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+==============================
+TableGen Language Introduction
+==============================
+
+.. 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 not 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). For
+the formal language specification, see :doc:`LangRef`.
+
+TableGen syntax
+===============
+
+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.
+
+TableGen primitives
+-------------------
+
+TableGen comments
+^^^^^^^^^^^^^^^^^
+
+TableGen supports C++ style "``//``" comments, which run to the end of the
+line, and it also supports **nestable** "``/* */``" comments.
+
+.. _TableGen type:
+
+The TableGen type system
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+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 `value
+definition`_ is required to have an associated type.
+
+TableGen supports a mixture of very low-level types (such as ``bit``) and very
+high-level types (such as ``dag``).  This flexibility is what allows it to
+describe a wide range of information conveniently and compactly.  The TableGen
+types are:
+
+``bit``
+    A 'bit' is a boolean value that can hold either 0 or 1.
+
+``int``
+    The 'int' type represents a simple 32-bit integer value, such as 5.
+
+``string``
+    The 'string' type represents an ordered sequence of characters of arbitrary
+    length.
+
+``bits<n>``
+    A 'bits' type is an 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.
+
+``list<ty>``
+    This type represents a list whose elements are some other type.  The
+    contained type is arbitrary: it can even be another list type.
+
+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 ``list<Register>`` can only contain definitions
+    derived from the "``Register``" class).
+
+``dag``
+    This type represents a nestable directed graph of elements.
+
+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.
+
+.. _TableGen expressions:
+
+TableGen values and expressions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+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:
+
+``?``
+    uninitialized field
+
+``0b1001011``
+    binary integer value
+
+``07654321``
+    octal integer value (indicated by a leading 0)
+
+``7``
+    decimal integer value
+
+``0x7F``
+    hexadecimal integer value
+
+``"foo"``
+    string value
+
+``[{ ... }]``
+    usually called a "code fragment", but is just a multiline string literal
+
+``[ X, Y, Z ]<type>``
+    list value.  <type> is the type of the list element and is usually optional.
+    In rare cases, TableGen is unable to deduce the element type in which case
+    the user must specify it explicitly.
+
+``{ a, b, c }``
+    initializer for a "bits<3>" value
+
+``value``
+    value reference
+
+``value{17}``
+    access to one bit of a value
+
+``value{15-17}``
+    access to multiple bits of a value
+
+``DEF``
+    reference to a record definition
+
+``CLASS<val list>``
+    reference to a new anonymous definition of CLASS with the specified template
+    arguments.
+
+``X.Y``
+    reference to the subfield of a value
+
+``list[4-7,17,2-3]``
+    A slice of the 'list' list, including elements 4,5,6,7,17,2, and 3 from it.
+    Elements may be included multiple times.
+
+``foreach <var> = [ <list> ] in { <body> }``
+
+``foreach <var> = [ <list> ] in <def>``
+    Replicate <body> or <def>, replacing instances of <var> with each value
+    in <list>.  <var> is scoped at the level of the ``foreach`` loop and must
+    not conflict with any other object introduced in <body> or <def>.  Currently
+    only ``def``\s are expanded within <body>.
+
+``foreach <var> = 0-15 in ...``
+
+``foreach <var> = {0-15,32-47} in ...``
+    Loop over ranges of integers. The braces are required for multiple ranges.
+
+``(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.
+
+``!strconcat(a, b)``
+    A string value that is the result of concatenating the 'a' and 'b' strings.
+
+``str1#str2``
+    "#" (paste) is a shorthand for !strconcat.  It may concatenate things that
+    are not quoted strings, in which case an implicit !cast<string> is done on
+    the operand of the paste.
+
+``!cast<type>(a)``
+    A symbol of type *type* obtained by looking up the string 'a' in the symbol
+    table.  If the type of 'a' does not match *type*, TableGen aborts with an
+    error. !cast<string> is a special case in that the argument must be an
+    object defined by a 'def' construct.
+
+``!subst(a, b, c)``
+    If 'a' and 'b' are of string type or are symbol references, substitute 'b'
+    for 'a' in 'c.'  This operation is analogous to $(subst) in GNU make.
+
+``!foreach(a, b, c)``
+    For each member 'b' of dag or list 'a' apply operator 'c.'  'b' is a dummy
+    variable that should be declared as a member variable of an instantiated
+    class.  This operation is analogous to $(foreach) in GNU make.
