Add aarch64_be-unknown-none-softfloat target

[Gelbpunkt]

This adds a new target for bare-metal big endian ARM64 without FPU. We want to use this in the Hermit unikernel because big endian ARM64 is the most accessible big endian architecture for us and it can be supported with our existing aarch64 code. I have compiled our kernel and bootloader with this target and they work as expected in QEMU.

Regarding the tier 3 target policy:

• A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

The maintainer(s) (currently just me) are listed in the markdown document that documents the target.

• Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.
  • Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.
  • If possible, use only letters, numbers, dashes and underscores for the name. Periods (.) are known to cause issues in Cargo.

The target name is consistent with the existing aarch64-unknown-none-softfloat target and the existing big endian aarch64 targets like aarch64_be-unknown-linux-gnu.

• Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users.
  • The target must not introduce license incompatibilities.
  • Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
  • The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.
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There are no licensing issues and any toolchain that can compile for aarch64-unknown-none-softfloat can also compile for aarch64_be-unknown-none-softfloat (well, at least GCC and LLVM). No proprietary components are required.

• Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.
  • This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Ack.

• Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

This target does not implement std and is equivalent to aarch64-unknown-none-softfloat in all these regards.

• The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

Ack, that is part of the markdown document.

• Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.
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Ack.

• Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.
  • In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

This doesn't break any existing targets.

• Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target. (Having support in a fork of the backend is not sufficient, it must be upstream.)

The LLVM backend works.

• If a tier 3 target stops meeting these requirements, or the target maintainers no longer have interest or time, or the target shows no signs of activity and has not built for some time, or removing the target would improve the quality of the Rust codebase, we may post a PR to remove it; any such PR will be CCed to the target maintainers (and potentially other people who have previously worked on the target), to check potential interest in improving the situation.

Ack.

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[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

These commits modify compiler targets.
(See the Target Tier Policy.)

[Noratrieb]

r? compiler_leads

cc @RalfJung will the NEON checks you added for another softfloat target work here too?

[Gelbpunkt]

I added some information regarding the target tier policy in the PR description. Hope that everything is clear, please let me know if anything's missing.

[davidtwco]

LGTM, complies with target policy.

@Noratrieb: I'm pretty sure that this works with Ralf's changes by default - those were disabling the flags/target features that enabled hardfloat targets to be made into softfloat targets, and thus being incompatible with the pre-built std. New targets that are only softfloat will have a softfloat std (if this target were tier two) and so there's no potential incompatibility.

[davidtwco]

@bors r+ rollup

[bors]

📌 Commit 070425e has been approved by davidtwco

It is now in the queue for this repository.

[Gelbpunkt]

@davidtwco Sorry for pushing another commit, I had forgotten the entry in STAGE0_MISSING_TARGETS. Everything is fine now.

[davidtwco]

@bors r-

[davidtwco]

@bors r+ rollup

[bors]

📌 Commit 6c96c94 has been approved by davidtwco

It is now in the queue for this repository.

[RalfJung]

r? compiler_leads

cc @RalfJung will the NEON checks you added for another softfloat target work here too?

The logic for that is here

rust/compiler/rustc_target/src/target_features.rs

Lines 990 to 1008 in 1dfc840

	"aarch64" | "arm64ec" => {
	// Aarch64 has no sane ABI specifier, and LLVM doesn't even have a way to force
	// the use of soft-float, so all we can do here is some crude hacks.
	match &*self.abi {
	"softfloat" => {
	// LLVM will use float registers when `fp-armv8` is available, e.g. for
	// calls to built-ins. The only way to ensure a consistent softfloat ABI
	// on aarch64 is to never enable `fp-armv8`, so we enforce that.
	// In Rust we tie `neon` and `fp-armv8` together, therefore `neon` is the
	// feature we have to mark as incompatible.
	FeatureConstraints { required: &[], incompatible: &["neon"] }
	}
	_ => {
	// Everything else is assumed to use a hardfloat ABI. neon and fp-armv8 must be enabled.
	// `FeatureConstraints` uses Rust feature names, hence only "neon" shows up.
	FeatureConstraints { required: &["neon"], incompatible: &[] }
	}
	}
	}

So if target.arch is "aarch64" and target.abi is "softfloat", then it should work.