[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 14:08:29 PDT 2020 

On 7/11/20 12:00 AM, JF Bastien wrote:
>
>
>> On Jul 10, 2020, at 1:55 PM, Hal Finkel <hfinkel at anl.gov 
>> <mailto:hfinkel at anl.gov>> wrote:
>>
>>
>> On 7/10/20 1:57 PM, Vassil Vassilev wrote:
>>> On 7/10/20 6:43 AM, JF Bastien wrote:
>>>> I like cling, and having it integrated with the rest of the project 
>>>> would be neat. I agree with Hal’s suggestion to explain the design 
>>>> of what remains. It sounds like a pretty small amount of code.
>>>
>>>
>>> JF, Hal, did you mean you want a design document of how cling in 
>>> general or a design RFC for the patches we have? A design document 
>>> for cling would be quite large and will take us some time to write 
>>> up. OTOH, we could relatively easily give a rationale for each patch.
>>
>>
>> I had in mind something that's probably in between. Something that 
>> explains the patches and enough about how they fit into a larger 
>> system that we can reason about the context.
>
> Maybe a purpose would be more useful to understand your request? I 
> assume you meant “I’d like us to understand what we’re signing up to 
> maintain, and why it’s useful to do things this way”. In particular, 
> if there’s undue burden in a particular component, and the code could 
> be changed to work differently with less support overhead, then we’d 
> want to identify this fact ahead of time.
>
> I’m guessing at what Hal is asking, LMK if that’s not what you had in 
> mind!


   Thanks for the clarification. Sure, we can do that I was hoping that 
to be part of the particular patch review process. Also, if that's the 
preference, we can write a short-ish doc with some patch classification 
and explanations. Btw, I've uploaded the cling-specific patches against 
the clang-9 codebase: 
https://github.com/vgvassilev/clang/commits/upgrade_llvm90 Our 
production LLVM-5 is patch free, I had to introduce some patches in 
llvm-9 but thanks to Lang I know how to get rid of them.


>
>
>>  -Hal
>>
>>
>>>
>>>
>>>>
>>>>
>>>>> On Jul 9, 2020, at 7:25 PM, Hal Finkel via cfe-dev 
>>>>> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>>>
>>>>> I think that it would be great to have infrastructure for 
>>>>> incremental C++ compilation, supporting interactive use, 
>>>>> just-in-time compilation, and so on. I think that the best way to 
>>>>> deal with the patches, etc., as well as IncrementalAction, is to 
>>>>> first send an RFC explaining the overall design.
>>>>>
>>>>> -Hal
>>>>>
>>>>> On 7/9/20 3:46 PM, Vassil Vassilev via cfe-dev 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
>>>>>>
>>>>>> _______________________________________________
>>>>>> cfe-dev mailing list
>>>>>> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>> --
>>>>> Hal Finkel
>>>>> Lead, Compiler Technology and Programming Languages
>>>>> Leadership Computing Facility
>>>>> Argonne National Laboratory
>>>>>
>>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>

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