diff --git a/llvm/docs/TableGen/LangIntro.rst b/llvm/docs/TableGen/LangIntro.rst
deleted file mode 100644
--- a/llvm/docs/TableGen/LangIntro.rst
+++ /dev/null
@@ -1,737 +0,0 @@
-==============================
-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 llvm-dev.
-
-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.
-
-``code``
-    The `code` type represents a code fragment, which can be single/multi-line
-    string literal.
-
-``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.
-    Note that this is sized by the number of bits given and will not be
-    silently extended/truncated.
-
-``7``
-    decimal integer value
-
-``0x7F``
-    hexadecimal integer value
-
-``"foo"``
-    a single-line string value, can be assigned to ``string`` or ``code`` variable.
-
-``[{ ... }]``
-    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, 0b10 }``
-    initializer for a "bits<4>" value.
-    1-bit from "a", 1-bit from "b", 2-bits from 0b10.
-
-``value``
-    value reference
-
-``value{17}``
-    access to one bit of a value
-
-``value{15-17}``
-    access to an ordered sequence of bits of a value, in particular ``value{15-17}``
-    produces an order that is the reverse of ``value{17-15}``.
-
-``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>.  Only
-    ``def``\s and ``defm``\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.
-
-``!con(a, b, ...)``
-    Concatenate two or more DAG nodes. Their operations must equal.
-
-    Example: !con((op a1:$name1, a2:$name2), (op b1:$name3)) results in
-    the DAG node (op a1:$name1, a2:$name2, b1:$name3).
-
-``!dag(op, children, names)``
-    Generate a DAG node programmatically. 'children' and 'names' must be lists
-    of equal length or unset ('?'). 'names' must be a 'list<string>'.
-
-    Due to limitations of the type system, 'children' must be a list of items
-    of a common type. In practice, this means that they should either have the
-    same type or be records with a common superclass. Mixing dag and non-dag
-    items is not possible. However, '?' can be used.
-
-    Example: !dag(op, [a1, a2, ?], ["name1", "name2", "name3"]) results in
-    (op a1:$name1, a2:$name2, ?:$name3).
-
-``!setop(dag, op)``
-    Return a DAG node with the same arguments as ``dag``, but with its
-    operator replaced with ``op``.
-
-    Example: ``!setop((foo 1, 2), bar)`` results in ``(bar 1, 2)``.
-
-``!getop(dag)``
-
-``!getop<type>(dag)``
-    Return the operator of the given DAG node.
-    Example: ``!getop((foo 1, 2))`` results in ``foo``.
-
-    The result of ``!getop`` can be used directly in a context where
-    any record value at all is acceptable (typically placing it into
-    another dag value). But in other contexts, it must be explicitly
-    cast to a particular class type. The ``!getop<type>`` syntax is
-    provided to make this easy.
-
-    For example, to assign the result to a class-typed value, you
-    could write either of these:
-    ``BaseClass b = !getop<BaseClass>(someDag);``
-
-    ``BaseClass b = !cast<BaseClass>(!getop(someDag));``
-
-    But to build a new dag node reusing the operator from another, no
-    cast is necessary:
-    ``dag d = !dag(!getop(someDag), args, names);``
-
-``!listconcat(a, b, ...)``
-    A list value that is the result of concatenating the 'a' and 'b' lists.
-    The lists must have the same element type.
-    More than two arguments are accepted with the result being the concatenation
-    of all the lists given.
-
-``!listsplat(a, size)``
-    A list value that contains the value ``a`` ``size`` times.
-    Example: ``!listsplat(0, 2)`` results in ``[0, 0]``.
-
-``!strconcat(a, b, ...)``
-    A string value that is the result of concatenating the 'a' and 'b' strings.
-    More than two arguments are accepted with the result being the concatenation
-    of all the strings given.
-
-``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)``
-    If 'a' is a string, a record of type *type* obtained by looking up the
-    string 'a' in the list of all records defined by the time that all template
-    arguments in 'a' are fully resolved.
-
-    For example, if !cast<type>(a) appears in a multiclass definition, or in a
-    class instantiated inside of a multiclass definition, and 'a' does not
-    reference any template arguments of the multiclass, then a record of name
-    'a' must be instantiated earlier in the source file. If 'a' does reference
-    a template argument, then the lookup is delayed until defm statements
-    instantiating the multiclass (or later, if the defm occurs in another
-    multiclass and template arguments of the inner multiclass that are
-    referenced by 'a' are substituted by values that themselves contain
-    references to template arguments of the outer multiclass).
-
-    If the type of 'a' does not match *type*, TableGen aborts with an error.
-
-    Otherwise, perform a normal type cast e.g. between an int and a bit, or
-    between record types. This allows casting a record to a subclass, though if
-    the types do not match, constant folding will be inhibited. !cast<string>
-    is a special case in that the argument can be an int or a record. In the
-    latter case, the record's name is returned.
-
-``!isa<type>(a)``
-    Returns an integer: 1 if 'a' is dynamically of the given type, 0 otherwise.
