Add links to the "Introduction to Clang for Clad contributors" doc

This commit updates the contributing guidelines and the developers
documentation to have link to the newly created doc for beginner
contributors.

CONTRIBUTING.md

@@ -69,4 +69,9 @@ review quickly and successfully share several common traits:
 ### Developer Documentation
 
 We have documented several useful hints that usually help when addressing issues
-as they come during developement time in our [developer documentation](https://clad.readthedocs.io/en/latest/user/DevelopersDocumentation.html).
+as they come during developement time in our [developer documentation](https://clad.readthedocs.io/en/latest/user/DevelopersDocumentation.html). 
+
+In case you are interested in seeing the big picture and are just getting 
+started with Clang, there is a document that might interest you: [Introduction to Clang for Clad contributors](https://clad.readthedocs.io/en/latest/user/IntroductionToClangForCladContributors.html). 
+It covers most of the basic concepts in Clang that Clad uses and describes the operation
+of the latter with examples for newcomers specifically.

docs/userDocs/source/index.rst

@@ -92,6 +92,7 @@ The User Guide
    user/UsingVectorMode.rst
    user/FAQ
    user/DevelopersDocumentation
+   user/IntroductionToClangForCladContributors
    user/FloatingPointErrorEstimation
 
 Citing Clad

docs/userDocs/source/user/DevelopersDocumentation.rst

@@ -126,6 +126,12 @@ to understand for the new developers.
 Internal documentation can be visited 
 `here </en/latest/internalDocs/html/index.html>`_
 
+In case you are interested in seeing the big picture and are just getting 
+started with Clang, there is a document that might help you:
+`Introduction to Clang for Clad contributors </en/latest/user/IntroductionToClangForCladContributors.html>`_. It covers 
+most of the basic concepts in Clang that Clad uses and describes the operation
+of the latter with examples for newcomers specifically.
+
 Clad's Tests
 ============
 

