During the last week I spent numerous hours on porting my favorite programming language Rust to operate on my favorite operating system DragonFly. While my first attempt months earlier failed, this time I was successful! In the following article I go through all the problems I had to deal with and how you can build Rust for DragonFly yourself. This might also be of interest to people who want to port Rust to further platforms like NetBSD or OpenBSD.
In short what needs to be done to port Rust to a new platform is the following:
rustccompiler to generate code for the platform you are porting to.
rustcdepends on LLVM this might require some patching here too, to be able to generate code for the target operating system (in my case segmented stacks were unsupported for DragonFly).
As Rust (still) requires segmented stacks there needs to be support for it in some way in the operating system. All Rust and LLVM need is a thread local field where they can store the size of the stack.
The Rust libraries (e.g. liballoc, liblibc, libstd, libnative) need to be ported. This basically involves adding some
#[cfg(target_os = "dragonfly")]somewhere, but also some system structures need to be adapted.
Once this is done we are ready to cross-compile Rust to DragonFly.
After trying to cross-compile Rust by specifying
--target x86_64-pc-dragonfly-elf to Rust's own
configure script and spending numerous hours just to note that the build fails, I gave up on this idea. Instead I took a different approach as shown below. Note that this all requires my dragonfly branch of rust. All stages depend sequentially on another. Below we are using the scripts from my rust-cross-dragonfly project.
Stage 1 (Linux, DragonFly)
On Linux: Compile a modified
rustcon Linux that supports DragonFly as target (rustc natively is able to generate code for different targets!).
On DragonFly: At the same time we can build the C libraries Rust depends on.
Actually I tried to cross-compile everything on Linux, but I failed for LLVM. So I decided to build everything that involved compiling C or C++ on the target (DragonFly) itself.
Stage 2 (Linux)
We have to copy the outcome of stage1-dragonfly (which is stage1-dragonfly.tgz) to the Linux box and extract it to create
stage1-dragonfly/. This includes the C libraries required for Rust as well as some system libraries.
sh stage2-linux.sh) cross-compiles all of Rust's libraries (e.g.
librustc) and generates the object file for the
rustc binary (
driver.o). I don't generate the executable here itself, which I could, but when I did, static initializations were omitted and the generated
rustc was unable to find statically registered command line options of LLVM. I spent hours to figure this out. The solution was to just generate the object file for
rustc and together with the Rust libraries pass it on to Stage3, which links the
rustc binary on DragonFly.
Copy the outcome of Stage2 (
stage2-linux.tgz) back to the DragonFly system.
Stage 3 (DragonFly)
Finally back again on DragonFly. Extract
stage2-linux.tgz to become
stage2-linux. In this stage we will construct a working
rustc binary (the Rust compiler) and test to compile a simple Hello World application
hw.rs natively on DragonFly.
All you have to do is to execute
Stage 4 (DragonFly)
Stage4 is the last stage and it builds my dragonfly branch with the compiler from Stage3. It uses Rust's own build infrastructure to do so and not my "hacked" build infrastructure.
All you have to do is to execute
sh stage4-dragonfly.sh and wait. It does not automatically install Rust for you, so you have to
gmake install inside directory
stage4-dragonfly/rust yourself (needing admin privileges).
LLVM does not support segmented stacks on DragonFly. So I had to patch LLVM and will submit the patch upstream soon.
The LLVM patch implies having a special field in the thread control block to record the stack size. Thanks to Matthew Dillon I was "allowed" to use a yet unused field for it and the patch got accepted.
Rust fails with a memory corruption when jemalloc is not used, see my bug report #16071. I noticed that earlier on Linux but ignored it. Later when I had a
rustccompiler working on DragonFly I got hit by this again as I built it without jemalloc. It cost me a whole night ktracing it and several reboots. The solution was to compile with jemalloc.
Intially the port of
liblibcwere basically copies of the related FreeBSD code. Several times I got hit by differences in system structures and return codes. The right solution would be to generate code like this in an automated fashion.
Cross-linking, that is linking an executable targeted for DragonFly on Linux, has some limitations, or more probable is that I did something wrong. Static initializations are handled incorrectly (they are not executed). It took me a while to figure this out after searching through the LLVM code.
Last but not least, compiling Rust takes an enormous amount of time. That is, the turn-around times are quite high. Compiling, fixing an error, compiling again. And so on and on.
Rust works on DragonFly, yippey! I work on uploading a snapshot and maybe setting up a regular build and add it to dports. Hopefully this document also helps others in porting Rust to further platforms, mainly OpenBSD and NetBSD.