llvm/llvm-project

[RISC-V] Update Linux Feature Detection

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#192,317 opened on Apr 15, 2026

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backend:RISC-Vgood first issue

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Description

There are new architecture hwprobe features exposed by recent linux versions. The docs are here: https://docs.kernel.org/arch/riscv/hwprobe.html

I think we have implemented up to RISCV_HWPROBE_EXT_ZAWRS, but no further. We should add support for the rest of the supported extensions.

This entails:

  • Proposing extension bitmask assignments for (standard) extensions that don't have them already. This is done via the https://github.com/riscv-non-isa/riscv-c-api-doc repo (note that there are some proposed but not yet landed bitmask assignments in the open PRs for that repo, which new proposals should not conflict with).
  • Add the RISCVExtensionBitmask to the relevant extensions.
  • Implement detection in llvm/lib/TargetParser/Host.cpp (used by -march=native) and compiler-rt/lib/builtins/cpu_model/riscv.c (used by ifuncs)

There is no compiler-rt interface for exposing vendor-specific extensions (which have a different hwprobe keys), but we may still be able to detect them in llvm/lib/TargetParser/Host.cpp for -march=native.

This would also be a good opportunity to move away from the deprecated RISCV_HWPROBE_KEY_CPUPERF_0, and to RISCV_HWPROBE_KEY_MISALIGNED_SCALAR_PERF, and also implement the same for RISCV_HWPROBE_KEY_MISALIGNED_VECTOR_PERF potentially (this can be a separate PR).

Contributor guide

Research direction
Study the Linux kernel hwprobe documentation for RISC V, examine existing detection implementations in llvm/lib/TargetParser/Host.cpp and compiler rt/lib/builtins/cpu model/riscv.c, propose bitmask assignments via the riscv c api doc repo, and implement detection for new extensions, also migrating from deprecated RISCV HWPROBE KEY CPUPERF 0 to RISCV HWPROBE KEY MISALIGNED SCALAR PERF.
Tech stack
None
Domain
backendtooling
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitC++RISC V architectureLinux kernel
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30