Support riscv64gc-unknown-linux-gnu target

[fintelia]

It was recently pointed out that Rust is one of the main blockers for the Debian RISC-V port. I wanted to ask what is actually missing on the Rust side? Since there is already support for the equivalent bare metal targets, is it simply a matter of porting the standard library? How much work would that actually be?

[mati865]

I can try building it but I won't be able to test it on real hardware.
One of issues for Debian would be LLVM. If they want to use system LLVM 8 they would need many backports for it.

EDIT: Failed with LLVM ERROR: Unable to lowerGlobalAddress, probably this should just wait until Rust rebases to LLVM 9.

[fintelia]

@mati865 Could you clarify what you tried? The riscv64imac-unknown-none-elf and similar targets do work but do have some issues. They also rely on some relatively recent upstream commits, so I won't expect them to work with any system LLVM yet.

[mati865]

I added new target riscv64gc-uknown-linux-gnu and tried to cross compile for it but hit LLVM ERROR: Unable to lowerGlobalAddress during stage1. With static relocation model it went further to stage2 and failed at libc, will be on my TODO.

AFAK Debian has strict packaging policy and packages must use system libraries.

[Disasm]

@mati865

Failed with LLVM ERROR: Unable to lowerGlobalAddress

This is already fixed in LLVM upstream master, but not released. At the moment I'm trying to grab some commits from master to fix this for riscv64*-unknown-none-elf targets. However, iirc for -linux- targets you also need TLS support (which is also in master, but not released).

[mati865]

@Disasm thanks. I haven't got that far yet to run executables. I'm waiting for #61393 anyway.

[davidlt]

Seems that Rust moved to LLVM 9.0.0 which has support for riscv64 (not experimental anymore). Thus targeting Linux should be possible now.

[laanwj]

Great progress! Does that mean it's possible to compile rust code for the hard-float (lp64d) ABI? Will the riscv64gc-none- target be changed to this too?

[sanxiyn]

@laanwj, hard-float is a separate issue, see #65024.

[laanwj]

@sanxiyn Thanks! Yes, I only mentioned it in this context because the existing gcc Linux riscv64gc target uses hardfloat, at least on Fedora Linux RISC-V, so it seems it's a requirement.

[archshift]

Matching what I mentioned in #65024, it is now at least possible to write a riscv64gc-unknown-linux target specification file with a hard-float ABI. The next step would probably be to port libc then.

[msizanoen1]

See #66661. In the meantime you can use https://github.com/msizanoen1/rust-riscv.