Add riscv64gc-unknown-hermit target

[mkroening]

This PR adds the new riscv64gc-unknown-hermit target, initially created by @simonschoening, a 64-bit RISC-V target for the Hermit unikernel project.

Furthermore, this cleans up the existing Hermit targets and adds a platform support documentation page for all Hermit targets and goes through the new tier 3 target policy process:

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.)

@stlankes as the Hermit project lead and I will be the target maintainers.

• 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 riscv64gc-unknown-hermit was derived from the existing x86_64-unknown-hermit and aarch64-unknown-hermit targets.

• 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.
  • Compiling, linking, and emitting functional binaries, libraries, or other
    code for the target (whether hosted on the target itself or cross-compiling
    from another target) must not depend on proprietary (non-FOSS) libraries.
    Host tools built for the target itself may depend on the ordinary runtime
    libraries supplied by the platform and commonly used by other applications
    built for the target, but those libraries must not be required for code
    generation for the target; cross-compilation to the target must not require
    such libraries at all. For instance, rustc built for the target may
    depend on a common proprietary C runtime library or console output library,
    but must not depend on a proprietary code generation library or code
    optimization library. Rust's license permits such combinations, but the
    Rust project has no interest in maintaining such combinations within the
    scope of Rust itself, even at tier 3.
  • "onerous" here is an intentionally subjective term. At a minimum, "onerous"
    legal/licensing terms include but are not limited to: non-disclosure
    requirements, non-compete requirements, contributor license agreements
    (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms,
    requirements conditional on the employer or employment of any particular
    Rust developers, revocable terms, any requirements that create liability
    for the Rust project or its developers or users, or any requirements that
    adversely affect the livelihood or prospects of the Rust project or its
    developers or users.

No dependencies were added to Rust.

• 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.

Understood.
I am not a member of a Rust team.

• 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.

Understood.
std is supported.

• 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.

Building is described in the platform support doc.

• 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.
  • Backlinks such as those generated by the issue/PR tracker when linking to
    an issue or PR are not considered a violation of this policy, within
    reason. However, such messages (even on a separate repository) must not
    generate notifications to anyone involved with a PR who has not requested
    such notifications.

Understood.

• 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.

I don't think this PR breaks anything.

r? compiler-team

[rustbot]

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

[davidtwco]

This looks good to me and I think all the requirements of the target tier policy have been satisfied.

[davidtwco]

@bors r+

[bors]

📌 Commit ab4d65e has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit ab4d65e with merge 48c0c25...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 48c0c25 to master...

[rust-timer]

Finished benchmarking commit (48c0c25): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.7%, 0.8%]	4
Regressions ❌ (secondary)	0.5%	[0.3%, 0.7%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.7%, 0.8%]	4

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 651.432s -> 653.19s (0.27%)

[rylev]

Perf: this is noise

@rustbot +perf-regression-triaged