Partially stabilize LoongArch target features

[heiher]

Stabilization PR for the LoongArch target features. This PR stabilizes some of the target features tracked by #44839.

Specifically, this PR stabilizes the following target features:

• frecipe
• lasx
• lbt
• lsx
• lvz

Docs PR: rust-lang/reference#1707

r? @Amanieu

[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

[RalfJung]

If "d" and "f" work anything like the RISC-V features of the same name, they should not be stabilized yet. We need to figure out #116344 and #131799 for LoongArch first.

Does any of the other features ever have any effect on calling conventions or ABI?

[heiher]

If "d" and "f" work anything like the RISC-V features of the same name, they should not be stabilized yet. We need to figure out #116344 and #131799 for LoongArch first.

Does any of the other features ever have any effect on calling conventions or ABI?

Thank you! Other than f and d, there are no other target features affecting the calling convention and ABI.

[ehuss]

This will need a PR for documentation at https://github.com/rust-lang/reference/blob/master/src/attributes/codegen.md, and a lang (+whoever?) FCP.

[Amanieu]

• relax refers to linker relaxation support and may not be appropriate to expose as a target feature.
• ual refers to support for unaligned memory access, similar to strict-align on ARM targets. It's probably fine to stabilize as-is, but we may want to double-check with other targets if we want to have a consistent feature name between all of them (alternatively it's also fine to make the feature target-specific).

[RalfJung]

Also to be clear, ual can only affect how unaligned accesses are codegen'd, and nothing else. See this recent discussion on Zulip.

If we have target feature docs somewhere, they should clarify hat unaligned accesses are still UB except when performed via {read,write}_unaligned.

[heiher]

I agree with skipping these two target features and stabilizing them at a more appropriate time.

[Amanieu]

r? lang

[heiher]

This will need a PR for documentation at https://github.com/rust-lang/reference/blob/master/src/attributes/codegen.md, and a lang (+whoever?) FCP.

PR: rust-lang/reference#1707

[bors]

☔ The latest upstream changes (presumably #134794) made this pull request unmergeable. Please resolve the merge conflicts.