-
-``!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 of dag or list 'b' apply operator 'c'. 'a' is the name
-    of a variable that will be substituted by members of 'b' in 'c'.
-    This operation is analogous to $(foreach) in GNU make.
-
-``!foldl(start, lst, a, b, expr)``
-    Perform a left-fold over 'lst' with the given starting value. 'a' and 'b'
-    are variable names which will be substituted in 'expr'. If you think of
-    expr as a function f(a,b), the fold will compute
-    'f(...f(f(start, lst[0]), lst[1]), ...), lst[n-1])' for a list of length n.
-    As usual, 'a' will be of the type of 'start', and 'b' will be of the type
-    of elements of 'lst'. These types need not be the same, but 'expr' must be
-    of the same type as 'start'.
-
-``!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.
-
-``!size(a)``
-    An integer indicating the number of elements in list 'a'.
-
-``!if(a,b,c)``
-  'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.
-
-``!cond(condition_1 : val1, condition_2 : val2, ..., condition_n : valn)``
-    Instead of embedding !if inside !if which can get cumbersome,
-    one can use !cond. !cond returns 'val1' if the result of 'int' or 'bit'
-    operator 'condition1' is nonzero. Otherwise, it checks 'condition2'.
-    If 'condition2' is nonzero, returns 'val2', and so on.
-    If all conditions are zero, it reports an error.  
-
-    For example, to convert an integer 'x' into a string:
-      !cond(!lt(x,0) : "negative", !eq(x,0) : "zero", 1 : "positive")
-
-``!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.
-
-``!ne(a,b)``
-    The negation of ``!eq(a,b)``.
-
-``!le(a,b), !lt(a,b), !ge(a,b), !gt(a,b)``
-    (Signed) comparison of integer values that returns bit 1 or 0 depending on
-    the result of the comparison.
-
-``!shl(a,b)`` ``!srl(a,b)`` ``!sra(a,b)``
-    The usual shift operators. Operations are on 64-bit integers, the result
-    is undefined for shift counts outside [0, 63].
-
-``!add(a,b,...)`` ``!mul(a,b,...)`` ``!and(a,b,...)`` ``!or(a,b,...)``
-    The usual arithmetic and binary operators.
-
-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:: text
-
-  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:: text
-
-  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.
-
-References between variables in a record are substituted late, which gives
-``let`` expressions unusual power. Consider this admittedly silly example:
-
-.. code-block:: text
-
-  class A<int x> {
-    int Y = x;
-    int Yplus1 = !add(Y, 1);
-    int xplus1 = !add(x, 1);
-  }
-  def Z : A<5> {
-    let Y = 10;
-  }
-
-The value of ``Z.xplus1`` will be 6, but the value of ``Z.Yplus1`` is 11. Use
-this power wisely.
-
-.. _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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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:: text
-
-  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.
-
diff --git a/llvm/docs/TableGen/LangRef.rst b/llvm/docs/TableGen/LangRef.rst
deleted file mode 100644
--- a/llvm/docs/TableGen/LangRef.rst
+++ /dev/null
@@ -1,556 +0,0 @@
-===========================
-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 llvm-dev.
-
-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.  TableGen also provides simple `Preprocessing Support`_.
-
-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.
-
-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:
-
-.. 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
-   if    then      else
-
-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       !and
-               :!or     !empty   !subst   !foreach   !strconcat
-               :!cast   !listconcat       !size      !foldl
-               :!isa    !dag     !le      !lt        !ge
-               :!gt     !ne      !mul     !listsplat !setop
-               :!getop
-
-TableGen also has !cond operator that needs a slightly different
-syntax compared to other "bang operators":
-
-.. productionlist::
-   CondOperator: !cond
-
-
-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` | `Defset` | `Defvar` | `Let` |
-           `MultiClass` | `Foreach` | `If`
-
-``class``\es
-------------
-
-.. productionlist::
-   Class: "class" `TokIdentifier` [`TemplateArgList`] `ObjectBody`
-   TemplateArgList: "<" `Declaration` ("," `Declaration`)* ">"
-
-A ``class`` declaration creates a record which other records can inherit
-from. A class can be parameterized 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 constituents 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 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
-------------
-
-.. 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 identifier.
-
-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;
-     }
-
-  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``
-
-.. 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` ")"
-              :| `CondOperator` "(" `CondVal` ("," `CondVal`)* ")"
-   CondVal: `Value` ":" `Value`
-
-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` ";"
-           :| `Defvar`
-
-The ``let`` form allows overriding the value of an inherited field.
