Added Documentation of cppyy Enhancements and how they relate to LibI…

…nterOp (#160)

docs/UsingCppInterOp.rst

@@ -15,7 +15,7 @@ The CppInterop comes with using it is a dynamic shared library,
     libInterop = ctypes.CDLL("./libclangCppInterOp.so")
     
 The above method of usage is for Python; for C, we can include the headers of 
-the library. Including this library in our programme enables the user to use 
+the library. Including this library in our program enables the user to use 
 the abilities of CppInterOp. CppInterOp helps programmers with multiple 
 verifications such as isClass, isBoolean, isStruct, and many more in different 
 languages. With the interop layer, we can access the scopes, namespaces of 
@@ -28,10 +28,10 @@ Using LLVM as external library
 
 In CppInterOp, we are leveraging Clang as a library for interoperability purposes.
 To use Clang, we need to pass the Clang configuration to the CMake build system,
-so that the build system recognises the configuration and enables usage of Clang
+so that the build system recognizes the configuration and enables usage of Clang
 and LLVM.
 We can consider clang-repl as a state manager, where CppInterOp allows you to
-query the state from the state manager. Thereafter, Cppyy uses this to create
+query the state from the state manager. Thereafter, cppyy uses this to create
 Python objects for C++.
 
 .. code-block:: bash
@@ -65,35 +65,257 @@ robust (simple function calls, no inheritance, etc.). The goal is to make it as
 close to the compiler API as possible, and each routine should do just one thing.
 that it was designed for.
 
-Further Enhancing the Dynamic/Automatic bindings in CPPYY
-=========================================================
-The main use case for CppInterOp is the CPPYY service. CPPYY is an
-automatic run-time bindings generator for Python and C++, and supports a wide
-range of C++ features (e.g., template instantiation). It operates on demand and
-generates only what is necessary. It requires a compiler (Cling or Clang-REPL).
-that can be available during programme runtime.
+How cppyy leverages CppInterOp
+===============================
+
+cppyy is a run-time Python-C++ bindings generator for calling C++ from Python
+and Python from C++. Interestingly, it uses C++ interactively by using the
+compiler as a service. This is made possible by the CppInterOp library.
+Following are some of the ways cppyy leverages CppInterOp for better
+performance and usability.
+
+1. **CppInterOp enables interoperability with C++ code**: CppInterOp provides a
+   minimalist and robust interface for language interoperability on the fly,
+   which helps CPPYY generate dynamic Python-C++ bindings by using a C++
+   interpreter (e.g., Clang-REPL/Cling) and LLVM.
+
+2. **Reducing dependencies**: Reducing domain-specific dependencies of cppyy
+   (e.g., on the Cling interpreter and the ROOT framework) to enable more
+   generalized usage.
+
+3. **LLVM Integration**: CppInterOp is designed to be used as a part of the
+   LLVM toolchain (as part of Clang-REPL) that can then be used as a runtime
+   compiler for CPPYY. This simplifies the codebase of CPPYY and enhances its
+   performance.
+
+ 4. **Making C++ More Social**: CppInterOp and cppyy help data scientists that
+    are working with legacy C++ code experiment with simpler, more interactive
+    languages, while also interacting with larger communities.
+
+**CppInterOp enables interoperability with C++ code**
+
+cppyy is a major use case for CppInterOp. cppyy is an automatic run-time
+bindings generator for Python and C++, and supports a wide range of C++
+features, including template instantiation. It operates on demand and generates
+only what is necessary. It requires a compiler (Cling or Clang-REPL) that can
+be available during program runtime.
+
+**Reducing Dependencies**
+
+Recent work done on cppyy has been focused on reducing dependencies on
+domain-specific infrastructure (e.g., the ROOT framework). Using an independent
+library such as CppInterOp helps accomplish that, while also improving the code
+consistency in cppyy.
+
+The CppInterOp library can be configured to use the newly developed Clang-Repl
+backend available in LLVM upstream (or to use the Cling legacy backend, for
+compatibility with High Energy Physics applications).
+
+Only a small set of APIs are needed to connect to the interpreter (Clang-Repl/
+Cling), since other APIs are already available in the standard compiler. This
+is one of the reasons that led to the creation of CppInterOp (a library of
+helper functions), that can help extract out things that are unnecessary for
+for core cppyy functionality.
+
+The cppyy API surface is now incomparably smaller and simpler than what it used
+to be.
+
+**LLVM Integration**
 