+
+``!head(a)``
+    The first element of list 'a.'
+
+``!tail(a)``
+    The 2nd-N elements of list 'a.'
+
+``!empty(a)``
+    An integer {0,1} indicating whether list 'a' is empty.
+
+``!if(a,b,c)``
+  'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.
+
+``!eq(a,b)``
+    'bit 1' if string a is equal to string b, 0 otherwise.  This only operates
+    on string, int and bit objects.  Use !cast<string> to compare other types of
+    objects.
+
+Note that all of the values have rules specifying how they convert to values
+for different types.  These rules allow you to assign a value like "``7``"
+to a "``bits<4>``" value, for example.
+
+Classes and definitions
+-----------------------
+
+As mentioned in the :doc:`introduction <index>`, 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 ``def`` or ``class`` keyword, the record
+name, and an optional list of "`template arguments`_".  If the record has
+superclasses, they are specified as a comma separated list that starts with a
+colon character ("``:``").  If `value definitions`_ or `let expressions`_ are
+needed for the class, they are enclosed in curly braces ("``{}``"); otherwise,
+the record ends with a semicolon.
+
+Here is a simple TableGen file:
+
+.. code-block:: llvm
+
+  class C { bit V = 1; }
+  def X : C;
+  def Y : C {
+    string Greeting = "hello";
+  }
+
+This example defines two definitions, ``X`` and ``Y``, both of which derive from
+the ``C`` class.  Because of this, they both get the ``V`` bit value.  The ``Y``
+definition also gets the Greeting member as well.
+
+In general, classes are useful for collecting together the commonality between a
+group of records and isolating it in a single place.  Also, classes permit the
+specification of default values for their subclasses, allowing the subclasses to
+override them as they wish.
+
+.. _value definition:
+.. _value definitions:
+
+Value definitions
+^^^^^^^^^^^^^^^^^
+
+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 `let expression`_.  A value is defined by
+specifying a `TableGen type`_ 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.
+
+.. _let expression:
+.. _let expressions:
+.. _"let" expressions within a record:
+
+'let' expressions
+^^^^^^^^^^^^^^^^^
+
+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 definition wants to override.  Let expressions consist of the
+'``let``' 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 ``V`` field for all of its subclasses:
+
+.. code-block:: llvm
+
+  class D : C { let V = 0; }
+  def Z : D;
+
+In this case, the ``Z`` definition will have a zero value for its ``V`` value,
+despite the fact that it derives (indirectly) from the ``C`` class, because the
+``D`` class overrode its value.
+
+.. _template arguments:
+
+Class template arguments
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+TableGen permits the definition of parameterized classes as well as normal
+concrete classes.  Parameterized TableGen classes specify a list of variable
+bindings (which may optionally have defaults) that are bound when used.  Here is
+a simple example:
+
+.. code-block:: llvm
+
+  class FPFormat<bits<3> val> {
+    bits<3> Value = val;
+  }
+  def NotFP      : FPFormat<0>;
+  def ZeroArgFP  : FPFormat<1>;
+  def OneArgFP   : FPFormat<2>;
+  def OneArgFPRW : FPFormat<3>;
+  def TwoArgFP   : FPFormat<4>;
+  def CompareFP  : FPFormat<5>;
+  def CondMovFP  : FPFormat<6>;
+  def SpecialFP  : FPFormat<7>;
+
+In this case, template arguments are used as a space efficient way to specify a
+list of "enumeration values", each with a "``Value``" field set to the specified
+integer.
+
+The more esoteric forms of `TableGen expressions`_ are useful in conjunction
+with template arguments.  As an example:
+
+.. code-block:: llvm
+
+  class ModRefVal<bits<2> val> {
+    bits<2> Value = val;
+  }
+
+  def None   : ModRefVal<0>;
+  def Mod    : ModRefVal<1>;
+  def Ref    : ModRefVal<2>;
+  def ModRef : ModRefVal<3>;
+
+  class Value<ModRefVal MR> {
+    // Decode some information into a more convenient format, while providing
+    // a nice interface to the user of the "Value" class.
+    bit isMod = MR.Value{0};
+    bit isRef = MR.Value{1};
+
+    // other stuff...
