[llvm-dev] ORC JIT Weekly #26 -- Orc library break-up, remote TargetProcessControl, and the beginnings of a runtime.

[Lang Hames via llvm-dev]

Hi All,

I'm back again after a couple of weeks hiatus, and I have some good news
for anyone interested in cross-process JITing with OrcV2: The remote
TargetProcessControl and Orc library breakup patch has landed
in 1d0676b54c4 [1]. Thanks very much to Dave Blaikie and Stefan Graenitz
for all their feedback on the review!

As described in my last email, this commit breaks the OrcJIT library into
three pieces:
- OrcJIT retains the bulk of the code.
- OrcTargetProcess contains code only needed by JIT execution processes,
and depends only on Support.
- OrcShared contains code required by both of these libraries -- mostly RPC
and error types.

For OrcV2 users who are running JIT'd code in-process the only essential
change is that you will need to link against OrcTargetProcess library as
well as OrcJIT. For users who want to JIT across processes/targets this
library break-up means that your target process only needs to link against
OrcTargetProcess, not LLVM's core or object libraries.

This patch also introduces a pair of classes, OrcRPCTargetProcessControl
and OrcRPCTPCServer, that enable Orc-RPC based out-of-process execution.
These classes serve two purposes: As an example of how to build
cross-process TargetProcessControl implementations (for anyone who wants to
build their own based on other XPC/IPC systems), and as a way to write
regression tests for out-of-process execution without introducing new
dependencies into LLVM.

Anyone looking to try out the new APIs should check out llvm-jitlink, which
has been updated to optionally use these new classes (see [2] for example
usage). Once the API for OrcRPCTargetProcessControl settles down I plan to
re-introduce Chapter 5 of the Building A JIT tutorial series to provide an
introductory example for these APIs.

The next step for Orc, required for both cross-process support and new
features, is a runtime library. The runtime should be loadable via the JIT
like any other static library, provide support for JIT re-entry (eventually
replacing the existing Orc-ABI reentry code), thread locals, eh-frame and
language runtime registration, execution of initializers, and more. I've
started a prototype of a runtime as a library within compiler-rt in the
orc-runtime-prototype branch of my llvm fork [3]. There's not much there
yet, but I will keep you updated on my progress, and any questions,
comments, or PRs are very welcome.

Finally: I've added a skeleton roadmap to the OrcV2 design document [4],
and plan to keep improving the documentation. I'd love to hear any specific
requests you have for documentation improvements, as this will help me
focus my efforts in this area.

-- Lang.

[1] https://reviews.llvm.org/D90132
[2] http://lists.llvm.org/pipermail/llvm-dev/2020-September/145070.html
[3] https://github.com/lhames/llvm-project/tree/orc-runtime-prototype
[4] https://llvm.org/docs/ORCv2.html#roadmap
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