RISC-V target feature B implies Zba, Zbb, Zbs

[tux3]

zba, zbb, and zbs are already stable, B is a convenient alias for all three, so it seems reasonable to stabilize it as well.
This feature name is supported by our current LLVM version (and LLVM uses the same name).

The fourth bitmanip extension, Zbc, is not implied by B.

For reference, the RISC-V unprivileged manual defines this in chapter 29 on the B extension: https://github.com/riscv/riscv-isa-manual/releases

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[fee1-dead]

r? compiler

[oli-obk]

who are the RISC-V maintainers?

[tux3]

I'm not sure if I'm looking in the right place, but I see these files list different sets of people:

https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/riscv32-unknown-none-elf.md#target-maintainers
https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/riscv64gc-unknown-linux-gnu.md#target-maintainers
https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/riscv64gc-unknown-linux-musl.md#target-maintainers
(and three more files like this)

[michaelmaitland]

LGTM. This is what LLVM does too. https://github.com/llvm/llvm-project/blob/305006179341d8f657b89fdc7d76502f9bc5f0aa/llvm/lib/Target/RISCV/RISCVFeatures.td#L497

[michaelmaitland]

Do you need to check for LLVM compatibility similar to 62612af#diff-9f3bfd2fdc228dcb1b352082a7ebc39b19c8bea15b73141f72d9f5021aa3c66cR266-R268?

[tux3]

It looks like FeatureStdExtB exists in rust-lang's fork of LLVM starting from LLVM 19: https://github.com/rust-lang/llvm-project/blob/rustc/19.1-2024-07-30/llvm/lib/Target/RISCV/RISCVFeatures.td

And it seems the minimum LLVM version was raised to 19 last week: 12167d7

So I think this should be good as is (the compat checks in 62612af also seem to have been removed)

[tux3]

I noticed this comment says we may need t-lang, but I'm not too familiar with procedures yet. Is this a bors command? =)

https://github.com/rust-lang/rust/blob/master/compiler/rustc_target/src/target_features.rs#L125

[Amanieu]

I believe this is fine to stabilize but I would like to double-check with @a4lg @taiki-e who have been working on RISC-V target features.

[traviscross]

In the lang call today, Amanieu proposed doing #139440 first here.

@rustbot labels -I-lang-nominated

r? Amanieu

[Amanieu]

#139440 also adds the B feature (along with some others), so I'm going to close this PR in favor of the other one.