Tracking Issue for asm_experimental_reg

This tracks support for additional registers in architectures where inline assembly is already stable.
The feature gate for the issue is #![feature(asm_experimental_reg)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implementation (see implementation history below)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

• s390x vreg input/output (related LLVM code)
  • Implementation https://github.com/rust-lang/rust/pull/131664
  • Stabilization PR https://github.com/rust-lang/rust/pull/154184
• RISC-V reg_pair
  • Implementation https://github.com/rust-lang/rust/pull/150973
• x86 {xyz}mm_reg {i,u}128 input/output (related LLVM code)
  • Implementation https://github.com/rust-lang/rust/pull/151059
  • Stabilization PR https://github.com/rust-lang/rust/pull/159525
• aarch64 vreg {i,u}128 input/output (related LLVM code)
  • Implementation https://github.com/rust-lang/rust/pull/154342
• loongarch vreg/xreg (related LLVM code)
  • Implementation https://github.com/rust-lang/rust/pull/158364

Other candidates:

• powerpc vreg/vsreg {i,u}128 input/output (related LLVM code) https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.4?topic=functions-vec-add-u128
• GPR pair on Arm and s390x
• AArch64 SVE types
• RISC-V RVV types

@rustbot label +A-inline-assembly +O-SystemZ +O-riscv +O-x86_32 +O-x86_64 +O-loongarch

Start with the stabilization guide linked in the issue and review the implementation history for the listed architectures, including the referenced stabilization PRs. The unchecked stabilization step is the remaining goal, but the issue does not identify a specific file or test; done means completing stabilization for the tracked register support.