CHERIoT-Platform/llvm-project

Replace various macros from compartment-macros with compiler builtins.

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#6 opened on Nov 24, 2023

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 (3 comments) (0 reactions) (0 assignees)LLVM (15 forks)auto 404
good first issue

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Description

A lot of things in compartment-macros.h that should be compiler builtins:

  • MMIO_CAPABILITY, should be __builtin_cheriot_mmio_capability(type, identifier), lowered to llvm.cheriot.mmio_capability, which is attributed so that optimisers know that it always returns the same value, and then expanded to something equivalent to the current assembly in the back end (ideally with the permissions handled more sensibly).
  • CHERIOT_EMIT_STATIC_SEALING_TYPE should be __builtin_cheriot_sealing_type(identifier), generating an LLVM intrinsic that then expanding in the back end to something equivalent to the current assembly.
  • DECLARE_STATIC_SEALED_VALUE should be replaced with a [[cheriot::sealed(identifier, identifier)]] attribute on a structure. This should use the attribute that will be introduced in #5, the only thing allowed on these globals is taking their address. STATIC_SEALED_VALUE then is replaced with simply taking the address of a global declared with the cheriot::sealed attribute.

Contributor guide

Research direction
Study how LLVM defines builtin functions and intrinsics. Look at existing builtins in clang and LLVM backend for CHERI. Implement the new builtins as LLVM intrinsics with appropriate attributes, then modify the backend to lower them to assembly. Update clang to handle the new attributes and builtins.
Tech stack
cpp
Domain
compilers
Issue type
Refactor
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.
3-5 days
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.
Mostly clear
Prerequisites
C++LLVMCHERI
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20