Add basic support for mips64/powerpc/powerpc64/riscv64

[alexcrichton]

Added for rust-lang/rust#84449

[alexcrichton]

I also went ahead and added SHF_EXCLUDE to the constants list for elf.rs as well since it's used by rustc.

[bjorn3]

Can you add riscv32 too?

[philipc]

Would Architecture::None be appropriate for rust-lang/rust#84449, so that you can support all architectures? Not sure if that is too much of a hack.

[alexcrichton]

I originally tried that actually but it panics in too many locations during object file writing, so I wasn't able to use it. (assuming you mean the Unknown variant). Since the object files make their way to the linker I think they need to have at least the right header as well, so I don't think that a placeholder will work in this case.

[philipc]

We would have to add a None variant that corresponds exactly to EM_NONE. The existing Unknown means it could be any unrecognized value, and so it isn't suitable for writing. But yeah the question is whether the linker would be happy with that. I wasn't sure if it mattered since the linker doesn't need anything from the file anyway.

[alexcrichton]

Ah I see! I wasn't aware of EM_NONE. (honestly I wasn't aware of basically any of this until now)

I tested out with object files that have EM_NONE but the linker (at least for s390x and mips) rejected the object files and printed an error. I think that means that we probably need to emit the right shape of object file in rustc at least. This isn't really too much of a problem though I think, there aren't really all that many architectures and we're only writing the bare minimum of an object file (just one data section with SHF_EXCLUDE)

[philipc]

I didn't find anything else that uses EM_NONE, so in hindsight that was unlikely to work.

I think this is good to merge if you can address bjorn3's comments.

[alexcrichton]

Sure thing, done now!

[philipc]

Let me know if you need a release for this any time soon.

[alexcrichton]

Ah yeah if you're up for it that'd be great, this'll be needed to land the rustc pr

[philipc]

Ok, this technically a breaking change. We already require rust 1.42 so I'll add in some non_exhaustive for future changes.

[philipc]

0.25.0 is published.

[alexcrichton]

Thanks! 👍