Version 0.5 of the "TableGen Backend Developer's Guide"

[Paul-C-Anagnostopoulos]

Summary

This is version 0.5 of the new document titled "TableGen Backend Developer's Guide." It is a first cut a describing how to write a new backend for TableGen. It describes the data structures available to the backend, but not in complete detail. That is left to the Doxygen pages. It also discusses various top-level concepts related to accessing classes and records, fields, and error messages.

Two other documents are updated to refer to the new document. And there is a backend skeleton C++ file.

[lattner]

Very cool! + @jpienaar in case you're interested.

llvm/docs/TableGen/BackEnds.rst

17

Might mention MLIR as well, which uses TableGen extensively as the basis for its ODS system:
https://mlir.llvm.org/docs/OpDefinitions/

llvm/lib/TableGen/TableGenBackendSkeleton.cpp

1

I think that adding an example backend is a good idea, but please make it valid C++ code that follows the coding standard etc.

[Paul-C-Anagnostopoulos]

llvm/docs/TableGen/BackEnds.rst

17

I will add a mention of MLIR.

llvm/lib/TableGen/TableGenBackendSkeleton.cpp

1

Hmm. Then I have to replace all the metasyntactic items with syntactically-valid items. The idea was to have the compilation fail if someone forgot to replace one of the meta items with their real item. How about if I use real items but uppercase them all so they stand out? I would note that convention in the initial comment.

[lattner]

I think that just naming it something obvious like "SkeletonEmitter" is enough. That allows you to follow the naming convention. The problem with doing something weird here is that people may propagate the weirdness. We would like to encourage people to follow best practices by default.

[Paul-C-Anagnostopoulos]

Okay, I will do that.

[Paul-C-Anagnostopoulos]

I addressed the first round of comments.

Is there a reason why MLIR patches can't be mixed with other patches? I need to update the "Operation Definition Specification" document to refer to the new TableGen documents.

[lattner]

No reason that i'm aware of. Feel free to mix mlir updates in.

[Paul-C-Anagnostopoulos]

Updated two MLIR documentation files to refer to the new TableGen documents.

[lattner]

very nice

[Paul-C-Anagnostopoulos]

Thanks! I've pushed the patch.

[jpienaar]

Indeed, thanks this is super nice (I missed the ping here but saw the commit on GitHub and pointed folks to it)

[Paul-C-Anagnostopoulos]

Thanks! If anyone has comments or suggestions, please feel free to post them at llvm-dev.

llvm/docs/TableGen/BackEnds.rst

 Introduction
 ============
 
-TableGen backends are at the core of TableGen's functionality. The source files
-provide the semantics to a generated (in memory) structure, but it's up to the
-backend to print this out in a way that is meaningful to the user (normally a
-C program including a file or a textual list of warnings, options and error
-messages).
-
-TableGen is used by both LLVM and Clang with very different goals. LLVM uses it
-as a way to automate the generation of massive amounts of information regarding
-instructions, schedules, cores and architecture features. Some backends generate
-output that is consumed by more than one source file, so they need to be created
-in a way that is easy to use pre-processor tricks. Some backends can also print
-C code structures, so that they can be directly included as-is.
+TableGen backends are at the core of TableGen's functionality. The source
+files provide the classes and records that are parsed and end up as a
+collection of record instances, but it's up to the backend to interpret and
+print the records in a way that is meaningful to the user (normally a C++
+include file or a textual list of warnings, options, and error messages).
+
+TableGen is used by both LLVM and Clang with very different goals. LLVM uses
+it as a way to automate the generation of massive amounts of information
+regarding instructions, schedules, cores, and architecture features. Some
+backends generate output that is consumed by more than one source file, so
+they need to be created in a way that makes it is easy for preprocessor
+tricks to be used. Some backends can also print C++ code structures, so that
+they can be directly included as-is.
 
 Clang, on the other hand, uses it mainly for diagnostic messages (errors,
 warnings, tips) and attributes, so more on the textual end of the scale.
 
