DEC Alpha support

[FractalFir]

This is an issue tracking the work on cross-compiling the Rust compiler to DEC Alpha.

Here is a list of things that need to be done to facilitate compiling any Rust code for DEC Alpha:

• Add bare-bones support for DEC Alpha in the object crate. This crate is used by Rust to embed metadata in executables.
• Done, PR waiting for merge Add support for the alpha architecture in cc-rs
• Create an Alpha target file - I have created a target file myself, but I would prefer to double-check if that target is correct.
• Fix the libgccjit !gdisp!0 error. For now, I workaround this issue by removing this erronious label from all assembled files. We should figure out why this is even generated.
• Fix the instruction jsr $23,($27),__%E3%j5 getting emited in compiler_builtins. From asking about, it seems like % can't be present in symbol names: it is a format specifier(like in printf). So, it seems like it just does not get expanded properly? This function should normally expand to a call to a division subroutine(DEC alpha lacks hardware division support).

Here is a list of compiler-side changes we will need to make to compile any code to Alpha. Those changes should be merged upstream.

• Add handling of the Alpha ABI to the Rust compiler, here.
• Add Alpha to the code inserting metadata into object files, here.
• Add Alpha as a recognized architecture to be used in #[cfg(target_arch = "alpha")]. This is not strictly needed for our work, but it is a required step before getting anything else upstream.

From some talks with the Rust folk, it seems like we can do those steps without the need for an official target. So, we will be able to just use our own target files, and add a tier 3 target once we have better support.

After those changes, we will be able to build code not depending on std.

• (Optional) We should also add the config for the x86_64 -> Alpha cross compiler to our configs. This will just make cross-compilation slightly easier for other people(creating those config files is not hard, but having prepared configs would not hurt).

Next, I will list the changes needed to build on std, and code depending on the standard library for Alpha.

• Add Alpha support to libc(a Rust library desciribing C types on the target). Doing so would require some work(tracking down the size / layout of all the libc types on Alpha). That work should get merged upstream. We may need to cordinate with the libc crate refactor project, to ensure our work does not collide.

• (Optional) Add Alpha unwinding support to std.

At this point, we should be able to build std, and at least a chunk of crates depending on it. There might be some issues with crates which build C dependencies, but we should be nontheless able to at least try cross-compling some Rust packages to Alpha. We would not be able to run the Rust compiler on Alpha yet, but we would be able to run a chunk of Rust code there.

I belive we should then cross-compile the Rust core, std and alloc test suite to Alpha, to check if everything is working correctly.

I don't yet know what would be required to get the Rust compiler to run on Alpha - will update this once I figure this out.

@glaubitz I think this issue might be of interest to you.

[antoyo]

Do you also need some fixes to use the correct linker?

[glaubitz]

Thank you so much for doing this! This is dearly needed and I am really looking forward to be able to bootstrap rustc on the remaining four architectures in Debian!

Do you also need some fixes to use the correct linker?

We can use mold on m68k and sh4 but we have to use ld.bfd on alpha and hppa. (alpha used to be supported in mold but it got removed, for hppa there is a WIP branch).

CC @danglin44 @gyrovorbis @hdeller @maciej-w-rozycki @rth7680 @sozud

[gyrovorbis]

Thank you SO MUCH for this from the Sega Dreamcast community as well...

Trust me, there are a WHOLE lot of us on "tier 3" devices who are still using the latest GCC 15.x and are drooling over potentially having Rust support... Truly, thank you.