stabilize s390x vector registers

[folkertdev]

tracking issue: #133416
reference PR: rust-lang/reference#2215

Stabilizes s390x vector registers, e.g.

unsafe fn vreg_128(x: i128) -> i128 {
    let y;
    asm!("vlr {}, {}", out(vreg) y, in(vreg) x);
    y
}

The types that are accepted for vreg registers are

• all float types f16, f32, f64, f128
• integer types i32, i64 and i128 and their unsigned counterparts
• integer vector types i8x16, i16x8, i32x4, i64x2 and their unsigned counterparts
• float vector types f16x8, f32x4 and f64x2

Support for all of these is tested in https://github.com/rust-lang/rust/blob/main/tests/assembly-llvm/asm/s390x-types.rs, and the types correspond with the LLVM definition in https://github.com/llvm/llvm-project/blob/df9eb79970c012990e829d174d181d575d414efe/llvm/lib/Target/SystemZ/SystemZRegisterInfo.td#L312-L339

The f16, f16x8 and f128 types are unstable, and so can't be used on stable in practice. They do show up in some error messages though.

vreg was previously only accepted as a clobber.

---

Currently the vector types in core::arch::s390x are still unstable. Separately stabilizing vreg is still useful because scalar types can also be put into vregs.

Implementation history

• Support input/output in vector registers of s390x inline assembly (under asm_experimental_reg feature) #131664
• Add f16 inline ASM support for s390x #150826

cc @uweigand @taiki-e
r? @Amanieu

[traviscross]

Thanks @folkertdev; I'll be looking to see that @Amanieu confirms this is OK after the back and force.

@rfcbot fcp merge lang

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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[scottmcm]

Happy to trust @Amanieu for this.

@rfcbot reviewed

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