Support for ARMv8.3+ / ARM64e targets

[Absolucy]

EDIT: Status

Currently, a blocker in a project of mine - Crabapple - is the lack of native arm64e support in Rust.

The aarch64-apple-ios target only emits arm64 (ARMv8) code, which will not work within an arm64e (ARMv8.3+) environment due to it's lack of awareness of Pointer Authentication, leading to segfaults when trying to access a signed pointer.

It is currently possible to work around this, by using a backtrace=false rustc, compiling with --emit=llvm-ir -Clto, and running the resulting IR through an arm64e-aware LLVM (such as apple/llvm-project). Pointer authentication may be worked around by linking to an FFI function that runs ptrauth_strip.

This may require LLVM changes if done, and quite possibly a new target (arm64e-apple-ios, aarch64-apple-ios-armv83, etc)

[Absolucy]

In addition, arm64e support important due to the upcoming Mac desktops running Bionic processors (such as an A12Z).

[Absolucy]

Upstream LLVM supports armv8.3, and most likely the Rust LLVM as well.

[jonas-schievink]

Upstream LLVM supports armv8.3, and most likely the Rust LLVM as well.

Weren't you working on building a custom LLVM since upstream doesn't support this yet? The link just goes to an RFC thread on the mailing list with no replies.

[Absolucy]

@jonas-schievink I was poorly attempting to jury-rig apple/llvm-project onto rustc. In short, I got arm64e and armv8.3 mixed up.

A quick grep and dig through rust-project/llvm reveals that it has the code for ARMv8.3 and Pointer Authentication support.

[Absolucy]

ARMv8.5 support would also be good, considering that there's a high likelihood than ARM Macs will run on ARMv8.5.

[Absolucy]

It is worth noting that the Rust LLVM branch has support for PAC (ARMv8.3) and BTI (ARMv8.5)

This isn't upstreamed, my bad.

[glandium]

Note that the macOS 11 SDK has both arm64-macos and arm64e-macos targets, although the arm64 variants don't have an uuid in the system library TBD files.

[glandium]

Upstream LLVM supports armv8.3, and most likely the Rust LLVM as well.

That's not merged in LLVM upstream.

[Absolucy]

@glandium my bad.

[emoon]

I'm interested in this as well because of Apple Silicon (i.e arm64 on macOS)

Going forward it will be mandated by Apple that executable released (at least for the App Store) supports both x86_64 and amd64 target. On macOS there is "fat binary" support which makes it possible to have both targets in the same exe/obj/dylib etc file. Clang for example supports that you can have -arch x86_64 and -arch amd64 on the command line to generate the correct binary for this to work. Then the OS will load the correct version depending on the OS version running.

There is a command line tool called lipo that can be used to combine two different compiles, but this (according to Apple) should only be used as a work-around and I think for Rust it would be better to support building for both targets at the same time.