TableGen Language Reference

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.

Lexical Analysis

TableGen supports BCPL (// ...) and nestable C-style (/* ... */) comments. TableGen also provides simple Preprocessing Support.

The following is a listing of the basic punctuation tokens:

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

Numeric literals take one of the following forms:

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.

Also note that :token:`BinInteger` creates a value of type bits<n> (where n is the number of bits). This will implicitly convert to integers when needed.

TableGen has identifier-like tokens:

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:

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

\\ \' \" \t \n

TableGen also has the following keywords:

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

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

TableGen also has !cond operator that needs a slightly different syntax compared to other "bang operators":

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.

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

classes

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:

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

You can declare an empty class by giving an 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.

Every class has an implicit template argument called NAME, which is set to the name of the instantiating def or defm. The result is undefined if the class is instantiated by an anonymous record.

Declarations

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

It assigns the value to the identifier.

Types

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

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:

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

• 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;
  }
  

  Values defined in superclasses can be accessed the same way.

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

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

• value local to a class, such as the use of Bar in:

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

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

  multiclass Foo<int Bar> {
    def : SomeClass<Bar>;
  }
  

• the iteration variable of a foreach, such as the use of i in:

  foreach i = 0-5 in
  def Foo#i;
  

• a variable defined by defset or defvar

• the implicit template argument NAME in a class or multiclass

This represents the numeric value of the integer.

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

The value is the string value of the code fragment.

? represents an "unset" initializer.

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

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.

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

Bodies

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.

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

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

def

Defines a record whose name is given by the optional :token:`Value`. The value is parsed in a special mode where global identifiers (records and variables defined by defset, and variables defined at global scope by defvar) are not recognized, and all unrecognized identifiers are interpreted as strings.

If no name is given, the record is anonymous. The final name of anonymous records is undefined, but globally unique.

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

When a non-anonymous record is defined in a multiclass and the given name does not contain a reference to the implicit template argument NAME, such a reference will automatically be prepended. That is, the following are equivalent inside a multiclass:

def Foo;
def NAME#Foo;

defm

The :token:`BaseClassList` is a list of at least one multiclass and any number of class's. The multiclass's must occur before any class's.

Instantiates all records defined in all given multiclass's and adds the given class's as superclasses.

The name is parsed in the same special mode used by def. If the name is missing, a globally unique string is used instead (but instantiated records are not considered to be anonymous, unless they were originally defined by an anonymous def) That is, the following have different semantics:

defm : SomeMultiClass<...>;    // some globally unique name
defm "" : SomeMultiClass<...>; // empty name string

When it occurs inside a multiclass, the second variant is equivalent to defm NAME : .... More generally, when defm occurs in a multiclass and its name does not contain a reference to the implicit template argument NAME, such a reference will automatically be prepended. That is, the following are equivalent inside a multiclass:

defm Foo : SomeMultiClass<...>;
defm NAME#Foo : SomeMultiClass<...>;

defset

All records defined inside the braces via def and defm are collected in a globally accessible list of the given name (in addition to being added to the global collection of records as usual). Anonymous records created inside initializier expressions using the Class<args...> syntax are never collected in a defset.

The given type must be list<A>, where A is some class. It is an error to define a record (via def or defm) inside the braces which doesn't derive from A.

defvar

The identifier on the left of the = is defined to be a global or local variable, whose value is given by the expression on the right of the =. The type of the variable is automatically inferred.

A defvar statement at the top level of the file defines a global variable, in the same scope used by defset. If a defvar statement appears inside any other construction, including classes, multiclasses and foreach statements, then the variable is scoped to the inside of that construction only.

In contexts where the defvar statement will be encountered multiple times, the definition is re-evaluated for each instance. For example, a defvar inside a foreach can construct a value based on the iteration variable, which will be different every time round the loop; a defvar inside a templated class or multiclass can have a definition depending on the template parameters.

Variables local to a foreach go out of scope at the end of each loop iteration, so their previous value is not accessible in the next iteration. (It won't work to defvar i=!add(i,1) each time you go round the loop.)

In general, defvar variables are immutable once they are defined. It is an error to define the same variable name twice in the same scope (but legal to shadow the first definition temporarily in an inner scope).

foreach

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.

Note that the productions involving RangeList and RangePiece have precedence over the more generic value parsing based on the first token.

if

The value expression after the if keyword is evaluated, and if it evaluates to true (in the same sense used by the !if operator), then the object definition(s) after the then keyword are executed. Otherwise, if there is an else keyword, the definition(s) after the else are executed instead.

Because the braces around the then clause are optional, this grammar rule has the usual ambiguity about dangling else clauses, and it is resolved in the usual way: in a case like if v1 then if v2 then {...} else {...}, the else binds to the inner if rather than the outer one.

Top-Level let

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

Preprocessing Support

TableGen's embedded preprocessor is only intended for conditional compilation. It supports the following directives:

:token:`PrepRegion` may occur anywhere in a TD file, as long as it matches the grammar specification.

:token:`PrepDefine` allows defining a :token:`MacroName` so that any following :token:`PrepIfdef` - :token:`PrepElse` preprocessing region part and :token:`PrepIfdef` - :token:`PrepEndif` preprocessing region are enabled for TableGen tokens parsing.

A preprocessing region, starting (i.e. having its :token:`PrepIfdef`) in a file, must end (i.e. have its :token:`PrepEndif`) in the same file.

A :token:`MacroName` may be defined externally by using { -D<NAME> } option of TableGen.