MCP: New Tier-3 target for x86_64h-apple-darwin
#599
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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. cc @rust-lang/compiler @rust-lang/compiler-contributors |
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Add support for the x86_64h-apple-darwin target See rust-lang/compiler-team#599 for MCP. r? compiler-team CC `@BlackHoleFox` who recently overhauled the apple target code in `rustc-target`. ## Target Support Checklist > - 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.) I'm the designated developer. > - 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. This uses the same naming conventions used for the other macOS targets (`-apple-darwin`), combined with the convention used by LLVM for the `x86_64h` targets. LLVM's convention matches the architecture name used when invoking various tools such as `lipo`, `arch`, and (IMO) there's not really a compelling reason to depart from it. > - 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. I don't think this is especially likely, although I suppose someone could mistake it for `x86_64-apple-darwin`. > - If possible, use only letters, numbers, dashes and underscores for the name. > Periods (`.`) are known to cause issues in Cargo. 👍 > - 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. It does not. > - Anything added to the Rust repository must be under the standard Rust > license (`MIT OR Apache-2.0`). It is. > - 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. There are no new dependencies that don't also apply to `x86_64-apple-darwin`. > - 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. This has the same requirements as the other macOS targets (e.g. `x86_64-apple-darwin` and similar). > - "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 change here. > - 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. 👍 > - 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 standard library tests seem to pass. > - 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. Documentation is provided. > - 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. Noted. This target is nearly identical to `x86_64-apple-darwin`, so this is unlikely to cause issues anyway. > - 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. 👍 > - 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. 👍
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Proposal
Add support for the
x86_64h-apple-darwintarget to rustc. Thex86_64h-apple-darwintarget is the "enhanced for Haswell" version ofx86_64-apple-darwin. The main reason to support this is for compiling code that can occupy thex86_64hentry of a universal binary (a "fat" mach-o file).Details
For background: when a universal binary containing entries for both
x86_64andx86_64his executed on a machine that supportsx86_64h, then the program loader will execute thex86_64hcode instead of thex86_64code. Additionally, if ax86_64machine withoutx86_64hsupport tries to load a MachO which only supportsx86_64h, then a error is emitted at load time, which is preferable to the alternative where the load succeeds, but then later tries to execute an instruction unsupported on that machine, resulting in UB (likely aSIGILL).IOW, this is useful for Rust projects that distribute precompiled binaries in at least two scenarios:
Projects that wish to distribute a universal binary that (possibly among others) contains entries for both
x86_64andx86_64h, thus allowing use of advanced CPU features (AVX2 and such) while avoiding the need for runtime target feature dispatch, or the need to to drop support for old machines.Projects that wish to explicitly drop support for macOS on older intel chips from their compiled distributions, possibly because it's so very dependent on SIMD performance.
Comparison to
x86-64-v3You may notice that this target is somewhat similar to a hypothetical
x86_64v3-apple-darwintarget. This is true, however there are at least two differences:Most obviously (but not most importantly): the set of target features this covers does not quite match those provided by
-Ctarget-cpu=x86-64-v3The precise invocation would need to be more like
-Ctarget-cpu=core-avx2 -Ctarget-features=-rdrnd,-aes,-pclmul,-rtm,-fsgsbase(or something along these lines), however this would be insufficient in practice, both due to the issues with disabling target features (generally requires a recompilation of the standard library), and more importantly, due to not providing the second benefit.More critically, it bottoms out at the
x86_64h-apple-macosxLLVM target, and thus the resulting mach-o may be used as thex86_64hentry in a universal binary.Note that inserting a binary compiled as
x86_64-apple-darwinin thex86_64hslot is not possible (as the CPU subtype of that part of the MachO must beCPU_SUBTYPE_X86_64_H). I was not able to solve that by manually modifying the CPU_SUBTYPE in the MachO, but did not investigate why (possibly due to incorrect load commands), as doing this the "right way" was simpler.Downsides
This may be confusing to some users, for example, ones who fail to notice
x86_64-apple-darwinand only see thex86_64h-apple-darwintarget in things like--print target-list, or who think that they should compile all their code asx86_64h-apple-darwin. This is true, but seems acceptable and is a risk for any new target.There is some additional maintenance burden, although it may be handled identically to
x86_64-apple-darwinin almost all cases, so that burden seems low. An initial implementation is available https://github.com/thomcc/rust/tree/x86_64h-targetalthough I have not submitted it as a PR (yet)(Edit: PR is Add support for the x86_64h-apple-darwin target rust#108795).Infra costs if it ever moves to tier-2: While I think it's plausible that this eventually becomes a tier 2 target, this MCP is just about tier-3 support (as the target tier documentation states targets should start as tier3 before moving up), and "is it worth tier-2" is something I'd rather defend another day.
Given the intended use case (universal binaries), I don't think there's much of a reason to ever support host tools — users should be able to use the
x86_64-apple-darwinhost tools just fine.Mentors or Reviewers
No mentor is likely needed (already implemented), but I'm unsure who would be the best reviewer.
Process
The main points of the Major Change Process are as follows:
@rustbot second.-C flag, then full team check-off is required.@rfcbot fcp mergeon either the MCP or the PR.You can read more about Major Change Proposals on forge.
Comments
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.
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