Add no_std Xtensa targets support

[SergioGasquez]

Adds no_std Xtensa targets. This enables using Rust on ESP32, ESP32-S2 and ESP32-S3 chips.

Tier 3 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.)

@MabezDev and I (@SergioGasquez) will maintain the targets.

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 target triple is consistent with other targets.

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.

We follow the same naming convention as other 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 does not introduce any legal issues.

The target must not introduce license incompatibilities.

There are no license incompatibilities

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

Everything added is under that licenses

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.

Requirements are not changed for any other target.

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.

The linker used by the targets is the GCC linker from the GCC toolchain cross-compiled for Xtensa. GNU GPL.

"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 such terms exist for this target

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

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.

The target already implements core.

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.

Here is how to build for the target https://docs.esp-rs.org/book/installation/riscv-and-xtensa.html and it also covers how to run binaries on the target.

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.

No other targets should be affected

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

It can produce assembly, but it requires a custom LLVM with Xtensa support (https://github.com/espressif/llvm-project/). The patches are trying to be upstreamed (espressif/llvm-project#4)

[rustbot]

r? @davidtwco

rustbot has assigned @davidtwco.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

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

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

cc @Nilstrieb

[Kobzol]

By the way, you can try to run the x86_64-gnu-llvm-17 CI job locally using

$ DEPLOY=1 ENABLE_GCC_CODEGEN=1 src/ci/docker/run.sh x86_64-gnu-llvm-17

[Kobzol]

I think that you should just run python3 x.py test --bless tests/ui and then commit the result.

[rustbot]

Some changes occurred in tests/ui/check-cfg

cc @Urgau

[Kobzol]

Hmm. It looks like it's a problem that rustc cannot even create a TargetMachine for this target, apparently LLVM does not know about it at all.

The existing csky-unknown-linux-gnuabiv2hf target, which is ignored in some tests, does not have a rustup component, however apparently LLVM does understand it. It seems like our tests aren't currently prepared for a situation where LLVM does not even know about the target (?).

@davidtwco might know more.

[davidtwco]

Can you complete the metadata for each of these targets? It's only Option for the targets we've not populated these for yet.

[SergioGasquez]

Hi! I think I just added it, but CI is still failing :/

[SergioGasquez]

What I don't understand is though why does this work for csky-unknown-linux-gnuabiv2, which LLVM seems to understand locally, but it prints an error on CI.

Yes, I really cant understand why it does complain about Xtensa, it should be the same as csky-unknown-linux-gnuabiv2. Any ideas @davidtwco @Kobzol?

[Kobzol]

(just to clarify) It is not the same, because CSky is actually in mainline LLVM, but xtensa is not (if I understand it correctly). So I would definitely expect Xtensa to fail on CI, I'm just not sure why CSky is also in the error log.

[MabezDev]

Xtensa is partially upstream, see: https://github.com/llvm/llvm-project/tree/main/llvm/lib/Target/Xtensa, It's just not complete enough to build useful things with it yet hence why we still need our fork.

Maybe we could try enabling Xtensa as an experimental target in the LLVM build and see if it helps? Not sure if that's something we can do here in this PR or if it needs some kind of MCP.

[davidtwco]

Maybe we could try enabling Xtensa as an experimental target in the LLVM build and see if it helps? Not sure if that's something we can do here in this PR or if it needs some kind of MCP.

I think it's okay to do this, I don't expect it would impact any other target.

[Kobzol]

Adding xtensa (whatever the name of the target is in LLVM) here should hopefully do the trick.

[RalfJung]

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

It can produce assembly, but it requires a custom LLVM with Xtensa support (https://github.com/espressif/llvm-project/). The patches are trying to be upstreamed (espressif/llvm-project#4)

FWIW, in my understanding we require one of the actual codegen backends shipped with rustc to be able to generate code. Having patches in a fork is not sufficient.

[Kobzol]

Hmm, CI failed again. With the LLVM component change, we do add the Xtensa experimental target, but this specific CI job just uses vanilla LLVM 17 (which we don't build), which probably doesn't enable the target by default.

I still don't understand why the test was green before when apparently the same happens for Csky (unless it is enabled in vanilla LLVM 17), it needs investigation.

[RalfJung]

The CSky component apparently exists on some runners but not on others. That may explain the strange behavior.

[Kobzol]

That makes sense, but I don't understand why wasn't CI failing before.

[MabezDev]

CI is green 🎉

FWIW, in my understanding we require one of the actual codegen backends shipped with rustc to be able to generate code. Having patches in a fork is not sufficient.

Upstream LLVM has landed enough to generate some code/asm for Xtensa targets, however, the LLVM used by Rust currently doesn't have these patches in it yet. Would we still be okay to merge given that it's already landed in LLVM, or would we have to wait? If we have to wait, do you know if there is a timescale for LLVM updates? (I tried looking in the rustc dev guide and didn't see anything there).

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

[Kobzol]

We update to major versions (so next update will be LLVM 19), and we support the two latest major versions. But as long as we can get at least basic support working (xtensa already is an experimental target, after all) if user brings their own LLVM, and CI is green, I don't think that we need to wait.