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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<head>
  <title>TableGen Fundamentals</title>
  <link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>

<div class="doc_title">TableGen Fundamentals</div>

<ul>
  <li><a href="#introduction">Introduction</a></li>
  <ol>
    <li><a href="#concepts">Basic concepts</a></li>
    <li><a href="#example">An example record</a></li>
    <li><a href="#running">Running TableGen</a></li>
  </ol>
  <li><a href="#syntax">TableGen syntax</a></li>
  <ol>
    <li><a href="#primitives">TableGen primitives</a></li>
    <ol>
      <li><a href="#comments">TableGen comments</a></li>
      <li><a href="#types">The TableGen type system</a></li>
      <li><a href="#values">TableGen values and expressions</a></li>
    </ol>
    <li><a href="#classesdefs">Classes and definitions</a></li>
    <ol>
      <li><a href="#valuedef">Value definitions</a></li>
      <li><a href="#recordlet">'let' expressions</a></li>
      <li><a href="#templateargs">Class template arguments</a></li>
    </ol>
    <li><a href="#filescope">File scope entities</a></li>
    <ol>
      <li><a href="#include">File inclusion</a></li>
      <li><a href="#globallet">'let' expressions</a></li>
    </ol>
  </ol>
  <li><a href="#backends">TableGen backends</a></li>
  <ol>
    <li><a href="#">x</a></li>
  </ol>
  <li><a href="#codegenerator">The LLVM code generator</a></li>
  <ol>
    <li><a href="#">x</a></li>
  </ol>
</ul>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="introduction">Introduction</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>TableGen's purpose is to help a human develop and maintain records of
domain-specific information.  Because there may be a large number of these
records, it is specifically designed to allow writing flexible descriptions and
for common features of these records to be factored out.  This reduces the
amount of duplication in the description, reduces the chance of error, and
makes it easier to structure domain specific information.</p>

<p>The core part of TableGen <a href="#syntax">parses a file</a>, instantiates
the declarations, and hands the result off to a domain-specific "<a
href="#backends">TableGen backend</a>" for processing.  The current major user
of TableGen is the <a href="#codegenerator">LLVM code generator</a>.
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="running">Basic concepts</a>
</div>

<div class="doc_text">

<p>
TableGen files consist of two key parts: 'classes' and 'definitions', both of
which are considered 'records'.
</p>

<p>
<b>TableGen records</b> have a unique name, a list of values, and a list of
superclasses.  The list of values is main data that TableGen builds for each
record, it is this that holds the domain specific information for the
application.  The interpretation of this data is left to a specific <a
href="#backends">TableGen backend</a>, but the structure and format rules are
taken care of and fixed by TableGen.
</p>

<p>
<b>TableGen definitions</b> are the concrete form of 'records'.  These generally
do not have any undefined values, and are marked with the '<tt>def</tt>'
keyword.
</p>

<p>
<b>TableGen classes</b> are abstract records that are used to build and describe
other records.  These 'classes' allow the end-user to build abstractions for
either the domain they are targetting (such as "Register", "RegisterClass", and
"Instruction" in the LLVM code generator) or for the implementor to help factor
out common properties of records (such as "FPInst", which is used to represent
floating point instructions in the X86 backend).  TableGen keeps track of all of
the classes that are used to build up a definition, so the backend can find all
definitions of a particular class, such as "Instruction".
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="example">An example record</a>
</div>

<div class="doc_text">

<p>
With no other arguments, TableGen parses the specified file and prints out all
of the classes, then all of the definitions.  This is a good way to see what the
various definitions expand to fully.  Running this on the <tt>X86.td</tt> file
prints this (at the time of this writing):
</p>

