[cfe-dev] [RFC] Moving (parts of) the Cling REPL in Clang
Vassil Vassilev via cfe-dev
cfe-dev at lists.llvm.org
Fri Jul 10 13:59:22 PDT 2020
Hi Richard,
On 7/10/20 11:10 PM, Richard Smith wrote:
> Hi Vassil,
>
> This is a very exciting proposal that I can imagine bringing important
> benefits to the existing cling users and also to the clang user and
> developer community. Thank you for all the work you and your team have
> done on cling so far and for offering to bring that work under the
> LLVM umbrella!
>
> Are you imagining cling being part of the clang repository, or a
> separate LLVM subproject (with only the changes necessary to support
> cling-style uses of the clang libraries added to the clang tree)?
Good question. In principle cling was developed with the idea to
become a separate LLVM subproject. Although I'd easily see it fit in
clang/tools/.
Nominally, cling has "high-energy physics"-specific features such as
the so called 'meta commands'. For example, `[cling] .L some_file` would
try to load a library called some_file.so and if it does not exist, try
#include-ing a header with that name; `[cling] .x script.C` includes
script.C and calls a function named `script`. I can imagine that broader
community may not like/use that. If we start trimming down features like
that then it won't really be cling anymore. Here is what I would imagine
as a way forward:
1. Land as many cling/"incremental compilation"-related patches as we
can in clang.
2. Build a simple tool, let's use a strawman name -- clang-repl,
which only does the basics. For example, one can feed it incremental C++
and execute it.
3. Rework cling to use that infrastructure -- ideally, implementing
it's specific meta commands and other domain-specific features such as
dynamic scopes.
We could move any of the cling features which the broader community
finds useful closer to clang. For the moment I am being conservative as
this will also give us the opportunity to rethink some of the features.
The hard part is what lives where. First bullet point is clear. The
second -- not so much. Clang has a clang-interpreter in its examples
folder and it looks a little unmaintained. Maybe we can start
repurposing that to match 2.
As for cling itself there are some challenges we should try to solve.
Our community lives downstream (currently llvm-5) and a straight-forward
llvm upgrade + bugfixing takes around 3 months due to the nature of our
software stacks. It would be a non-trivial task to move the cling-based
development in llvm upstream. My worry is that HEP-cling will soon
depart from LLVM-cling if we don't get both communities on the same
codebase (we have experienced such a problem with the getFullyQualified*
interfaces). I am hoping that a middleman, such as clang-repl, can help.
When we move parts of cling in clang we will develop and test the
required functionality using clang-repl. This way users will enjoy
cling-like experience and when cling upgrades its llvm its codebase will
become smaller in size.
Am I making sense?
>
> On Thu, 9 Jul 2020 at 13:46, Vassil Vassilev via cfe-dev
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
> Motivation
> ===
>
> Over the last decade we have developed an interactive, interpretative
> C++ (aka REPL) as part of the high-energy physics (HEP) data analysis
> project -- ROOT [1-2]. We invested a significant effort to
> replace the
> CINT C++ interpreter with a newly implemented REPL based on llvm --
> cling [3]. The cling infrastructure is a core component of the data
> analysis framework of ROOT and runs in production for approximately 5
> years.
>
> Cling is also a standalone tool, which has a growing community
> outside
> of our field. Cling’s user community includes users in finance,
> biology
> and in a few companies with proprietary software. For example,
> there is
> a xeus-cling jupyter kernel [4]. One of the major challenges we
> face to
> foster that community is our cling-related patches in llvm and clang
> forks. The benefits of using the LLVM community standards for code
> reviews, release cycles and integration has been mentioned a
> number of
> times by our "external" users.
>
> Last year we were awarded an NSF grant to improve cling's
> sustainability
> and make it a standalone tool. We thank the LLVM Foundation Board for
> supporting us with a non-binding letter of collaboration which was
> essential for getting this grant.
>
>
> Background
> ===
>
> Cling is a C++ interpreter built on top of clang and llvm. In a
> nutshell, it uses clang's incremental compilation facilities to
> process
> code chunk-by-chunk by assuming an ever-growing translation unit [5].
> Then code is lowered into llvm IR and run by the llvm jit. Cling has
> implemented some language "extensions" such as execution
> statements on
> the global scope and error recovery. Cling is in the core of HEP
> -- it
> is heavily used during data analysis of exabytes of particle physics
> data coming from the Large Hadron Collider (LHC) and other particle
> physics experiments.
>
>
> Plans
> ===
>
> The project foresees three main directions -- move parts of cling
> upstream along with the clang and llvm features that enable them;
> extend
> and generalize the language interoperability layer around cling; and
> extend and generalize the OpenCL/CUDA support in cling. We are at the
> early stages of the project and this email intends to be an RFC
> for the
> first part -- upstreaming parts of cling. Please do share your
> thoughts
> on the rest, too.