 Once CppInterOp is integrated with LLVM's Clang-REPL component (that can then
-be used as a runtime compiler for CPPYY), it will further enhance CPPYY's
+be used as a runtime compiler for cppyy), it will further enhance cppyy's
 performance in the following ways:
 
 
-**Simpler codebase:** The removal of string parsing logic will lead to a simpler
-code base.
+- *Simpler codebase:* The removal of string parsing logic will lead to a
+  simpler code base.
+
+- *Built into the LLVM toolchain:* The CppInterOp depends only on the LLVM
+  toolchain (as part of Clang-REPL).
+
+- *Better C++ Support:* Finer-grained control over template instantiation is
+  available through CppInterOp.
+
+- *Fewer Lines of Code:* A lot of dependencies and workarounds will be
+  removed, reducing the lines of code required to execute cppyy.
+
+- *Well tested interoperability Layer:* The CppInterOp interfaces have full
+  unit test coverage.
+
+**Making C++ More Social**
+
+cppyy is the first use case demonstrating how CppInterOp can enable C++ to be
+more easily interoperable with other languages. This helps many data scientists
+that are working with legacy C++ code and would like to use simpler, more
+interactive languages.
+
+The goal of these enhancements is to eventually land these interoperability
+tools (including CppInterOp) to broader communities like LLVM and Clang, to
+enable C++ to interact with other languages besides Python.
+
+Example: Template Instantiation
+-------------------------------
+
+The developmental cppyy version can run basic examples such as the one
+here. Features such as standalone functions and basic classes are also
+supported.
+
+C++ code (Tmpl.h)
+
+::
+
+   template <typename T>
+   struct Tmpl {
+     T m_num;
+     T add (T n) {
+       return m_num + n;
+   }
+   };
+
+Python Interpreter
+
+::
+
+   >>> import cppyy
+   >>> cppyy.include("Tmpl.h")
+   >>> tmpl = Tmpl[int]()
+   >>> tmpl.m_num = 4
+   >>> print(tmpl.add(5))
+   9
+   >>> tmpl = Tmpl[float]()
+   >>> tmpl.m_num = 3.0
+   >>> print(tmpl.add(4.0))
+   7.0
+
+Where does the cppyy code reside?
+---------------------------------
+
+Following are the main components where cppyy logic (with Compiler Research
+Organization’s customizations started by `sudo-panda`_) resides:
+
+-  `cppyy <https://github.com/compiler-research/cppyy>`_
+-  `cppyy-backend <https://github.com/compiler-research/cppyy-backend>`_
+-  `CPyCppyy <https://github.com/compiler-research/CPyCppyy>`_
+
+..
+
+   Note: These are forks of the `upstream cppyy`_ repos created by `wlav`_.
+
+CppInterOp is a separate library that helps these packages communicate with C++
+code.
+
+-  `CppInterOp <https://github.com/compiler-research/CppInterOp/tree/main>`_
 
-**LLVM Integration:** The CppInterOp interfaces will be part of the LLVM 
-toolchain (as part of Clang-REPL).
+How cppyy components interact with each other
+---------------------------------------------
 
-**Better C++ Support:** C++ features such as Partial Template Specialisation will
-be available through CppInterOp.
+cppyy is made up of the following packages: 
 
-**Fewer Lines of Code:** A lot of dependencies and workarounds will be removed,
-reducing the lines of code required to execute CPPYY.
+- A frontend: cppyy, 
 
-**Well tested interoperability Layer:** The CppInterOp interfaces have full
-unit test coverage.
+- A backend: cppyy-backend, and 
 