+See the :doc:`TableGen Programmer's Reference <./ProgRef>` for an in-depth
+description of TableGen, and :doc:`TableGen Backend Developer's Guide
+<./BackGuide>` for a guide to writing a new backend.
+
 LLVM BackEnds
 =============
 

   These values are not expected to be needed by backends. The standard
   ``printable`` field can be used to extract a representation of them
   in TableGen source syntax if necessary.
-
-How to write a back-end
-=======================
-
-TODO.
-
-Until we get a step-by-step HowTo for writing TableGen backends, you can at
-least grab the boilerplate (build system, new files, etc.) from Clang's
-r173931.
-
-TODO: How they work, how to write one.  This section should not contain details
-about any particular backend, except maybe ``-print-enums`` as an example.  This
-should highlight the APIs in ``TableGen/Record.h``.
-

llvm/docs/TableGen/BackGuide.rst

+===================================
+TableGen Backend Developer's Guide
+===================================
+
+.. sectnum::
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+The purpose of TableGen is to generate complex output files based on
+information from source files that are significantly easier to code than the
+output files would be, and also easier to maintain and modify over time. The
+information is coded in a declarative style involving classes and records,
+which are then processed by TableGen. The internalized records are passed on
+to various backends, which extract information from a subset of the records
+and generate an output file. These output files are typically ``.inc`` files
+for C++, but may be any type of file that the backend developer needs.
+
+This document is a guide to writing a backend for TableGen. It is not a
+complete reference manual, but rather a guide to using the facilities
+provided by TableGen for the backends. For a complete reference to the
+various data structures and functions involved, see the Doxygen
+documentation.
+
+This document assumes that you have read the :doc:`TableGen Programmer's
+Reference <./ProgRef>`, which provides a detailed reference for coding
+TableGen source files. This document and the relevant Doxygen pages will be
+improved over time.
+
+Data Structures
+===============
+
+The following sections describe the data structures that contain the classes
+and records that are collected from the TableGen source files by the
+TableGen parser. Note that the term *class* refers to an abstract record
+class, while the term *record* refers to a concrete record.
+
+Unless otherwise noted, functions associated with classes are instance
+functions.
+
+``RecordKeeper``
+----------------
+
+An instance of the ``RecordKeeper`` class acts as the container for all the
+classes and records parsed and collected by TableGen. The ``RecordKeeper``
+instance is passed to the backend when it is invoked by TableGen. This class
+is usually abbreviated ``RK``.
+
+There are two maps in the recordkeeper, one for classes and one for records
+(the latter often referred to as *defs*). Each map maps the class or record
+name to an instance of the ``Record`` class (see `Record`_), which contains
+all the information about that class or record. The ``RecordKeeper`` class
+defines a type that must be used to declare these maps if they are requested
+directly.
+
+.. code-block:: text
+
+  using RecordMap = std::map<std::string, std::unique_ptr<Record>,
+                             std::less<>>;
+
+In addition to the two maps, the ``RecordKeeper`` instance contains:
+
+* A map that maps the names of global variables to their values.
+  Global variables are defined in TableGen files with outer
+  ``defvar`` statements.
+
+* A counter for naming anonymous records.
+
+The ``RecordKeeper`` class provides a few useful functions.
+
+* Functions to get the complete class and record maps.
+
+* Functions to get a subset of the records based on their parent classes.
+
+* Functions to get individual classes, records, and globals.
+
+A ``RecordKeeper`` instance can be printed to an output stream with the ``<<``
+operator.
+
+``Record``
+----------
+
+Each class or record built by TableGen is represented by an instance of
+the ``Record`` class. The ``RecordKeeper`` instance contains one map for the
+classes and one for the records. The primary data members of a record are
+the record name, the vector of field names and their values, and the vector of
+superclasses of the record.
+
+The record name is stored as a pointer to an ``Init`` (see `Init`_), which
+is a class whose instances hold TableGen values (sometimes refered to as
+*initializers*). The field names and values are stored in a vector of
+``RecordVal`` instances (see `RecordVal`_), each of which contains both the
+field name and its value. The superclass vector contains a sequence of
+pairs, with each pair including the superclass record and its source
+file location.
+