>
>
> Moving Parts of Cling Upstream
> ---
>
> Over the years we have slowly moved some patches upstream. However we
> still have around 100 patches in the clang fork. Most of them are
> in the
> context of extending the incremental compilation support for
> clang. The
> incremental compilation poses some challenges in the clang
> infrastructure. For example, we need to tune CodeGen to work with
> multiple llvm::Module instances, and finalize per each
> end-of-translation unit (we have multiple of them). Other changes
> include small adjustments in the FileManager's caching mechanism, and
> bug fixes in the SourceManager (code which can be reached mostly from
> within our setup). One conclusion we can draw from our research is
> that
> the clang infrastructure fits amazingly well to something which
> was not
> its main use case. The grand total of our diffs against clang-9
> is: `62
> files changed, 1294 insertions(+), 231 deletions(-)`. Cling is
> currently
> being upgraded from llvm-5 to llvm-9.
>
> A major weakness of cling's infrastructure is that it does not
> work with
> the clang Action infrastructure due to the lack of an
> IncrementalAction. A possible way forward would be to implement a
> clang::IncrementalAction as a starting point. This way we should
> be able
> to reduce the amount of setup necessary to use the incremental
> infrastructure in clang. However, this will be a bit of a testing
> challenge -- cling lives downstream and some of the new code may be
> impossible to pick straight away and use. Building a mainline example
> tool such as clang-repl which gives us a way to test that incremental
> case or repurpose the already existing clang-interpreter may be
> able to
> address the issue. The major risk of the task is avoiding code in the
> clang mainline which is untested by its HEP production environment.
> There are several other types of patches to the ROOT fork of Clang,
> including ones in the context of performance,towards C++ modules
> support (D41416), and storage (does not have a patch yet but has
> an open
> projects entry and somebody working on it). These patches can be
> considered in parallel independently on the rest.
>
> Extend and Generalize the Language Interoperability Layer Around Cling
> ---
>
> HEP has extensive experience with on-demand python interoperability
> using cppyy[6], which is built around the type information
> provided by
> cling. Unlike tools with custom parsers such as swig and sip and
> tools
> built on top of C-APIs such as boost.python and pybind11, cling can
> provide information about memory management patterns (eg refcounting)
> and instantiate templates on the fly.We feel that functionality
> may not
> be of general interest to the llvm community but we will prepare
> another
> RFC and send it here later on to gather feedback.
>
>
> Extend and Generalize the OpenCL/CUDA Support in Cling
> ---
>
> Cling can incrementally compile CUDA code [7-8] allowing easier
> set up
> and enabling some interesting use cases. There are a number of
> planned
> improvements including talking to HIP [9] and SYCL to support more
> hardware architectures.
>
>
>
> The primary focus of our work is to upstreaming functionality
> required
> to build an incremental compiler and rework cling build against
> vanilla
> clang and llvm. The last two points are to give the scope of the work
> which we will be doing the next 2-3 years. We will send here RFCs for
> both of them to trigger technical discussion if there is interest in
> pursuing this direction.
>
>
> Collaboration
> ===
>
> Open source development nowadays relies on reviewers. LLVM is no
> different and we will probably disturb a good number of people in the
> community ;)We would like to invite anybody interested in joining our
> incremental C++ activities to our open every second week calls.
> Announcements will be done via google group:
> compiler-research-announce
> (https://groups.google.com/g/compiler-research-announce).
>
>
>
> Many thanks!
>
>
> David & Vassil
>
> References
> ===
> [1] ROOT GitHub https://github.com/root-project/root
> [2] ROOT https://root.cern
> [3] Cling https://github.com/root-project/cling
> [4] Xeus-Cling
> https://blog.jupyter.org/xeus-is-now-a-jupyter-subproject-c4ec5a1bf30b
> [5] Cling – The New Interactive Interpreter for ROOT 6,
> https://iopscience.iop.org/article/10.1088/1742-6596/396/5/052071
> [6] High-performance Python-C++ bindings with PyPy and Cling,
> https://dl.acm.org/doi/10.5555/3019083.3019087
> [7]
> https://indico.cern.ch/event/697389/contributions/3085538/attachments/1712698/2761717/2018_09_10_cling_CUDA.pdf
> [8] CUDA C++ in Jupyter: Adding CUDA Runtime Support to Cling',
> https://zenodo.org/record/3713753#.Xu8jqvJRXxU
> [9] HIP Programming Guide
> https://rocmdocs.amd.com/en/latest/Programming_Guides/HIP-GUIDE.html
>
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