Updating LLVM version

[Disasm]

I'd like to add tier2 platforms for RV64IMAC and RV64GC targets. However, simple target implementation cannot be built against current rustc/8.0-2019-01-16 LLVM version. We need to get some LLVM patches for RV64 from upstream (which are already there). At the moment I have no idea how to achieve this. Hypothetical choices are:

• Cherry-pick only the necessary patches into Rust LLVM tree
• Determine some point in upstream LLVM which contains all needed patches and merge it into Rust LLVM tree
• Rebase Rust LLVM tree to the point mentioned above

I will experiment with different choices, but I need to know which approach is actually used and what should I do next.

[davidlt]

Maybe Alex @asb could provide a good suggestion. He also provided an update on RISC-V support in LLVM at FOSDEM 2019: https://fosdem.org/2019/schedule/event/riscvllvmclang/

[nikic]

This depends on how many patches we're talking. If it's just two or three, they can be cherry-picked (you can submit a PR at https://github.com/rust-lang/llvm-project). If a lot of changes are necessary, then the Rust LLVM fork will have to be rebased.

[Disasm]

@nikic At the moment I can build RV64 support with 16 patches, but I can try to reduce this patch set. Theoretical minimum is 4 patches, but I haven't tried this yet.

[michaelwoerister]

Can we update to LLVM 8 proper? The rc2 for that is already out.

[nikic]

We are already on LLVM 8. These changes are part of LLVM 9.

[michaelwoerister]

Ah, OK. I hope we stay on LLVM 8 until the next beta branches off.