llvm/docs/TableGen/LangRef.rst

===========================
TableGen Language Reference
===========================

.. contents::
   :local:

.. warning::
   This document is extremely rough. If you find something lacking, please
   fix it, file a documentation bug, or ask about it on llvmdev.

Introduction
============

This document is meant to be a normative spec about the TableGen language
in and of itself (i.e. how to understand a given construct in terms of how
it affects the final set of records represented by the TableGen file). If
you are unsure if this document is really what you are looking for, please
read the :doc:`introduction to TableGen <index>` first.

Notation
========

The lexical and syntax notation used here is intended to imitate
`Python's`_. In particular, for lexical definitions, the productions
operate at the character level and there is no implied whitespace between
elements. The syntax definitions operate at the token level, so there is
implied whitespace between tokens.

.. _`Python's`: http://docs.python.org/py3k/reference/introduction.html#notation

Lexical Analysis
================

TableGen supports BCPL (``// ...``) and nestable C-style (``/* ... */``)
comments.

The following is a listing of the basic punctuation tokens::

   - + [ ] { } ( ) < > : ; .  = ? #

Numeric literals take one of the following forms:

.. TableGen actually will lex some pretty strange sequences an interpret
   them as numbers. What is shown here is an attempt to approximate what it
   "should" accept.

.. productionlist::
   TokInteger: `DecimalInteger` | `HexInteger` | `BinInteger`
   DecimalInteger: ["+" | "-"] ("0"..."9")+
   HexInteger: "0x" ("0"..."9" | "a"..."f" | "A"..."F")+
   BinInteger: "0b" ("0" | "1")+

One aspect to note is that the :token:`DecimalInteger` token *includes* the
``+`` or ``-``, as opposed to having ``+`` and ``-`` be unary operators as
most languages do.

TableGen has identifier-like tokens:

.. productionlist::
   ualpha: "a"..."z" | "A"..."Z" | "_"
   TokIdentifier: ("0"..."9")* `ualpha` (`ualpha` | "0"..."9")*
   TokVarName: "$" `ualpha` (`ualpha` |  "0"..."9")*

Note that unlike most languages, TableGen allows :token:`TokIdentifier` to
begin with a number. In case of ambiguity, a token will be interpreted as a
numeric literal rather than an identifier.

TableGen also has two string-like literals:

.. productionlist::
   TokString: '"' <non-'"' characters and C-like escapes> '"'
   TokCodeFragment: "[{" <shortest text not containing "}]"> "}]"

:token:`TokCodeFragment` is essentially a multiline string literal
delimited by ``[{`` and ``}]``.

.. note::
   The current implementation accepts the following C-like escapes::

      \\ \' \" \t \n

TableGen also has the following keywords::

   bit   bits      class   code         dag
   def   foreach   defm    field        in
   int   let       list    multiclass   string

TableGen also has "bang operators" which have a
wide variety of meanings:

.. productionlist::
   BangOperator: one of
               :!eq     !if      !head    !tail      !con
               :!add    !shl     !sra     !srl
               :!cast   !empty   !subst   !foreach   !strconcat

Syntax
======

TableGen has an ``include`` mechanism. It does not play a role in the
syntax per se, since it is lexically replaced with the contents of the
included file.

.. productionlist::
   IncludeDirective: "include" `TokString`

TableGen's top-level production consists of "objects".

.. productionlist::
   TableGenFile: `Object`*
   Object: `Class` | `Def` | `Defm` | `Let` | `MultiClass` | `Foreach`

``class``\es
------------

.. productionlist::
   Class: "class" `TokIdentifier` [`TemplateArgList`] `ObjectBody`

A ``class`` declaration creates a record which other records can inherit
from. A class can be parametrized by a list of "template arguments", whose
values can be used in the class body.

A given class can only be defined once. A ``class`` declaration is
considered to define the class if any of the following is true:

.. break ObjectBody into its consituents so that they are present here?