+In addition to those members, a ``Record`` instance contains:
+
+* A vector of source file locations that includes the record definition
+  itself, plus the locations of any multiclasses involved in its definition.
+
+* For a class record, a vector of the class's template arguments.
+
+* An instance of ``DefInit`` (see `DefInit`_) corresponding to this record.
+
+* A unique record ID.
+
+* A boolean that specifies whether this is a class definition.
+
+* A boolean that specifies whether this is an anonymous record.
+
+The ``Record`` class provides many useful functions.
+
+* Functions to get the record name, fields, source file locations,
+  template arguments, and unique ID.
+
+* Functions to get all the record's superclasses or just its direct
+  superclasses.
+
+* Functions to get a particular field value by specifying its name in various
+  forms, and returning its value in various forms
+  (see `Getting Record Names and Fields`_).
+
+* Boolean functions to check the various attributes of the record.
+
+A ``Record`` instance can be printed to an output stream with the ``<<``
+operator.
+
+
+``RecordVal``
+-------------
+
+Each field of a record is stored in an instance of the ``RecordVal`` class.
+The ``Record`` instance includes a vector of these value instances. A
+``RecordVal`` instance contains the name of the field, stored in an ``Init``
+instance. It also contains the value of the field, likewise stored in an
+``Init``. (A better name for this class might be ``RecordField``.)
+
+In addition to those primary members, the ``RecordVal`` has other data members.
+
+* The source file location of the field definition.
+
+* The type of the field, stored as an instance
+  of the ``RecTy`` class (see `RecTy`_).
+
+The ``RecordVal`` class provides some useful functions.
+
+* Functions to get the name of the field in various forms.
+
+* A function to get the type of the field.
+
+* A function to get the value of the field.
+
+* A function to get the source file location.
+
+Note that field values are more easily obtained directly from the ``Record``
+instance (see `Record`_).
+
+A ``RecordVal`` instance can be printed to an output stream with the ``<<``
+operator.
+
+``RecTy``
+---------
+
+The ``RecTy`` class is used to represent the types of field values. It is
+the base class for a series of subclasses, one for each of the
+available field types. The ``RecTy`` class has one data member that is an
+enumerated type specifying the specific type of field value. (A better
+name for this class might be ``FieldTy``.)
+
+The ``RecTy`` class provides a few useful functions.
+
+* A virtual function to get the type name as a string.
+
+* A virtual function to check whether all the values of this type can
+  be converted to another given type.
+
+* A virtual function to check whether this type is a subtype of
+  another given type.
+
+* A function to get the corresponding ``list``
+  type for lists with elements of this type. For example, the function
+  returns the ``list<int>`` type when called with the ``int`` type.
+
+The subclasses that inherit from ``RecTy`` are
+``BitRecTy``,
+``BitsRecTy``,
+``CodeRecTy``,
+``DagRecTy``,
+``IntRecTy``,
+``ListRecTy``,
+``RecordRecTy``, and
+``StringRecTy``.
+Some of these classes have additional members that
+are described in the following subsections.
+
+*All* of the classes derived from ``RecTy`` provide the ``get()`` function.
+It returns an instance of ``Recty`` corresponding to the derived class. 
+Some of the ``get()`` functions require an argument to
+specify which particular variant of the type is desired. These arguments are
+described in the following subsections.
+
+A ``RecTy`` instance can be printed to an output stream with the ``<<``
+operator.
+
+.. warning::
+  It is not specified whether there is a single ``RecTy`` instance of a
+  particular type or multiple instances.
+
+
+``BitsRecTy``
+~~~~~~~~~~~~~
+
+This class includes a data member with the size of the ``bits`` value and a
+function to get that size.
+
+The ``get()`` function takes the length of the sequence, *n*, and returns the
+``BitsRecTy`` type corresponding to ``bits<``\ *n*\ ``>``.
+
+``ListRecTy``
+~~~~~~~~~~~~~
+
+This class includes a data member that specifies the type of the list's
+elements and a function to get that type.
+
+The ``get()`` function takes the ``RecTy`` *type* of the list members and
+returns the ``ListRecTy`` type corresponding to ``list<``\ *type*\ ``>``.
+
+
+``RecordRecTy``
+~~~~~~~~~~~~~~~
+
+This class includes data members that contain the list of parent classes of
+this record. It also provides a function to obtain the array of classes and
+two functions to get the iterator ``begin()`` and ``end()`` values. The
+class defines a type for the return values of the latter two functions.
+
+.. code-block:: text
+
+  using const_record_iterator = Record * const *;
+
+The ``get()`` function takes an ``ArrayRef`` of pointers to the ``Record``
