Tracking Issue for RISC-V Ratified Extensions Intrinsics

[coastalwhite]

This issue tracks the implementation of all the intrinsics linked to the Ratified RISC-V Extensions. The, not yet stable intrinsics are behind the riscv_ext_intrinsics nightly feature.

Steps

• Implement all extensions in stdarch
  • Cryptography Extensions Volume I: Scalar & Entropy Source Instructions
  • Zkr. Entropy source extension
  • Bit-Manipulation
  • Zicntr and Zihpm
  • Zicsr
  • Half-Precision
  • Float in Integer
  • Vector Extension
• Adjust module level documentation
• Stabilization PR

Unresolved Questions

• How to deal with instructions that are already dealt with by LLVM intrinsics and library features (e.g. cpop and u*::count_ones)? Include them or not?
• Use usize for riscv32 or riscv64 specific intrinsics or use u32 and u64 respectively?
• How to deal with intrinsics involving immediate values?
  1. Use the asm_const feature outlined in Tracking Issue for asm_const #93332.
  2. Use the trick described in Implement all x86 vendor intrinsics stdarch#40 and illustrated in _mm_cmpestri.
• How to deal with CSR registers?
• Can we source titles and descriptions from the specifications? License issues?
• Dealing with the Vector extension is very difficult due to dynamically sized registers (RFC: Add a scalable representation to allow support for scalable vectors rfcs#3268).

Implementation History

• Implement RISC-V Zk intrinsics stdarch#1453. Implements the Zk extension, except Entropy Source and Zkt.
• Implement RISC-V Zb intrinsics  stdarch#1461. Implements the Zb extension.
• Stabilize Ratified RISC-V Target Features #116485. Stabilizes several RISC-V target features.

[coastalwhite]

Two additional points:

1. Upcoming PRs should use the rustc_legacy_const_generics for immediate values instead.
2. Prefer LLVM intrinsics to assembly. Intrinsics can be found here.

[coastalwhite]

Entropy Source of the Cryptographic Scalar Extension are not implemented in LLVM. Therefore, it should be implemented with inline assembly and is not part of the implemented intrinsics.

[coastalwhite]

Extensions such as Zfinx, Zdinx and Zhinx are all done by LLVM without intrinsics and therefore don't have to be done.