+  }
+
+  // Example uses
+  def bork : Value<Mod>;
+  def zork : Value<Ref>;
+  def hork : Value<ModRef>;
+
+This is obviously a contrived example, but it shows how template arguments can
+be used to decouple the interface provided to the user of the class from the
+actual internal data representation expected by the class.  In this case,
+running ``llvm-tblgen`` on the example prints the following definitions:
+
+.. code-block:: llvm
+
+  def bork {      // Value
+    bit isMod = 1;
+    bit isRef = 0;
+  }
+  def hork {      // Value
+    bit isMod = 1;
+    bit isRef = 1;
+  }
+  def zork {      // Value
+    bit isMod = 0;
+    bit isRef = 1;
+  }
+
+This shows that TableGen was able to dig into the argument and extract a piece
+of information that was requested by the designer of the "Value" class.  For
+more realistic examples, please see existing users of TableGen, such as the X86
+backend.
+
+Multiclass definitions and instances
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+While classes with template arguments are a good way to factor commonality
+between two instances of a definition, multiclasses allow a convenient notation
+for defining multiple definitions at once (instances of implicitly constructed
+classes).  For example, consider an 3-address instruction set whose instructions
+come in two forms: "``reg = reg op reg``" and "``reg = reg op imm``"
+(e.g. SPARC). In this case, you'd like to specify in one place that this
+commonality exists, then in a separate place indicate what all the ops are.
+
+Here is an example TableGen fragment that shows this idea:
+
+.. code-block:: llvm
+
+  def ops;
+  def GPR;
+  def Imm;
+  class inst<int opc, string asmstr, dag operandlist>;
+
+  multiclass ri_inst<int opc, string asmstr> {
+    def _rr : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
+                   (ops GPR:$dst, GPR:$src1, GPR:$src2)>;
+    def _ri : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
+                   (ops GPR:$dst, GPR:$src1, Imm:$src2)>;
+  }
+
+  // Instantiations of the ri_inst multiclass.
+  defm ADD : ri_inst<0b111, "add">;
+  defm SUB : ri_inst<0b101, "sub">;
+  defm MUL : ri_inst<0b100, "mul">;
+  ...
+
+The name of the resultant definitions has the multidef fragment names appended
+to them, so this defines ``ADD_rr``, ``ADD_ri``, ``SUB_rr``, etc.  A defm may
+inherit from multiple multiclasses, instantiating definitions from each
+multiclass.  Using a multiclass this way is exactly equivalent to instantiating
+the classes multiple times yourself, e.g. by writing:
+
+.. code-block:: llvm
+
+  def ops;
+  def GPR;
+  def Imm;
+  class inst<int opc, string asmstr, dag operandlist>;
+
+  class rrinst<int opc, string asmstr>
+    : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
+           (ops GPR:$dst, GPR:$src1, GPR:$src2)>;
+
+  class riinst<int opc, string asmstr>
+    : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
+           (ops GPR:$dst, GPR:$src1, Imm:$src2)>;
+
+  // Instantiations of the ri_inst multiclass.
+  def ADD_rr : rrinst<0b111, "add">;
+  def ADD_ri : riinst<0b111, "add">;
+  def SUB_rr : rrinst<0b101, "sub">;
+  def SUB_ri : riinst<0b101, "sub">;
+  def MUL_rr : rrinst<0b100, "mul">;
+  def MUL_ri : riinst<0b100, "mul">;
+  ...
+
+A ``defm`` can also be used inside a multiclass providing several levels of
+multiclass instantiations.
+
+.. code-block:: llvm
+
+  class Instruction<bits<4> opc, string Name> {
+    bits<4> opcode = opc;
+    string name = Name;
+  }
+
+  multiclass basic_r<bits<4> opc> {
+    def rr : Instruction<opc, "rr">;
+    def rm : Instruction<opc, "rm">;
+  }
+
+  multiclass basic_s<bits<4> opc> {
+    defm SS : basic_r<opc>;
+    defm SD : basic_r<opc>;
+    def X : Instruction<opc, "x">;
+  }
+
+  multiclass basic_p<bits<4> opc> {
+    defm PS : basic_r<opc>;
+    defm PD : basic_r<opc>;
+    def Y : Instruction<opc, "y">;
+  }
+
+  defm ADD : basic_s<0xf>, basic_p<0xf>;
+  ...
+
+  // Results
+  def ADDPDrm { ...
+  def ADDPDrr { ...
+  def ADDPSrm { ...
+  def ADDPSrr { ...
+  def ADDSDrm { ...
+  def ADDSDrr { ...
+  def ADDY { ...