<p>
<pre>
...
def ADDrr8 {    // Instruction X86Inst I2A8 Pattern
  string Name = "add";
  string Namespace = "X86";
  list&lt;Register&gt; Uses = [];
  list&lt;Register&gt; Defs = [];
  bit isReturn = 0;
  bit isBranch = 0;
  bit isCall = 0;
  bit isTwoAddress = 1;
  bit isTerminator = 0;
  dag Pattern = (set R8, (plus R8, R8));
  bits&lt;8&gt; Opcode = { 0, 0, 0, 0, 0, 0, 0, 0 };
  Format Form = MRMDestReg;
  bits&lt;5&gt; FormBits = { 0, 0, 0, 1, 1 };
  ArgType Type = Arg8;
  bits&lt;3&gt; TypeBits = { 0, 0, 1 };
  bit hasOpSizePrefix = 0;
  bit printImplicitUses = 0;
  bits&lt;4&gt; Prefix = { 0, 0, 0, 0 };
  FPFormat FPForm = ?;
  bits&lt;3&gt; FPFormBits = { 0, 0, 0 };
}
...
</pre><p>

<p>
This definition corresponds to an 8-bit register-register add instruction in the
X86.  The string after the '<tt>def</tt>' string indicates the name of the
record ("<tt>ADDrr8</tt>" in this case), and the comment at the end of the line
indicates the superclasses of the definition.  The body of the record contains
all of the data that TableGen assembled for the record, indicating that the
instruction is part of the "X86" namespace, should be printed as "<tt>add</tt>"
in the assembly file, it is a two-address instruction, has a particular
encoding, etc.  The contents and semantics of the information in the record is
specific to the needs of the X86 backend, and is only shown as an example.
</p>

<p>
As you can see, a lot of information is needed for every instruction supported
by the code generator, and specifying it all manually would be unmaintainble,
prone to bugs, and tiring to do in the first place.  Because we are using
TableGen, all of the information was derived from the following definition:
</p>

<p><pre>
def ADDrr8   : I2A8&lt;"add", 0x00, MRMDestReg&gt;,
               Pattern&lt;(set R8, (plus R8, R8))&gt;;
</pre></p>

<p>
This definition makes use of the custom I2A8 (two address instruction with 8-bit
operand) class, which is defined in the X86-specific TableGen file to factor out
the common features that instructions of its class share.  A key feature of
TableGen is that it allows the end-user to define the abstractions they prefer
to use when describing their information.
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="running">Running TableGen</a>
</div>

<div class="doc_text">

<p>
TableGen runs just like any other LLVM tool.  The first (optional) argument
specifies the file to read.  If a filename is not specified, <tt>tblgen</tt>
reads from standard input.
</p>

<p>
To be useful, one of the <a href="#backends">TableGen backends</a> must be used.
These backends are selectable on the command line (type '<tt>tblgen --help</tt>'
for a list).  For example, to get a list of all of the definitions that subclass
a particular type (which can be useful for building up an enum list of these
records), use the <tt>--print-enums</tt> option:
</p>

<p><pre>
$ tblgen X86.td -print-enums -class=Register
AH, AL, AX, BH, BL, BP, BX, CH, CL, CX, DH, DI, DL, DX,
EAX, EBP, EBX, ECX, EDI, EDX, ESI, ESP, FP0, FP1, FP2, FP3, FP4, FP5, FP6,
SI, SP, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, 

$ tblgen X86.td -print-enums -class=Instruction 
ADCrr32, ADDri16, ADDri16b, ADDri32, ADDri32b, ADDri8, ADDrr16, ADDrr32,
ADDrr8, ADJCALLSTACKDOWN, ADJCALLSTACKUP, ANDri16, ANDri16b, ANDri32, ANDri32b,
ANDri8, ANDrr16, ANDrr32, ANDrr8, BSWAPr32, CALLm32, CALLpcrel32, ...
</pre></p>

<p>
The default backend prints out all of the records, as described <a
href="#example">above</a>.
</p>

<p>
If you plan to use TableGen for some purpose, you will most likely have to <a
href="#backends">write a backend</a> that extracts the information specific to
what you need and formats it in the appropriate way.
</p>

</div>


<!-- *********************************************************************** -->
<div class="doc_section"><a name="syntax">TableGen syntax</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
TableGen doesn't care about the meaning of data (that is up to the backend to
define), but it does care about syntax, and it enforces a simple type system.
This section describes the syntax and the constructs allowed in a TableGen file.
</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="primitives">TableGen primitives</tt></a>
</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="comments">TableGen comments</tt></a>
</div>

<div class="doc_text">

<p>TableGen supports BCPL style "<tt>//</tt>" comments, which run to the end of
the line, and it also supports <b>nestable</b> "<tt>/* */</tt>" comments.</p>

