The -Cllvm-args rustc flag is repurposed by rustc_codegen_gcc to pass arguments directly to the GCC backend. You can use it via the CG_RUSTFLAGS environment variable. For example, to pass a -f flag to GCC:

CG_RUSTFLAGS="-Cllvm-args=-fflag-name" ../y.sh cargo build

How to see the personality functions in the asm dump

CG_RUSTFLAGS="-Clink-arg=-save-temps -v -Clink-arg=-dA" ../y.sh cargo build

How to see the LLVM IR for a sysroot crate

cargo build -v --target x86_64-unknown-linux-gnu -Zbuild-std
# Take the command from the output and add --emit=llvm-ir

To prevent the linker from unmangling symbols

Run with:

COLLECT_NO_DEMANGLE=1

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 use a custom sysroot source path

If you wish to build a custom sysroot, pass the path of your sysroot source to --sysroot-source during the prepare step, like so:

./y.sh prepare --sysroot-source /path/to/custom/source

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 have 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 the instructions on this repo.

Configuring rustc_codegen_gcc

Run ./y.sh prepare --cross so that the sysroot is patched for the cross-compiling case.
Set the path to the cross-compiling libgccjit in gcc-path (in config.toml).
Make sure you have the linker for your target (for instance m68k-unknown-linux-gnu-gcc) in your $PATH. You can specify which linker to use via CG_RUSTFLAGS="-Clinker=<linker>", for instance: CG_RUSTFLAGS="-Clinker=m68k-unknown-linux-gnu-gcc". Specify the target when building the sysroot: ./y.sh build --sysroot --target-triple m68k-unknown-linux-gnu.
Build your project by specifying the target and the linker to use: CG_RUSTFLAGS="-Clinker=m68k-unknown-linux-gnu-gcc" ../y.sh cargo build --target m68k-unknown-linux-gnu.

If the target is not yet supported by the Rust compiler, create a target specification file (note that the arch specified in this file must be supported by the rust compiler). Then, you can use it the following way:

Add the target specification file using --target as an absolute path to build the sysroot: ./y.sh build --sysroot --target-triple m68k-unknown-linux-gnu --target $(pwd)/m68k-unknown-linux-gnu.json
Build your project by specifying the target specification file: ../y.sh cargo build --target path/to/m68k-unknown-linux-gnu.json.

If you get the following error:

/usr/bin/ld: unrecognised emulation mode: m68kelf

Make sure you set gcc-path (in config.toml) to the install directory.

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