[cfe-dev] [RFC] Moving (parts of) the Cling REPL in Clang
Vassil Vassilev via cfe-dev
cfe-dev at lists.llvm.org
Sat Jul 11 00:02:26 PDT 2020
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).
>>
>>
>>
>> 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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