+instances of the *direct* superclasses of the record and returns the ``RecordRecTy``
+corresponding to the record inheriting from those superclasses.
+
+``Init``
+--------
+
+The ``Init`` class is used to represent TableGen values.  The name derives
+from *initialization value*. This class should not be confused with the
+``RecordVal`` class, which represents record fields, both their names and
+values. The ``Init`` class is the base class for a series of
+subclasses, one for each of the available value types.
+
+The primary data member
+of ``Init`` is an enumerated type that represents the specific type of the
+value.
+
+The ``Init`` class provides a few useful functions.
+
+* A boolean virtual function to determine whether a value is completely
+  specified; that is, has no uninitialized subvalues.
+
+* Virtual functions to get the value as a string.
+
+* Virtual functions to cast the value to other types, implement the bit
+  range feature of TableGen, and implement the list slice feature.
+
+The subclasses that inherit directly from ``Init`` are
+``UnsetInit`` and ``TypedInit``.
+
+An ``Init`` instance can be printed to an output stream with the ``<<``
+operator.
+
+.. warning::
+  It is not specified whether two separate initialization values with
+  the same underlying type and value (e.g., two strings with the value
+  "Hello") are represented by two ``Init``\ s or share the same ``Init``.
+
+``UnsetInit``
+~~~~~~~~~~~~~
+
+This class, a subclass of ``Init``, represents the unset (uninitialized)
+value. The static function ``get()`` can be used to obtain the singleton
+``Init`` of this type.
+
+
+``TypedInit``
+~~~~~~~~~~~~~
+
+This class, a subclass of ``Init``, acts as the parent class of the classes
+that represent specific value types (except for the unset value). These
+classes include ``BitInit``, ``BitsInit``, ``CodeInit``, ``DagInit``,
+``DefInit``, ``IntInit``, ``ListInit``, and ``StringInit``. (There are
+additional derived types used by the TableGen parser.)
+
+This class includes a data member that specifies the ``RecTy`` type of the
+value. It provides a function to get that ``RecTy`` type.
+
+``BitInit``
+~~~~~~~~~~~
+
+The ``BitInit`` class is a subclass of ``TypedInit``. Its instances
+represent the possible values of a bit: 0 or 1. It includes a data member
+that contains the bit.
+
+*All* of the classes derived from ``TypedInit`` provide the following functions.
+
+* A static function named ``get()`` that returns an ``Init`` representing
+  the specified value(s). In the case of ``BitInit``, ``get(true)`` returns
+  an instance of ``BitInit`` representing true, while ``get(false)`` returns
+  an instance
+  representing false. As noted above, it is not specified whether there
+  is exactly one or more than one ``BitInit`` representing true (or false).
+
+* A function named ``GetValue()`` that returns the value of the instance
+  in a more direct form, in this case as a ``bool``.
+
+``BitsInit``
+~~~~~~~~~~~~
+
+The ``BitsInit`` class is a subclass of ``TypedInit``. Its instances
+represent sequences of bits, from high-order to low-order. It includes a
+data member with the length of the sequence and a vector of pointers to
+``Init`` instances, one per bit.
+
+The class provides the usual ``get()`` function. It does not provide the
+``getValue()`` function.
+
+The class provides the following additional functions.
+
+* A function to get the number of bits in the sequence.
+
+* A function that gets a bit specified by an integer index.
+
+``CodeInit``
+~~~~~~~~~~~~
+
+The ``CodeInit`` class is a subclass of ``TypedInit``. Its instances
+represent arbitrary-length strings produced from ``code`` literals in the
+TableGen files. It includes a data member that contains a ``StringRef`` of
+the value. It also includes a data member specifying the source code
+location of the code stirng.
+
+The class provides the usual ``get()`` and ``getValue()`` functions. The
+latter function returns the ``StringRef``.
+
+The ``getLoc()`` function returns the source code location.
+
+
+``DagInit``
+~~~~~~~~~~~
+
+The ``DagInit`` class is a subclass of ``TypedInit``. Its instances
+represent the possible direct acyclic graphs (``dag``). 
+
+The class includes a pointer to an ``Init`` for the DAG operator and a
+pointer to a ``StringInit`` for the operator name. It includes the count of
+DAG operands and the count of operand names. Finally, it includes a vector of
+pointers to ``Init`` instances for the operands and another to
+``StringInit`` instances for the operand names.
+(The DAG operands are also referred to as *arguments*.)
+
+The class provides two forms of the usual ``get()`` function. It does not
+provide the usual ``getValue()`` function.
+
+The class provides many additional functions:
+
+* Functions to get the operator in various forms and to get the
+  operator name in various forms.
+
+* Functions to determine whether there are any operands and to get the
