Add bare metal riscv32 target.

[royb3]

I asked in the embedded Rust matrix if it would be OK to clone a PR to add another riscv32 configuration. The riscv32ima in this case. @MabezDev was open to this suggestion as a maintainer for the Riscv targets.

I now took #117958 for inspiration and added/edited the appropriate files.

Tier 3 target policy

At this tier, the Rust project provides no official support for a target, so we place minimal requirements on the introduction of targets.

A proposed new tier 3 target must be reviewed and approved by a member of the compiler team based on these requirements. The reviewer may choose to gauge broader compiler team consensus via a Major Change Proposal (MCP).

A proposed target or target-specific patch that substantially changes code shared with other targets (not just target-specific code) must be reviewed and approved by the appropriate team for that shared code before acceptance.

• 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 target being added is using riscv32 as a basis, with added extensions. The riscv32 targets already have a maintainer and are named in the description file.

• 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.

Name is derived from the extensions used in the target.

• 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.

Same conditions apply compared to other riscv32 targets.

• Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).

Same conditions apply compared to other riscv32 targets.

• 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.

Same conditions apply compared to other riscv32 targets.

• 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.

Same conditions apply compared to other riscv32 targets.

• "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.

Same conditions apply compared to other riscv32 targets.

• 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.

Same conditions apply compared to other riscv32 targets.

• 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 is build on top of existing riscv32 targets and inherits these implementations.

• 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.

The documentation of this target is shared along with targets that target riscv32 with a different configuration of extensions.

• 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.

I now understand, apologies for the mention before.

• 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.

I now understand, apologies for the link to a similar PR before.

• 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 should not cause issues, as the target has similarities to other configurations of the riscv32 targets.

• Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target.

This should not cause issues, as the target has similarities to other configurations of the riscv32 targets.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @wesleywiser (or someone else) some time within the next two weeks.

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[rustbot]

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

[royb3]

r? compiler-team

[workingjubilee]

The target being added is using riscv32 as a basis, with added extensions. The riscv32 targets already have a maintainer and are named in the description file.

The maintainer policy requiring explicit assent exists to minimize a target having someone nominally expected to be taken care of it but that person not actually being able to fulfill their duties. It would be extremely unprecedented, as far as I am aware, for the project to accept someone signing up another person for additional work.

[workingjubilee]

@royb3 Please fix the merge conflicts. We no longer permit the "unqualified" compiletest pragmas.

[Noratrieb]

In other words, @almindor @dkhayes117 @romancardenas @MabezDev @jessebraham
are you fine with adding and maintaining this target?

[workingjubilee]

And why do we need a new target for this? Why do we need a target for ima and imac? Why do we need a target for im, for that matter? -Ctarget-feature exists.

[MabezDev]

I'm happy to maintain this.

I've just opened #122700 which should be merged before this, to fix the doc issue @workingjubilee spotted.

And why do we need a new target for this? Why do we need a target for ima and imac? Why do we need a target for im, for that matter? -Ctarget-feature exists.

We need this because build-std isn't stable, and these options affect how core is compiled; you can't run code built with c extension support on a core that doesn't support it. We discussed this in the r-e matrix channel earlier today: https://matrix.to/#/!BHcierreUuwCMxVqOf:matrix.org/$c6Ci5hqAvu6o9POkeolvr5Bl-o1HDzaJgsw9nX2RK0M?via=matrix.org&via=catircservices.org&via=tchncs.de

Edit: also see: rust-lang/wg-cargo-std-aware#86

[workingjubilee]

@MabezDev Ah, thank you for confirming.

@MabezDev @royb3 Hm. This is more of a philosophical question, but I have to ask:

Is it desirable that we add another target instead of pushing on advancing -Zbuild-std? Especially when this target will also require building it, as it will be tier 3? I do not believe you will be able to simply rustup target add it?

Maybe we should seriously consider stabilizing -Cbuild-std just for RV32...

[romancardenas]

Yep, the number of possible combinations for RISC-V extensions does not scale well if we need a specific target for each one. Stabilizing build-std would be a huge improvement for RISC-V.

[royb3]

@MabezDev Ah, thank you for confirming.

@MabezDev @royb3 Hm. This is more of a philosophical question, but I have to ask:

Is it desirable that we add another target instead of pushing on advancing -Zbuild-std? Especially when this target will also require building it, as it will be tier 3? I do not believe you will be able to simply rustup target add it?

Maybe we should seriously consider stabilizing -Cbuild-std just for RV32...

Stabilizing build-std would be a solution as well. It is actually what I currently use. The reason for this PR is to get the target to stable Rust, assuming it would get an upgrade to tier 2. Stabilizing build std would achieve the same.

[MabezDev]

@MabezDev Ah, thank you for confirming.

@MabezDev @royb3 Hm. This is more of a philosophical question, but I have to ask:

Is it desirable that we add another target instead of pushing on advancing -Zbuild-std? Especially when this target will also require building it, as it will be tier 3? I do not believe you will be able to simply rustup target add it?

Maybe we should seriously consider stabilizing -Cbuild-std just for RV32...

I would love to see build-std, in some form stabilized. I use it for every target I maintain (and it has benefits for the entire Rust ecosystem, not just embedded). The problem is I have no sway in that area, I've commented in rust-lang/wg-cargo-std-aware#86 but other than that I'm not sure what I can do; I don't have time to get involved with cargo sadly :(. I would happily provide evidence of the feature working as intended and or help review/test the feature as it stabilizes.

For now, I don't want to block @royb3 from using stable Rust on their target, if we can just as easily add the target and then open an MCP to promote it to tier 2; but I am happy to wait if we can instead put more effort behind stabilizing build-std.

[petrochenkov]

I'm fine with merging this as a tier 3 target, but I also wouldn't want to multiply tier 2 targets.

@ehuss What are the blockers for stabilizing -Zbuild-std for bare metal targets specifically (libcore only)?

[ehuss]

Stabilization blockers are tracked in https://github.com/rust-lang/wg-cargo-std-aware/labels/stabilization%20blocker and https://github.com/rust-lang/wg-cargo-std-aware/labels/plan%20before%20stabilization.

[workingjubilee]

I should note that this is a pretty bad issue for x86-64 too in various ways but we do not have an x86_64v2 target. ( We do for v3, for apple-darwin, but that is a somewhat amusing special case re: ABI stuff, for a case of "it actually is much harder to work with if it isn't its own target." )

[petrochenkov]

@royb3
Will having this target as a built-in in rustc be useful for you, if it stays tier 3 and is never promoted to tier 2?

[royb3]

@royb3 Will having this target as a built-in in rustc be useful for you, if it stays tier 3 and is never promoted to tier 2?

I think so. That way I can get rid of the rust flags that add extra extensions and atomics support.
The main goal was for stable rust to be available for the target, so nightly could be dropped. If that could be achieved by stabilization of cargo build-std, I would be happy too. Having this target as tier 3 would then still save me from having to specify the target flags, correct?

On the other hand, I do agree that the list of riscv32 targets is starting to get a bit long and that we can't keep adding to is forever.

[petrochenkov]

Ok, let's merge this as a tier 3 target then.
The PR needs a rebase.
@rustbot author

[petrochenkov]

@royb3
Could you squash commits as well?

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit 2fca27c has been approved by petrochenkov

It is now in the queue for this repository.