rustc_target/asm: add LoongArch LSX/LASX inline asm register support

[heiher]

Add support for LoongArch LSX and LASX registers in inline assembly by introducing the vreg and xreg register classes, along with their associated vector types and operand modifiers. The new register classes are gated behind the asm_experimental_reg feature. Also model the overlap between FPU, LSX, and LASX registers so register conflict checking works correctly for aliased registers.

[rustbot]

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[rustbot]

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

cc @antoyo, @GuillaumeGomez

[wesleywiser]

This looks fine to me but I'm not sure whether we typically need T-lang to sign off on extensions to stable inline asm.

@Amanieu do you know if this needs a T-lang FCP?

[taiki-e]

AFAIK, a common way to add support for new registers in architectures where inline assembly is stable is to use asm_experimental_reg (#133416).

[heiher]

AFAIK, a common way to add support for new registers in architectures where inline assembly is stable is to use asm_experimental_reg (#133416).

Thanks for pointing that out. I think gating new inline assembly register support behind asm_experimental_reg is a great design choice. It allows new functionality to be introduced and validated without immediately becoming part of Rust's stable interface, which helps avoid turning early design mistakes into long-term compatibility burdens.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[heiher]

The newly added vreg and xreg register classes are gated behind the asm_experimental_reg feature rather than being stabilized. I've updated the test cases and the unstable book accordingly. Thanks!