</div>


<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="types">The TableGen type system</tt></a>
</div>

<div class="doc_text">
<p>
TableGen files are strongly typed, in a simple (but complete) type-system.
These types are used to perform automatic conversions, check for errors, and to
help interface designers constrain the input that they allow.  Every <a
href="#valuedef">value definition</a> is required to have an associated type.
</p>

<p>
TableGen supports a mixture of very low-level types (such as <tt>bit</tt>) and
very high-level types (such as <tt>dag</tt>).  This flexibility is what allows
it to describe a wide range of information conveniently and compactly.  The
TableGen types are:
</p>

<p>
<ul>
<li>"<tt>bit</tt>" - A 'bit' is a boolean value that can hold either 0 or
1.</li>

<li>"<tt>int</tt>" - The 'int' type represents a simple 32-bit integer value, such as 5.</li>

<li>"<tt>string</tt>" - The 'string' type represents an ordered sequence of
characters of arbitrary length.</li>

<li>"<tt>bits&lt;n&gt;</tt>" - A 'bits' type is a arbitrary, but fixed, size
integer that is broken up into individual bits.  This type is useful because it
can handle some bits being defined while others are undefined.</li>

<li>"<tt>list&lt;ty&gt;</tt>" - This type represents a list whose elements are
some other type.  The contained type is arbitrary: it can even be another list
type.</li>

<li>Class type - Specifying a class name in a type context means that the
defined value must be a subclass of the specified class.  This is useful in
conjunction with the "list" type, for example, to constrain the elements of the
list to a common base class (e.g., a <tt>list&lt;Register&gt;</tt> can only
contain definitions derived from the "<tt>Register</tt>" class).</li>

<li>"<tt>code</tt>" - This represents a big hunk of text.  NOTE: I don't
remember why this is distinct from string!</li>

<li>"<tt>dag</tt>" - This type represents a nestable directed graph of
elements.</li>
</ul>
</p>

<p>
To date, these types have been sufficient for describing things that TableGen
has been used for, but it is straight-forward to extend this list if needed.
</p>

</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="values">TableGen values and expressions</tt></a>
</div>

<div>
<p>
TableGen allows for a pretty reasonable number of different expression forms
when building up values.  These forms allow the TableGen file to be written in a
natural syntax and flavor for the application.  The current expression forms
supported include:
</p>

<p><ul>
<li>? - Uninitialized field.</li>
<li>0b1001011 - Binary integer value.</li>
<li>07654321 - Octal integer value (indicated by a leading 0).</li>
<li>7 - Decimal integer value.</li>
<li>0x7F - Hexadecimal integer value.</li>
<li>"foo" - String value.</li>
<li>[{ .... }] - Code fragment.</li>
<li>[ X, Y, Z ] - List value.</li>
<li>{ a, b, c } - Initializer for a "bits&lt;3&gt;" value.</li>
<li>value - Value reference.</li>
<li>value{17} - Access to one or more bits of a value.</li>
<li>DEF - Reference to a record definition.</li>
<li>X.Y - Reference to the subfield of a value.</li>

<li>(DEF a, b) - A dag value.  The first element is required to be a record
definition, the remaining elements in the list may be arbitrary other values,
including nested 'dag' values.</li>

</ul></p>

<p>
Note that all of the values have rules specifying how they convert to to values
for different types.  These rules allow you to assign a value like "7" to a
"bits&lt;4&gt;" value, for example.
</p>



</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="classesdefs">Classes and definitions</tt></a>
</div>

<div>
<p>
As mentioned in the <a href="#concepts">intro</a>, classes and definitions
(collectively known as 'records') in TableGen are the main high-level unit of
information that TableGen collects.  Records are defined with a <tt>def</tt> or
<tt>class</tt> keyword, the record name, and an optional list of "<a
href="templateargs">template arguments</a>".  If the record has superclasses,
they are specified as a comma seperated list that starts with a colon character
(":").  If <a href="#valuedef">value definitions</a> or <a href="#recordlet">let
expressions</a> are needed for the class they are enclosed in curly braces
("{}"), otherwise the record ends with a semicolon.  Here is a simple TableGen
file:
</p>

