[cfe-dev] [RFC] Moving (parts of) the Cling REPL in Clang

Mon Jul 13 05:57:20 PDT 2020

Vassil is right, it's just one Clang instance per expression. This is by design as it allows LLDB's expression evaluator to be flexible and it also makes the code simpler and clearer.

Regarding the REPL part: LLDB's expression parser for C++ isn't meant to be a full REPL. There is only some limited data sharing between each expression (e.g., result types and a few specifically marked declarations from the user) as the goal is to make an AST that fits the context where the expression is evaluated. That means that we need to support that a user can type "MyStruct" and in one expression it might refer to a struct type, but in the next expression (which could by at some other point in the program) it might be a typedef, or a macro, or a Objective-C class, or a global/local variable name, or a member variable as the evaluation context changed into a class, or a keyword in the current C-language, or also a struct but with a different definition or not even anything at all.

I really don't see a sane way to support just this one simple feature with with Cling's single shared AST + incremental CodeGen approach.

Also LLDB's expression parser doesn't really have a lot of non-LLDB specific code left that could be shared with other projects. We used to have a bunch of Clang in the expression parser but most of it is be gone by now. The rest is really LLDB-specific (e.g., configuring Clang to match the target we are trying to debug, a lot of code for setting up the right evaluation context of the location where the program is stopped).

Having said that, I think Cling should be upstream any shared code we can find between Cling and LLDB should be shared. Feel free to add me to patches and I'll see what I can find.

- Raphael

> On 11 Jul 2020, at 09:02, Vassil Vassilev <v.g.vassilev at gmail.com> wrote:
> 
> On 7/11/20 12:58 AM, Richard Smith wrote:
>> On Fri, 10 Jul 2020 at 13:59, Vassil Vassilev via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> 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?
>> 
>> Yes, the above all makes sense to me. I agree that there should be only one thing named 'cling', and that it should broadly have the feature set that current 'cling' has. I think there are a couple of ways we can get there while still providing the a minimalist interpreter to a broader audience: either we can build a simpler clang-interpreter and a more advanced cling binary from a common set of libraries, or we could produce a configurable binary that's able to serve both rules depending on configuration or a plugin or scripting system.
> 
>   Good point. We could make it extendable, and actually that should be a design goal. The question how exactly is not very clear to me. Can you elaborate on what you had in mind as configuration or scripting system (plugin system I think I know what you meant). I will give an example with 3 distinct features in cling which we have implemented over the years and had different requirements:
> 
>   * AST-based automatic differentiation <https://llvm.org/devmtg/2013-11/slides/Vassilev-Poster.pdf> with the clad library <https://github.com/vgvassilev/clad> -- here we essentially extend cling's runtime by providing a `clad::differentiate`, `clad::gradient`, `clad::hessian` and `clad::jacobian` primitives. Each primitive is a specially annotated wrapper over a function, say `double pow2(double x) { return x*x; }; auto pow2dx = clad::differentiate(pow2,/*wrt*/0);`. Here we let clang build a valid AST and the plugin creates the first order derivative and swaps the DeclRefExpr just before codegen so that we call the derivative instead. This is achievable by the current clang plugin system ( a bit problematic on windows as clang plugins do not work there ).
> 
>   * Language extensions which require Sema support -- we have a legacy feature which should define a variable on the prompt if not defined (something like implicit auto) `cling[] i = 13;` should be translated into `cling[] auto i = 13;` if I is undefined. We solve that by adding some last resort lookup callback which marks `i` of dependent type so that we can produce an AST which we can later 'fix'.
> 
>   * Language extensions which require delayed lookup rules (aka dynamic scope) -- ROOT has an I/O system bound to cling people can write:`if (TFile::Open("file_that_has_hist_cpp_obj.root")) hist->Draw();`. Here we use the approach from the previous bullet and synthesize `if (TFile::Open("file_that_has_hist_cpp_obj.root")) eval<void>("hist->Draw()", /*escape some context*/...);`.
> 
> 
> 
>   The implementation of these three features can be considered as possible with current clang. The issue is that it seems more like hacking clang rather than extending it. If we can come up with a sound way of implementing these features that would be awesome.
> 
> 
> 
>> 
>> One other thing I think we should consider: there will be substantial overlap between the incremental compilation, code generation, REPL, etc. of cling and that of lldb.
> 
>   I would love to hear opinions from the lldb folks. We have chatted number of times and I have looked at how they do it. I think lldb spawns (used to spawn last time I looked) a compiler instance per input line. That is not acceptable for cling due to its high-performance requirements. Most of the issues that need solving for lldb comes from materializing debug information to AST. LLDB folks, correct me if I am wrong.
> 
>   That being said doesn't mean that we should not aim for centralizing the incremental compilation for both projects. We should but may be challenging because of the different focus which defines project priorities.
> 
> 
> 
>> For the initial integration of cling into LLVM, there's probably not much we can do about that, but it would seem beneficial for both cling and lldb if common parts could be shared where possible. As an extreme example, if we could fully unify the projects to the point where a user could switch into an 'lldb mode' in the middle of a cling session to do step-by-step debugging of code entered into the REPL, that would seem like an incredibly useful feature. Perhaps there's some common set of base functionality that can be factored out of lldb and cling and unified. It would likely be a good idea to start talking to the lldb folks about that early, in case it guides your work porting cling to trunk.
> 
>   Indeed. There have been user requests to be able to run step-by-step in cling. That would be the ultimate long term goal!
> 
> 
> 
>>> 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 <https://groups.google.com/g/compiler-research-announce>).
>>> 
>>> 
>>> 
>>> Many thanks!
>>> 
>>> 
>>> David & Vassil
>>> 
>>> References
>>> ===
>>> [1] ROOT GitHub https://github.com/root-project/root <https://github.com/root-project/root>
>>> [2] ROOT https://root.cern <https://root.cern/>
>>> [3] Cling https://github.com/root-project/cling <https://github.com/root-project/cling>
>>> [4] Xeus-Cling 
>>> https://blog.jupyter.org/xeus-is-now-a-jupyter-subproject-c4ec5a1bf30b <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 <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 <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 <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 <https://zenodo.org/record/3713753#.Xu8jqvJRXxU>
>>> [9] HIP Programming Guide 
>>> https://rocmdocs.amd.com/en/latest/Programming_Guides/HIP-GUIDE.html <https://rocmdocs.amd.com/en/latest/Programming_Guides/HIP-GUIDE.html>
>>> 
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>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>> 
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