docs/userDocs/source/user/IntroductionToClangForCladContributors.rst

@@ -0,0 +1,159 @@
+Introduction to Clang for Clad contributors
+**********************************************
+
+Since there’s a lack of official documentation for the LLVM Clang, while the
+existing docs are not meant to explain how to modify the Clang AST, we are 
+writing this text to help the newcomers get into the technical aspects of Clang
+useful for contributing to Clad and manipulating the Clang AST in general.
+
+The big picture
+=================
+So, as you probably know, Clang is a compiler. More precisely, it is a compiler
+front end that operates with the LLVM compiler backend. This means that the 
+code you're trying to compile will first be processed by Clang, which parses 
+the source code and performs initial processing before passing it to the LLVM 
+backend for optimisation and code generation. In this picture, Clad is an 
+extension for Clang to generate derivatives of C++ functions along the way. 
+That is, **Clad synthesises the code for the derivatives** while Clang is 
+processing the actual code you have written.
+
+The AST
+=========
+.. _clang::Expr: https://clang.llvm.org/doxygen/classclang_1_1ValueStmt__inherit__graph.png
+.. |clang::Expr| replace:: ``clang::Expr`` 
+.. _clang::Decl: https://clang.llvm.org/doxygen/classclang_1_1Decl.html
+.. |clang::Decl| replace:: ``clang::Decl`` 
+.. _clang::Stmt: https://clang.llvm.org/doxygen/classclang_1_1Stmt.html
+.. |clang::Stmt| replace:: ``clang::Stmt``
+.. _clang::ValueStmt: https://clang.llvm.org/doxygen/classclang_1_1ValueStmt.html
+.. |clang::ValueStmt| replace:: ``clang::ValueStmt`` 
+.. _clang::CompoundStmt: https://clang.llvm.org/doxygen/classclang_1_1CompoundStmt.html
+.. |clang::CompoundStmt| replace:: ``clang::CompoundStmt``
+
+Clang, parses the input code into an **Abstract Syntax Tree (AST)**. It is at 
+this stage that Clad operates as well. The nodes in the Clang AST represent 
+various constructs of the source code, including: **declarations, statements, 
+expressions**, etc. As one can guess from the name, **declarations (** 
+|clang::Decl|_ **)** represent places in the source code where something is 
+declared (a variable (clang::VarDecl), a function, etc.); **expressions (** 
+|clang::Expr|_ **)** are basically something that can be evaluated to a 
+meaningful value, for instance binary operations (``clang::BinaryOperator``) 
+like addition or comparison; **statements (** |clang::Stmt|_ **)** could be a 
+lot of different things, like loops (``clang::WhileStmt``), conditions 
+(``clang::IfStmt``), breaks, etc – pretty much anything you have in your code. 
+An important note is that declarations (``clang::Decl``) and statements 
+(``clang::Stmt``) are completely different classes (so, **a declaration is not 
+a statement**), but there’s a separate declaration statement class, 
+|clang::Decl|_, which describes a declaration statement to intertwine the 
+statements and declarations. An expression (``clang::Expr``), on the other 
+hand, is always a statement, since ``clang::Expr`` inherits from 
+|clang::ValueStmt|_ which, in turn, inherits from ``clang::Stmt`` and is a 
+statement.
+Statements can be grouped into one with a compound statement 
+(|clang::CompoundStmt|_).
+
+Flow graph of Clad’s compilation
+==================================
+The graph below indicates the order of execution during compilation of the 
+source code. Clang offers some API functions that work as entry points to the 
+AST’s processing. These functions can be modified by the user and are invoked 
+by clang. In our case, Clad has altered these functions accordingly, in order 
+to compute and include the function’s gradient to the original source code. 
+`This source <https://clang.llvm.org/docs/RAVFrontendAction.html>`__ helps to 
+distinguish the clang functions, their purpose and how they are connected. The
+ most useful ones are explained in better detail in later sections.
+
+.. figure:: ../_static/clad-flow-graph.png
+   :width: 850px
+   :align: center
+
+Identifying functions to derive
+=================================
+During parsing of the source code through ``HandleTopLevelDecl()``, a 
+``RecursiveASTVisitor`` is created. This visitor traverses the nodes by using 
+the equivalent ``Visit()`` functions. For instance, a call expression, which 
+corresponds to a function call node, is processed through ``VisitCallExpr()`` 
+etc. Clad distinguishes the functions related to a differentiation request by 
+annotating a compiler attribute that matches the first letter of the 
+differentiation method to be used (e.g. “G” for gradient function). This way, 
+when ``VisitCallExpr()`` is called upon those nodes, the differentiation 
+request is identified, initialised and added to a list. 
+Similarly, any statement supported by Clad has a corresponding ``Visit()`` 
+method. An if statement would be visited by ``VisitIfStmt()``, a return 
+statement by ``VisitReturnStmt()``, etc. The general rule is that the ``Visit``
+method is called ``Visit+the name of the statement in Clang``.
+
+Code generation and insertion
+===============================
+After the parsing is completed and all differentiation requests have been 
+identified, the system can begin processing to compute the derived functions 
+through ``HandleTranslationUnit()``. A ``DerivativeBuilder`` is created and, 
+depending on the Clad function used and its corresponding mode (forward, 
+reverse, etc.), the appropriate ``Visitor`` is defined. The ``Visitor``’s 
+``Derive()`` method is invoked, which contains a nested call to 
+``DifferentiateWithClad()``. This triggers a secondary processing of the nodes 
+in the original function. 
+During this traversal, for each node encountered, the corresponding node in the
+derived function is created and returned to the parent node. The parent node 
+then emplaces this newly created node into the current code block of the 
+derived function. This process repeats until the traversal reaches the top 
+level of the original function, resulting in the complete computation of the 
+derived function.
+Once the derived function is fully computed, its declaration is created, 
+including the appropriate body and parameters. A reference to this derived 
+function is then added in ``updateCall()``, where the function to be returned 
+by ``clad::differentiate()`` / ``clad::gradient()`` is replaced with its 
+derivative.
+
+Statement differentiation
+===========================
+Visit methods that we have just talked about earlier typically return a 
+``clad::StmtDiff`` object. This is a Clad’s structure, so you can find its 
+definition in the code and explore its methods. It is basically a type that 
+contains 4 values, each one being a pointer to a ``clang::Stmt``. Of these, the
+following two values are crucial to understand:
+
+- ``clad::StmtDiff::getStmt()`` - this method gives you the statement contained
+  in this StmtDiff object.
+
+- ``clad::StmtDiff::getStmt_dx()`` - this method gives you the derivative of 
+  the statement contained by this object.
+
+Let’s see how ``clang::NullStmt`` is handled in the reverse mode (reverse mode
+differentiation is called by calling ``clad::gradient``) as an example. Since 
+the null statement (which you can have by writing ``;;`` at the end of a line 
+in the source code) represents an empty statement in Clang, it doesn’t really 
+affect the derivative in any way. So if this happens to be in the source code, 
+we just want to ignore it. In the reverse mode, Clad generates derivative 
+functions that consist of two parts: a forward pass and a reverse pass. Clad 
+produces a ``clad::StmtDiff`` object for each statement of the original 
+function where the object’s first part is what will be put in the forward pass
+part of the produced derivative function and should basically do the same thing
+as the original statement, whereas the object’s second part is basically the 
+derivative, which is put into the reverse pass. So, in the case of the null 
+statement, both of these should be nothing! Which is expressed by returning an 
+empty ``clad::StmtDiff`` from the visitor.
+
+.. code-block:: c++
+
+    StmtDiff VisitNullStmt(const clang::NullStmt* NS) { 
+        return StmtDiff{}; 
+    };
+
+Less relevant notes
+=====================
+This section is not that important, but it’s just a bunch of facts that might 
+confuse a newcomer.
+
+- Clang AST might not seem like a tree (:D)! In a mathematical sense, a tree is
+  a graph without cycles. While the AST itself is intended to be a tree and 
+  thus acyclic, practical considerations and additional structures can 
+  introduce references that might resemble cycles. So, some recursive 
+  constructs or templates  in the source code might confuse you.
+
+- Clang can compile many C-like languages, specifically: C, C++, Objective-C, 
+  and Objective-C++. So, if you look at the documentation, not only are you 
+  going to see some general Clang classes and classes designed specifically for
+  C++ compilation (those mostly start with a CXX- prefix), but also things that
+  have an ObjC- prefix like ``clang::ObjCAtTryStmt`` that represent statements 
+  of the Objective-C programming language that you likely don’t need.