<p><pre>
class C { bit V = 1; }
def X : C;
def Y : C {
  string Greeting = "hello";
}
</pre></p>

<p>
This example defines two definitions, <tt>X</tt> and <tt>Y</tt>, both of which
derive from the <tt>C</tt> class.  Because of this, they both get the <tt>V</tt>
bit value.  The <tt>Y</tt> definition also gets the Greeting member as well.
</p>

</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="valuedef">Value definitions</tt></a>
</div>

<div class="doc_text">
<p>
Value definitions define named entries in records.  A value must be defined
before it can be referred to as the operand for another value definition, or
before the value is reset with a <a href="#recordlet">let expression</a>.  A
value is defined by specifying a <a href="#types">TableGen type</a> and a name.
If an initial value is available, it may be specified after the type with an
equal sign.  Value definitions require terminating semicolons.
</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="recordlet">'let' expressions</tt></a>
</div>

<div class="doc_text">
<p>
A record-level let expression is used to change the value of a value definition
in a record.  This is primarily useful when a superclass defines a value that a
derived class or definitions wants to override.  Let expressions consist of the
'<tt>let</tt>' keyword, followed by a value name, an equal sign ("="), and a new
value for example, a new class could be added to the example above, redefining
the <tt>V</tt> field for all of its subclasses:</p>

<p><pre>
class D : C { let V = 0; }
def Z : D;
</pre></p>

<p>
In this case, the <tt>Z</tt> definition will have a zero value for its "V"
value, despite the fact that it derives (indirectly) from the <tt>C</tt> class,
because the <tt>D</tt> class overrode its value.
</p>

</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="templateargs">Class template arguments</tt></a>
</div>

<div class="doc_text">
and default values...
</div>



<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="filescope">File scope entities</tt></a>
</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="include">File inclusion</tt></a>
</div>

<div class="doc_text">
<p>
TableGen supports the '<tt>include</tt>' token, which textually substitutes the
specified file in place of the include directive.  The filename should be
specified as a double quoted string immediately after the '<tt>include</tt>'
keyword.  Example:

<p><pre>
  include "foo.td"
</pre></p>

</div>

<!----------------------------------------------------------------------------->
<div class="doc_subsubsection">
  <a name="globallet">'let' expressions</tt></a>
</div>

<div class="doc_text">
<p>
"let" expressions at file scope are similar to <a href="#recordlet">"let"
expressions within a record</a>, except they can specify a value binding for
multiple records at a time, and may be useful in certain other cases.
File-scope let expressions are really just another way that TableGen allows the
end-user to factor out commonality from the records.
</p>

<p>
File-scope "let" expressions take a comma-seperated list of bindings to apply,
and one of more records to bind the values in.  Here are some examples:
</p>

<p><pre>
let isTerminator = 1, isReturn = 1 in
  def RET : X86Inst&lt;"ret", 0xC3, RawFrm, NoArg&gt;;

let isCall = 1 in
  // All calls clobber the non-callee saved registers...
  let Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6] in {
    def CALLpcrel32 : X86Inst&lt;"call", 0xE8, RawFrm, NoArg&gt;;
    def CALLr32     : X86Inst&lt;"call", 0xFF, MRMS2r, Arg32&gt;;
    def CALLm32     : X86Inst&lt;"call", 0xFF, MRMS2m, Arg32&gt;;
  }
</pre></p>

<p>
File-scope "let" expressions are often useful when a couple of definitions need
to be added to several records, and the records do not otherwise need to be
opened, as in the case with the CALL* instructions above.
</p>
</div>


<!-- *********************************************************************** -->
<div class="doc_section"><a name="backends">TableGen backends</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
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.
</p>

</div>


<!-- *********************************************************************** -->
<div class="doc_section"><a name="codegenerator">The LLVM code generator</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
This is just a temporary, convenient, place to put stuff about the code
generator before it gets its own document.  This should describe all of the
tablegen backends used by the code generator and the classes/definitions they
expect.
</p>

</div>



<!-- *********************************************************************** -->
<hr>
<div class="doc_footer">
  <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
  <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
  <br>
  Last modified: $Date$
</div>

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