+  number of operands.
+
+* Functions to the get the operands, both individually and together.
+
+* Functions to determine whether there are any names and to
+  get the number of names
+
+* Functions to the get the names, both individually and together.
+
+* Functions to get the operand iterator ``begin()`` and ``end()`` values.
+
+* Functions to get the name iterator ``begin()`` and ``end()`` values.
+
+The class defines two types for the return values of the operand and name
+iterators.
+
+.. code-block:: text
+
+  using const_arg_iterator = SmallVectorImpl<Init*>::const_iterator;
+  using const_name_iterator = SmallVectorImpl<StringInit*>::const_iterator;
+
+
+``DefInit``
+~~~~~~~~~~~
+
+The ``DefInit`` class is a subclass of ``TypedInit``. Its instances
+represent the records that were collected by TableGen. It includes a data
+member that is a pointer to the record's ``Record`` instance.
+
+The class provides the usual ``get()`` function. It does not provide
+``getValue()``. Instead, it provides ``getDef()``, which returns the
+``Record`` instance.
+
+``IntInit``
+~~~~~~~~~~~
+
+The ``IntInit`` class is a subclass of ``TypedInit``. Its instances
+represent the possible values of a 64-bit integer. It includes a data member
+that contains the integer.
+
+The class provides the usual ``get()`` and ``getValue()`` functions. The
+latter function returns the integer as an ``int64_t``.
+
+The class also provides a function, ``getBit()``, to obtain a specified bit
+of the integer value.
+
+``ListInit``
+~~~~~~~~~~~~
+
+The ``ListInit`` class is a subclass of ``TypedInit``. Its instances
+represent lists of elements of some type. It includes a data member with the
+length of the list and a vector of pointers to ``Init`` instances, one per
+element.
+
+The class provides the usual ``get()`` and ``getValues()`` functions. The
+latter function returns an ``ArrayRef`` of the vector of pointers to ``Init``
+instances.
+
+The class provides these additional functions.
+
+* A function to get the element type.
+
+* Functions to get the length of the vector and to determine whether
+  it is empty.
+
+* Functions to get an element specified by an integer index and return
+  it in various forms.
+
+* Functions to get the iterator ``begin()`` and ``end()`` values. The
+  class defines a type for the return type of these two functions.
+
+.. code-block:: text
+
+  using const_iterator = Init *const *;
+ 
+
+``StringInit``
+~~~~~~~~~~~~~~
+
+The ``StringInit`` class is a subclass of ``TypedInit``. Its instances
+represent arbitrary-length strings. It includes a data member
+that contains a ``StringRef`` of the value.
+
+The class provides the usual ``get()`` and ``getValue()`` functions. The
+latter function returns the ``StringRef``.
+
+Creating a New Backend
+======================
+
+The following steps are required to create a new backend for TableGen.
+
+#. Invent a name for your backend C++ file, say ``GenAddressModes``.
+
+#. Write the new backend, using the file ``TableGenBackendSkeleton.cpp``
+   as a starting point.
+
+#. Determine which instance of TableGen requires the new backend. There is
+   one instance for Clang and another for LLVM. Or you may be building
+   your own instance.
+
+#. Modify the selected ``tablegen.cpp`` to include your new backend.
+
+  a. Add the name to the enumerated type ``ActionType``.
+
+  #. Add a keyword to the ``ActionType`` command option using the
+     ``clEnumValN()`` function.
+
+  #. Add a case to the ``switch`` statement in the *xxx*\ ``TableGenMain()``
+     function. It should invoke the "main function" of your backend, which
+     in this case, according to convention, is named ``EmitAddressModes``.
+
+5. Add a declaration of your "main function" to the corresponding 
+   ``TableGenBackends.h`` header file.
+
+#. Add your backend C++ file to the appropriate ``CMakeLists.txt`` file so
+   that it will be built.
+
+#. Add your C++ file to the system.
+
+The Backend Skeleton
+====================
+
+The file ``TableGenBackendSkeleton.cpp`` provides a skeleton C++ translation
+unit for writing a new TableGen backend. Here are a few notes on the file.
+
+* The list of includes is the minimal list required by most backends.
+
+* As with all LLVM C++ files, it has a ``using namespace llvm;`` statement.
+  It also has an anonymous namespace that contains all the file-specific
+  data structure definitions, along with the class embodying the emitter
+  data members and functions. Continuing with the ``GenAddressModes`` example,
+  this class is named ``AddressModesEmitter``.
+
+* The constructor for the emitter class accepts a ``RecordKeeper`` reference,
+  typically named ``RK``. The ``RecordKeeper`` reference is saved in a data
+  member so that records can be obtained from it. This data member is usually
+  named ``Records``.
+
+* One function is named ``run``. It is invoked by the backend's "main
