orc-rt

orc-rt/ is the runtime support library for ORC ("On-Request Compilation"), LLVM's modern JIT framework. The compile-time side of ORC lives in llvm/lib/ExecutionEngine/Orc/; orc-rt is the runtime side: code that ships with a JIT-using application and provides the helpers ORC needs at runtime.

Purpose

The legacy MCJIT and ORCv1 worked by compiling code in-process and providing thin runtime support. ORCv2 split the responsibilities more carefully: the compiler-side libraries handle module compilation and symbol resolution; the runtime-side library provides things like the platform-specific bootstrap, exception-handling registration of JIT'd code, atexit-style finalization, and the dlopen-like APIs JIT clients use to manage materialization units.

Directory layout

orc-rt/
├── include/        # Public headers
├── lib/            # Implementation
├── unittests/
├── test/
├── docs/
├── cmake/
└── ...

The subproject is small (~7,000 source lines at this snapshot). It was carved out of compiler-rt's lib/orc/ and elevated to a top-level subproject as the ORC runtime story matured.

What it provides

• EH frame registration / deregistration for JIT'd code so the unwinder can find DWARF CFI for compiled-on-the-fly functions.
• atexit-style finalizers that fire when materializations are torn down.
• Per-platform bootstrap — small platform-specific routines used to initialize the JIT linker's view of the host process.
• Generic helpers that ORC's Platform plugins call into.

The exact public API is still settling — orc-rt is the youngest top-level subproject in the repository.

Integration points

• LLVM core (llvm) — llvm::orc::LLJIT, llvm::orc::ExecutionSession, the Platform interface in llvm/include/llvm/ExecutionEngine/Orc/ are the consumers.
• libunwind (libunwind) — JIT'd code needs to register its .eh_frame so unwinding works through it.
• Compiler-rt (compiler-rt) — the older lib/orc/ implementation lives there for compatibility.

Entry points for modification

• Adding a runtime helper: implementation in orc-rt/lib/, header in orc-rt/include/, and a public API contract on the LLVM side under llvm/include/llvm/ExecutionEngine/Orc/.
• Platform plumbing: per-OS shims live alongside the generic implementation; see how the existing platforms split out.

Reference

• LLVM ORC documentation
• LLJIT design notes