#. The :token:`TemplateArgList` is present.
#. The :token:`Body` in the :token:`ObjectBody` is present and is not empty.
#. The :token:`BaseClassList` in the :token:`ObjectBody` is present.

You can declare an empty class by giving and empty :token:`TemplateArgList`
and an empty :token:`ObjectBody`. This can serve as a restricted form of
forward declaration: note that records deriving from the forward-declared
class will inherit no fields from it since the record expansion is done
when the record is parsed.

.. productionlist::
   TemplateArgList: "<" `Declaration` ("," `Declaration`)* ">"

Declarations
------------

.. Omitting mention of arcane "field" prefix to discourage its use.

The declaration syntax is pretty much what you would expect as a C++
programmer.

.. productionlist::
   Declaration: `Type` `TokIdentifier` ["=" `Value`]

It assigns the value to the identifer.

Types
-----

.. productionlist::
   Type: "string" | "code" | "bit" | "int" | "dag"
       :| "bits" "<" `TokInteger` ">"
       :| "list" "<" `Type` ">"
       :| `ClassID`
   ClassID: `TokIdentifier`

Both ``string`` and ``code`` correspond to the string type; the difference
is purely to indicate programmer intention.

The :token:`ClassID` must identify a class that has been previously
declared or defined.

Values
------

.. productionlist::
   Value: `SimpleValue` `ValueSuffix`*
   ValueSuffix: "{" `RangeList` "}"
              :| "[" `RangeList` "]"
              :| "." `TokIdentifier`
   RangeList: `RangePiece` ("," `RangePiece`)*
   RangePiece: `TokInteger`
             :| `TokInteger` "-" `TokInteger`
             :| `TokInteger` `TokInteger`

The peculiar last form of :token:`RangePiece` is due to the fact that the
"``-``" is included in the :token:`TokInteger`, hence ``1-5`` gets lexed as
two consecutive :token:`TokInteger`'s, with values ``1`` and ``-5``,
instead of "1", "-", and "5".
The :token:`RangeList` can be thought of as specifying "list slice" in some
contexts.


:token:`SimpleValue` has a number of forms:


.. productionlist::
   SimpleValue: `TokIdentifier`

The value will be the variable referenced by the identifier. It can be one
of:

.. The code for this is exceptionally abstruse. These examples are a
   best-effort attempt.

* name of a ``def``, such as the use of ``Bar`` in::

     def Bar : SomeClass {
       int X = 5;
     }

     def Foo {
       SomeClass Baz = Bar;
     }

* value local to a ``def``, such as the use of ``Bar`` in::

     def Foo {
       int Bar = 5;
       int Baz = Bar;
     }

* a template arg of a ``class``, such as the use of ``Bar`` in::

     class Foo<int Bar> {
       int Baz = Bar;
     }

* value local to a ``multiclass``, such as the use of ``Bar`` in::

     multiclass Foo {
       int Bar = 5;
       int Baz = Bar;
     }

* a template arg to a ``multiclass``, such as the use of ``Bar`` in::

     multiclass Foo<int Bar> {
       int Baz = Bar;
     }

.. productionlist::
   SimpleValue: `TokInteger`

This represents the numeric value of the integer.

.. productionlist::
   SimpleValue: `TokString`+

Multiple adjacent string literals are concatenated like in C/C++. The value
is the concatenation of the strings.

.. productionlist::
   SimpleValue: `TokCodeFragment`

The value is the string value of the code fragment.

.. productionlist::
   SimpleValue: "?"

``?`` represents an "unset" initializer.

.. productionlist::
   SimpleValue: "{" `ValueList` "}"
   ValueList: [`ValueListNE`]
   ValueListNE: `Value` ("," `Value`)*

This represents a sequence of bits, as would be used to initialize a
``bits<n>`` field (where ``n`` is the number of bits).