+  function" to collect records and emit the output file. It accepts an instance
+  of the ``raw_ostream`` class, typically named ``OS``. The output file is
+  emitted by writing to this stream.
+
+* The ``run`` function should use the ``emitSourceFileHeader`` helper function
+  to include a standard header in the emitted file.
+
+* The only function in the ``llvm`` namespace is the backend "main function."
+  In this example, it is named ``EmitAddressModes``. It creates an instance
+  of the ``AddressModesEmitter`` class, passing the ``RecordKeeper``
+  instance, then invokes the ``run`` function, passing the ``raw_ostream``
+  instance.
+
+All the examples in the remainder of this document will assume the naming
+conventions used in the skeleton file.
+
+Getting Classes
+===============
+
+The ``RecordKeeper`` class provides two functions for getting the
+``Record`` instances for classes defined in the TableGen files.
+
+* ``getClasses()`` returns a ``RecordMap`` reference for all the classes.
+
+* ``getClass(``\ *name*\ ``)`` returns a ``Record`` reference for the named
+  class.
+
+If you need to iterate over all the class records:
+
+.. code-block:: text
+
+  for (auto ClassPair : Records.getClasses()) {
+    Record *ClassRec = ClassPair.second.get();
+    ...
+  }
+
+``ClassPair.second`` gets the class's ``unique_ptr``, then ``.get()`` gets the
+class ``Record`` itself.
+
+
+Getting Records
+===============
+
+The ``RecordKeeper`` class provides four functions for getting the
+``Record`` instances for concrete records defined in the TableGen files.
+
+* ``getDefs()`` returns a ``RecordMap`` reference for all the concrete
+  records.
+
+* ``getDef(``\ *name*\ ``)`` return a ``Record`` reference for the named
+  concrete record.
+
+* ``getAllDerivedDefinitions(``\ *classname*\ ``)`` returns a vector of
+  ``Record`` references for the concrete records that derive from the
+  given class.
+
+* ``getAllDerivedDefinitionsTwo(``\ *classname1*\ ``,`` *classname2*\ ``)`` returns
+  a vector of ``Record`` references for the concrete records that derive from
+  *both* of the given classes. [function to come]
+
+This statement obtains all the records that derive from the ``Attribute``
+class and iterates over them.
+
+.. code-block:: text
+
+  auto AttrRecords = Records.getAllDerivedDefinitions("Attribute");
+  for (Record *AttrRec : AttrRecords) {
+    ...
+  }
+
+Getting Record Names and Fields
+===============================
+
+As described above (see `Record`_), there are multiple functions that
+return the name of a record. One particularly useful one is
+``getNameInitAsString()``, which returns the name as a ``std::string``.
+
+There are also multiple functions that return the fields of a record. To
+obtain and iterate over all the fields:
+
+.. code-block:: text
+
+  for (const RecordVal &Field : SomeRec->getValues()) {
+    ...
+  }
+
+You will recall that ``RecordVal`` is the class whose instances contain
+information about the fields in records.
+
+The ``getValue()`` function returns the ``RecordVal`` instance for a field
+specified by name. There are multiple overloaded functions, some taking a
+``StringRef`` and others taking a ``const Init *``. Some functions return a
+``RecordVal *`` and others return a ``const RecordVal *``. If the field does
+not exist, a fatal error message is printed.
+
+More often than not, you are interested in the value of the field, not all
+the information in the ``RecordVal``. There is a large set of functions that
+take a field name in some form and return its value. One function,
+``getValueInit``, returns the value as an ``Init *``. Another function,
+``isValueUnset``, returns a boolean specifying whether the value is unset
+(uninitialized).
+
+Most of the functions return the value in some more useful form. For
+example:
+
+.. code-block:: text
+
+  std::vector<int64_t> RegCosts =
+      SomeRec->getValueAsListOfInts("RegCosts");
+
+The field ``RegCosts`` is assumed to be a list of integers. That list is
+returned as a ``std::vector`` of 64-bit integers. If the field is not a list
+of integers, a fatal error message is printed.
+
+Here is a function that returns a field value as a ``Record``, but returns
+null if the field does not exist.
+
+.. code-block:: text
+
+  if (Record *BaseRec = SomeRec->getValueAsOptionalDef(BaseFieldName)) {
+    ...
+  }
+
+The field is assumed to have another record as its value. That record is returned
+as a pointer to a ``Record``. If the field does not exist or is unset, the
+functions returns null.  
+
+Getting Record Superclasses
+===========================
+
+The ``Record`` class provides a function to obtain the superclasses of a
+record. It is named ``getSuperClasses`` and returns an ``ArrayRef`` of an
+array of ``std::pair`` pairs. The superclasses are in post-order: the order
+in which the superclasses were visited while copying their fields into the
