Create a new Tier 3 target: `powerpc-unknown-none`

[amy-kwan]

Proposal

This compiler MCP proposes adding a new Tier 3 bare-metal target: powerpc-unknown-none.

powerpc-unknown-none provides a minimal Rust entry point for big-endian 32-bit PowerPC in freestanding environments such as firmware, kernels, and other no_std software.

The implementation is intentionally minimal as the implementation (that will be subsequently posted) adds a built-in target entry in rustc_target::spec, a target specification file for powerpc-unknown-none, and the required bootstrap STAGE0_MISSING_TARGETS entry. The target is cross-compilation only (host_tools: false) and does not provide std support (std: false). This keeps the proposal within the usual scope for a Tier 3 bare-metal target.

Target Tier 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.)

Target maintainers for this proposal:

• @amy-kwan
• @daltenty
• @gilamn5tr

These maintainers will be the points of contact for target-specific issues and follow-up maintenance.

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.

The proposed name follows existing Rust naming conventions for PowerPC and for bare-metal targets. It reuses the existing powerpc architecture naming already present in Rust, and uses the unknown-none suffix consistent with other bare-metal Rust 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.

There are no known legal or licensing issues associated with this target. The proposal uses ordinary Rust target machinery and upstream LLVM PowerPC backend support. It does not add proprietary dependencies or impose unusual legal terms on the Rust project or its users.

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.

Acknowledged.

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 proposal is intentionally for a bare-metal no_std target. It is expected to support core as the baseline library. This proposal does not require a default allocator. std is not supported because the target is freestanding and does not assume an operating system or equivalent hosted runtime.

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.

Acknowledged. The accompanying Rust PR will add target-specific documentation under src/doc/rustc/src/platform-support/.

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.

Acknowledged.

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.

Acknowledged. The implementation is additive and isolated. It adds a new target specification and related target registration/bootstrap entries, but does not intentionally change semantics for 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.)

This requirement is satisfied by upstream LLVM PowerPC backend support. The proposed target uses Rust’s standard LLVM-based code generation path and does not rely on a forked backend.

Mentors or Reviewers

I intend to implement support for this target.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections can formally be registered here by adding a comment.

@rustbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rustbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[tgross35]

FCP as part of #988 (comment)

[linkmauve]

This will be very useful to replace the json target file for https://github.com/rust-wii/luma too, thanks!