+  def ADDX { ...
+
+``defm`` declarations can inherit from classes too, the rule to follow is that
+the class list must start after the last multiclass, and there must be at least
+one multiclass before them.
+
+.. code-block:: llvm
+
+  class XD { bits<4> Prefix = 11; }
+  class XS { bits<4> Prefix = 12; }
+
+  class I<bits<4> op> {
+    bits<4> opcode = op;
+  }
+
+  multiclass R {
+    def rr : I<4>;
+    def rm : I<2>;
+  }
+
+  multiclass Y {
+    defm SS : R, XD;
+    defm SD : R, XS;
+  }
+
+  defm Instr : Y;
+
+  // Results
+  def InstrSDrm {
+    bits<4> opcode = { 0, 0, 1, 0 };
+    bits<4> Prefix = { 1, 1, 0, 0 };
+  }
+  ...
+  def InstrSSrr {
+    bits<4> opcode = { 0, 1, 0, 0 };
+    bits<4> Prefix = { 1, 0, 1, 1 };
+  }
+
+File scope entities
+-------------------
+
+File inclusion
+^^^^^^^^^^^^^^
+
+TableGen supports the '``include``' 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 '``include``' keyword.
+Example:
+
+.. code-block:: llvm
+
+  include "foo.td"
+
+'let' expressions
+^^^^^^^^^^^^^^^^^
+
+"Let" expressions at file scope are similar to `"let" expressions within a
+record`_, 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.
+
+File-scope "let" expressions take a comma-separated list of bindings to apply,
+and one or more records to bind the values in.  Here are some examples:
+
+.. code-block:: llvm
+
+  let isTerminator = 1, isReturn = 1, isBarrier = 1, hasCtrlDep = 1 in
+    def RET : I<0xC3, RawFrm, (outs), (ins), "ret", [(X86retflag 0)]>;
+
+  let isCall = 1 in
+    // All calls clobber the non-callee saved registers...
+    let Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6, ST0,
+                MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7,
+                XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7, EFLAGS] in {
+      def CALLpcrel32 : Ii32<0xE8, RawFrm, (outs), (ins i32imm:$dst,variable_ops),
+                             "call\t${dst:call}", []>;
+      def CALL32r     : I<0xFF, MRM2r, (outs), (ins GR32:$dst, variable_ops),
+                          "call\t{*}$dst", [(X86call GR32:$dst)]>;
+      def CALL32m     : I<0xFF, MRM2m, (outs), (ins i32mem:$dst, variable_ops),
+                          "call\t{*}$dst", []>;
+    }
+
+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.
+
+It's also possible to use "let" expressions inside multiclasses, providing more
+ways to factor out commonality from the records, specially if using several
+levels of multiclass instantiations. This also avoids the need of using "let"
+expressions within subsequent records inside a multiclass.
+
+.. code-block:: llvm
+
+  multiclass basic_r<bits<4> opc> {
+    let Predicates = [HasSSE2] in {
+      def rr : Instruction<opc, "rr">;
+      def rm : Instruction<opc, "rm">;
+    }
+    let Predicates = [HasSSE3] in
+      def rx : Instruction<opc, "rx">;
+  }
+
+  multiclass basic_ss<bits<4> opc> {
+    let IsDouble = 0 in
+      defm SS : basic_r<opc>;
+
+    let IsDouble = 1 in
+      defm SD : basic_r<opc>;
+  }
+
+  defm ADD : basic_ss<0xf>;
+
+Looping
+^^^^^^^
+
+TableGen supports the '``foreach``' block, which textually replicates the loop
+body, substituting iterator values for iterator references in the body.
+Example:
+
+.. code-block:: llvm
+
+  foreach i = [0, 1, 2, 3] in {
+    def R#i : Register<...>;
+    def F#i : Register<...>;
+  }
+
+This will create objects ``R0``, ``R1``, ``R2`` and ``R3``.  ``foreach`` blocks
+may be nested. If there is only one item in the body the braces may be
+elided:
+
+.. code-block:: llvm
+
+  foreach i = [0, 1, 2, 3] in
+    def R#i : Register<...>;
+
+Code Generator backend info
+===========================
+
+Expressions used by code generator to describe instructions and isel patterns:
+
+``(implicit a)``
+    an implicitly defined physical register.  This tells the dag instruction
+    selection emitter the input pattern's extra definitions matches implicit
+    physical register definitions.
+