+record. Each pair consists of a pointer to the ``Record`` instance for a
+superclass record and an instance of the ``SMRange`` class. The range
+indicates the source file locations of the beginning and end of the class
+definition.
+
+This example obtains the superclasses of the ``Prototype`` record and then
+iterates over the pairs in the returned array.
+
+.. code-block:: text
+
+  ArrayRef<std::pair<Record *, SMRange>>
+      Superclasses = Prototype->getSuperClasses();
+  for (const auto &SuperPair : Superclasses) {
+    ...
+  }
+
+The ``Record`` class also provides a function, ``getDirectSuperClasses``, to
+append the *direct* superclasses of a record to a given vector of type
+``SmallVectorImpl<Record *>``.
+
+Emitting Text to the Output Stream
+==================================
+
+The ``run`` function is passed a ``raw_ostream`` to which it prints the
+output file. By convention, this stream is saved in the emitter class member
+named ``OS``, although some ``run`` functions are simple and just use the
+stream without saving it. The output can be produced by writing values
+directly to the output stream, or by using the ``std::format()`` or
+``llvm::formatv()`` functions.
+
+.. code-block:: text
+
+  OS << "#ifndef " << NodeName << "\n";
+
+  OS << format("0x%0*x, ", Digits, Value);
+
+Instances of the following classes can be printed using the ``<<`` operator:
+``RecordKeeper``,
+``Record``,
+``RecTy``,
+``RecordVal``, and
+``Init``.
+
+A constant and two helper functions are provided for producing the output
+file.  The constant ``MAX_LINE_LEN`` specifies the maximum length of output
+lines.  The helper function ``printLine`` prints a horizontal line comment.
+The helper function ``emitSourceFileHeader`` prints the header comment that
+should be included at the top of every output file.
+
+Printing Error Messages
+=======================
+
+TableGen records are often derived from multiple classes and also often
+defined through a sequence of multiclasses. Because of this, it can be
+difficult for backends to report clear error messages with accurate source
+file locations.  To make error reporting easier, five error reporting
+functions are provided, each with four overloads. [all combinations to come]
+
+* ``PrintWarning`` prints a message tagged as a warning.
+
+* ``PrintError`` prints a message tagged as an error.
+
+* ``PrintFatalError`` prints a message tagged as an error and then terminates.
+
+* ``PrintNote`` prints a note. It is often used after one of the previous
+  functions to provide more information.
+
+* ``PrintFatalNote`` prints a note and then terminates.
+
+Each of these five functions is overloaded four times. 
+
+* ``PrintError(const Twine &Msg)``:
+  Prints the message with no source file location.
+
+* ``PrintError(ArrayRef<SMLoc> ErrorLoc, const Twine &Msg)``: 
+  Prints the message followed by the specified source line,
+  along with a pointer to the item in error. The array of
+  source file locations is typically taken from a ``Record`` instance.
+
+* ``PrintError(const Record *Rec, const Twine &Msg)``:
+  Prints the message followed by the source line associated with the
+  specified record (see `Record`_).
+
+* ``PrintError(const RecordVal *RecVal, const Twine &Msg)``:
+  Prints the message followed by the source line associated with the
+  specified record field (see `RecordVal`_).
+
+Using these functions, the goal is to produce the most specific error report
+possible.
+
+Debugging Tools
+===============
+
+TableGen provides some tools to aid in debugging backends.
+
+The ``PrintRecords`` Backend
+----------------------------
+
+The TableGen command option ``--print-records`` invokes a simple backend
+that prints all the classes and records defined in the source files. This is
+the default backend option. The output looks like this:
+
+.. code-block:: text
+
+  ------------- Classes -----------------
+  ...
+  class XEntry<string XEntry:str = ?, int XEntry:val1 = ?> { // XBase
+    string Str = XEntry:str;
+    bits<8> Val1 = { !cast<bits<8>>(XEntry:val1){7}, ... };
+    bit Val3 = 1;
+  }
+  ...
+  ------------- Defs -----------------
+  def ATable {  // GenericTable
+    string FilterClass = "AEntry";
+    string CppTypeName = "AEntry";
+    list<string> Fields = ["Str", "Val1", "Val2"];
+    list<string> PrimaryKey = ["Val1", "Val2"];
+    string PrimaryKeyName = "lookupATableByValues";
+    bit PrimaryKeyEarlyOut = 0;
+  }
+  ...
+  def anonymous_0 { // AEntry
+    string Str = "Bob";
+    bits<8> Val1 = { 0, 0, 0, 0, 0, 1, 0, 1 };
+    bits<10> Val2 = { 0, 0, 0, 0, 0, 0, 0, 0, 1, 1 };
+  }
+
+Classes are shown with their template arguments, parent classes (following
+``//``), and fields. Records are shown with their parent classes and
+fields. Note that anonymous records are named ``anonymous_0``,
+``anonymous_1``, etc.
+
+
+
+The ``PrintDetailedRecords`` Backend
+------------------------------------
+
+[to come]