.. productionlist::
   SimpleValue: `ClassID` "<" `ValueListNE` ">"

This generates a new anonymous record definition (as would be created by an
unnamed ``def`` inheriting from the given class with the given template
arguments) and the value is the value of that record definition.

.. productionlist::
   SimpleValue: "[" `ValueList` "]" ["<" `Type` ">"]

A list initializer. The optional :token:`Type` can be used to indicate a
specific element type, otherwise the element type will be deduced from the
given values.

.. The initial `DagArg` of the dag must start with an identifier or
   !cast, but this is more of an implementation detail and so for now just
   leave it out.

.. productionlist::
   SimpleValue: "(" `DagArg` `DagArgList` ")"
   DagArgList: `DagArg` ("," `DagArg`)*
   DagArg: `Value` [":" `TokVarName`] | `TokVarName`

The initial :token:`DagArg` is called the "operator" of the dag.

.. productionlist::
   SimpleValue: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"

Bodies
------

.. productionlist::
   ObjectBody: `BaseClassList` `Body`
   BaseClassList: [":" `BaseClassListNE`]
   BaseClassListNE: `SubClassRef` ("," `SubClassRef`)*
   SubClassRef: (`ClassID` | `MultiClassID`) ["<" `ValueList` ">"]
   DefmID: `TokIdentifier`

The version with the :token:`MultiClassID` is only valid in the
:token:`BaseClassList` of a ``defm``.
The :token:`MultiClassID` should be the name of a ``multiclass``.

.. put this somewhere else

It is after parsing the base class list that the "let stack" is applied.

.. productionlist::
   Body: ";" | "{" BodyList "}"
   BodyList: BodyItem*
   BodyItem: `Declaration` ";"
           :| "let" `TokIdentifier` [`RangeList`] "=" `Value` ";"

The ``let`` form allows overriding the value of an inherited field.

``def``
-------

.. TODO::
   There can be pastes in the names here, like ``#NAME#``. Look into that
   and document it (it boils down to ParseIDValue with IDParseMode ==
   ParseNameMode). ParseObjectName calls into the general ParseValue, with
   the only different from "arbitrary expression parsing" being IDParseMode
   == Mode.

.. productionlist::
   Def: "def" `TokIdentifier` `ObjectBody`

Defines a record whose name is given by the :token:`TokIdentifier`. The
fields of the record are inherited from the base classes and defined in the
body.

Special handling occurs if this ``def`` appears inside a ``multiclass`` or
a ``foreach``.

``defm``
--------

.. productionlist::
   Defm: "defm" `TokIdentifier` ":" `BaseClassListNE` ";"

Note that in the :token:`BaseClassList`, all of the ``multiclass``'s must
precede any ``class``'s that appear.

``foreach``
-----------

.. productionlist::
   Foreach: "foreach" `Declaration` "in" "{" `Object`* "}"
          :| "foreach" `Declaration` "in" `Object`

The value assigned to the variable in the declaration is iterated over and
the object or object list is reevaluated with the variable set at each
iterated value.

Top-Level ``let``
-----------------

.. productionlist::
   Let:  "let" `LetList` "in" "{" `Object`* "}"
      :| "let" `LetList` "in" `Object`
   LetList: `LetItem` ("," `LetItem`)*
   LetItem: `TokIdentifier` [`RangeList`] "=" `Value`

This is effectively equivalent to ``let`` inside the body of a record
except that it applies to multiple records at a time. The bindings are
applied at the end of parsing the base classes of a record.

``multiclass``
--------------

.. productionlist::
   MultiClass: "multiclass" `TokIdentifier` [`TemplateArgList`]
             : [":" `BaseMultiClassList`] "{" `MultiClassObject`+ "}"
   BaseMultiClassList: `MultiClassID` ("," `MultiClassID`)*
   MultiClassID: `TokIdentifier`
   MultiClassObject: `Def` | `Defm` | `Let` | `Foreach`