+- An extension: CPyCppyy.
+
+Besides these, the ``CppInterOp`` library serves as an additional layer on top
+of Cling/Clang-REPL that helps these packages in communicating with C++ code.
+
+**1. cppyy-backend**
+
+The `cppyy-backend`_ package forms a layer over ``cppyy``, for example,
+modifying some functionality to provide the functions required for
+``CPyCppyy``. 
+
+  `CPyCppyy`_ is a CPython extension module built on top of the same backend
+  API as PyPy/_cppyy. It thus requires the installation of the cppyy-backend
+  for use, which will pull in Cling. 
+
+``cppyy-backend`` also adds some `utilities`_ to help with repackaging and
+redistribution.
+
+For example, ``cppyy-backend`` initializes the interpreter (using the
+``clingwrapper::ApplicationStarter`` function), adds the required ``include``
+paths, and adds the headers required for cppyy to work. It also adds some
+checks and combines two or more functions to help CPyCppyy work.
+
+These changes help ensure that any change in ``cppyy`` doesn’t directly
+affect ``CPyCppyy``, and the API for ``CPyCppyy`` remains unchanged.
+
+**2. CPyCppyy**
+
+The ``CPyCppyy`` package uses the functionality provided by ``cppyy-backend``
+and provides Python objects for C++ entities. ``CPyCppyy`` uses separate proxy
+classes for each type of object. It also includes helper classes, for example,
+``Converters.cxx`` helps convert Python type objects to C++ type objects, while
+``Executors.cxx`` is used to execute a function and convert its return value to
+a Python object, so that it can be used inside Python.
+
+**3. cppyy**
+
+The cppyy package provides the front-end for Python. It is `included in code`_
+(using ``import cppyy``) to import cppyy in Python. It initializes things on
+the backend side, provides helper functions (e.g., ``cppdef()``, ``cppexec()``,
+etc.) that the user can utilize, and it calls the relevant backend functions
+required to initialize cppyy.
+
+
+Further Reading
+---------------
+
+-  `High-performance Python-C++ bindings with PyPy and
+   Cling <http://cern.ch/wlav/Cppyy_LavrijsenDutta_PyHPC16.pdf>`_
+
+-  `Efficient and Accurate Automatic Python Bindings with cppyy &
+   Cling <https://arxiv.org/abs/2304.02712>`_
+
+-  cppyy documentation:
+   `cppyy.readthedocs.io <http://cppyy.readthedocs.io/>`_.
+
+-  Notebook-based tutorial: `Cppyy
+   Tutorial <https://github.com/wlav/cppyy/blob/master/doc/tutorial/CppyyTutorial.ipynb>`_.
+
+-  `C++ Language Interoperability
+   Layer <https://compiler-research.org/libinterop/>`_
+
+**Credits:**
+
+-  `Wim Lavrijsen <https://github.com/wlav>`_ (Lawrence Berkeley National Lab.)
+   for his original work in cppyy and mentorship towards student contributors.
+
+-  `Vassil Vasilev <https://github.com/vgvassilev>`_ (Princeton University)
+   for his mentorship towards Compiler Research Org's student contributors.
+
+-  `Baidyanath Kundu <https://github.com/sudo-panda>`_ (Princeton University)
+   for his research work on cppyy and Numba with `Compiler Research Organization`_ 
+   (as discussed in this document).
+
+- `Aaron Jomy <https://github.com/maximusron>`_ (Princeton University) for
+  continuing this research work with `Compiler Research Organization`_.
 
 In case you haven't already installed CppInterop, please do so before proceeding
 with the Installation And Usage Guide.
-:doc:`Installation and usage <InstallationAndUsage>`
+:doc:`Installation and usage <InstallationAndUsage>`
+
+.. _Compiler Research Organization: https://compiler-research.org/
+
+.. _upstream cppyy: https://github.com/wlav/cppyy
+
+.. _wlav: https://github.com/wlav
+
+.. _utilities: https://cppyy.readthedocs.io/en/latest/utilities.html
+
+.. _included in code: https://cppyy.readthedocs.io/en/latest/starting.html
+
+.. _sudo-panda: https://github.com/sudo-panda
+
+.. _cppyy: https://cppyy.readthedocs.io/en/latest/index.html
+
+.. _CppInterOp: https://github.com/compiler-research/CppInterOp
+
+.. _ROOT meta: https://github.com/root-project/root/tree/master/core/meta
+
+.. _enhancements in cppyy: https://arxiv.org/abs/2304.02712
+
+.. _CPyCppyy: https://github.com/wlav/CPyCppyy
+
+.. _cppyy-backend: https://github.com/wlav/cppyy-backend