llvm/docs/TableGen/ProgRef.rst

 
 An example of a backend is ``RegisterInfo``, which generates the register
 file information for a particular target machine, for use by the LLVM
-target-independent code generator. See :doc:`TableGen Backends <./BackEnds>` for
-a description of the LLVM TableGen backends. Here are a few of the things
-backends can do.
+target-independent code generator. See :doc:`TableGen Backends <./BackEnds>`
+for a description of the LLVM TableGen backends, and :doc:`TableGen
+Backend Developer's Guide <./BackGuide>` for a guide to writing a new
+backend.
+
+Here are a few of the things backends can do.
 
 * Generate the register file information for a particular target machine.
 

llvm/lib/TableGen/TableGenBackendSkeleton.cpp

+//===- [SkeletonEmitter.cpp - Skeleton TableGen backend]     -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// [This Tablegen backend emits ...]
+// [For lack of anything better, metasyntactic items in this skeleton file
+//  are enclosed in brackets.]
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/Format.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/TableGen/Error.h"
+#include "llvm/TableGen/Record.h"
+#include "llvm/TableGen/TableGenBackend.h"
+#include <algorithm>
+#include <set>
+#include <string>
+#include <vector>
+
+#define DEBUG_TYPE "[skeleton-emitter]"
+
+using namespace llvm;
+
+namespace {
+
+// [Any helper data structures can be defined here. Some backends use
+//  structs to collect information from the records.]
+
+class [SkeletonEmitter] {
+private:
+  RecordKeeper &Records;
+
+public:
+  [SkeltonEmitter](RecordKeeper &RK) : Records(RK) {}
+
+  void run(raw_ostream &OS);
+} // End emitter class.
+
+} // End anonymous namespace.
+
+void [SkeletonEmitter]::run(raw_ostream &OS) {
+  emitSourceFileHeader("[Skeleton data structures]", OS);
+}
+
+namespace llvm {
+
+// [The only thing that should be in the llvm namespace is the
+//  emitter entry point function.]
+
+void [EmitSkeleton](RecordKeeper &RK, raw_ostream &OS) {
+  // Instantiate the emitter class and invoke run().
+  [SkeletonEmitter](RK).run(OS);
+}
+
+} // End llvm namespace.