-
-``def``
--------
-
-.. productionlist::
-   Def: "def" [`Value`] `ObjectBody`
-
-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``
---------
-
-.. productionlist::
-   Defm: "defm" [`Value`] ":" `BaseClassListNE` ";"
-
-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``
-----------
-.. productionlist::
-   Defset: "defset" `Type` `TokIdentifier` "=" "{" `Object`* "}"
-
-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``
-----------
-.. productionlist::
-   Defvar: "defvar" `TokIdentifier` "=" `Value` ";"
-
-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``
------------
-
-.. productionlist::
-   Foreach: "foreach" `ForeachDeclaration` "in" "{" `Object`* "}"
-          :| "foreach" `ForeachDeclaration` "in" `Object`
-   ForeachDeclaration: ID "=" ( "{" `RangeList` "}" | `RangePiece` | `Value` )
-
-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``
-------
-
-.. productionlist::
-   If: "if" `Value` "then" `IfBody`
-     :| "if" `Value` "then" `IfBody` "else" `IfBody`
-   IfBody: "{" `Object`* "}" | `Object`
-
-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``
------------------
-
-.. 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`
-
-Preprocessing Support
-=====================
-
-TableGen's embedded preprocessor is only intended for conditional compilation.
-It supports the following directives:
-
-.. productionlist::
-   LineBegin: ^
-   LineEnd: "\n" | "\r" | EOF
-   WhiteSpace: " " | "\t"
-   CStyleComment: "/*" (.* - "*/") "*/"
-   BCPLComment: "//" (.* - `LineEnd`) `LineEnd`
-   WhiteSpaceOrCStyleComment: `WhiteSpace` | `CStyleComment`
-   WhiteSpaceOrAnyComment: `WhiteSpace` | `CStyleComment` | `BCPLComment`
-   MacroName: `ualpha` (`ualpha` | "0"..."9")*
-   PrepDefine: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
-             : "#define" (`WhiteSpace`)+ `MacroName`
-             : (`WhiteSpaceOrAnyComment`)* `LineEnd`
-   PrepIfdef: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
-            : "#ifdef" (`WhiteSpace`)+ `MacroName`
-            : (`WhiteSpaceOrAnyComment`)* `LineEnd`
-   PrepElse: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
-           : "#else" (`WhiteSpaceOrAnyComment`)* `LineEnd`
-   PrepEndif: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
-            : "#endif" (`WhiteSpaceOrAnyComment`)* `LineEnd`
-   PrepRegContentException: `PrepIfdef` | `PrepElse` | `PrepEndif` | EOF
-   PrepRegion: .* - `PrepRegContentException`
-             :| `PrepIfdef`
-             :  (`PrepRegion`)*
-             :  [`PrepElse`]
-             :  (`PrepRegion`)*
-             :  `PrepEndif`
-
-: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.
diff --git a/llvm/docs/TableGen/ProgRef.rst b/llvm/docs/TableGen/ProgRef.rst
new file mode 100644
--- /dev/null
+++ b/llvm/docs/TableGen/ProgRef.rst
@@ -0,0 +1,1709 @@
+===============================
+TableGen Programmer's Reference
+===============================
+
+.. 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 one or more output files. These output files are typically
+``.inc`` files for C++, but may be any type of file that the backend
+developer needs.
+
+This document describes the LLVM TableGen facility in detail. It is intended
+for the programmer who is using TableGen to produce tables for a project. If
+you are looking for a simple overview, check out :doc:`TableGen Overview <./index>`.
+
+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.
+
+* Generate the register file information for a particular target machine.
+
+* Generate the instruction definitions for a target.
+
+* Generate the patterns that the code generator uses to match instructions
+  to intermediate representation (IR) nodes.
+
+* Generate semantic attribute identifiers for Clang.
+
+* Generate abstract syntax tree (AST) declaration node definitions for Clang.
+
+* Generate AST statement node definitions for Clang.
+
+
+Concepts
+--------
+
+TableGen source files contain two primary items: *abstract records* and
+*concrete records*. In this and other TableGen documents, abstract records
+are called *classes.* (These classes are different from C++ classes and do
+not map onto them.) In addition, concrete records are usually just called
+records, although sometimes the term *record* refers to both classes and
+concrete records. The distinction should be clear in context.
+
+Classes and concrete records have a unique *name*, either chosen by
+the programmer or generated by TableGen. Associated with that name
+is a list of *fields* with values and an optional list of *superclasses*
+(sometimes called base or parent classes). The fields are the primary data that
+backends will process. Note that TableGen assigns no meanings to fields; the
+meanings are entirely up to the backends and the programs that incorporate
+the output of those backends.
+
+A backend processes some subset of the concrete records built by the
+TableGen parser and emits the output files. These files are usually C++
+``.inc`` files that are included by the programs that require the data in
+those records. However, a backend can produce any type of output files. For
+example, it could produce a data file containing messages tagged with
+identifiers and substitution parameters. In a complex use case such as the
+LLVM code generator, there can be many concrete records and some of them can
+have an unexpectedly large number of fields, resulting in large output files.
+
+In order to reduce the complexity of TableGen files, classes are used to
+abstract out groups of record fields. For example, a few classes may
+abstract the concept of a machine register file, while other classes may
+abstract the instruction formats, and still others may abstract the
+individual instructions. TableGen allows an arbitrary hierarchy of classes,
+so that the abstract classes for two concepts can share a third superclass that
+abstracts common "sub-concepts" from the two original concepts.
+
+In order to make classes more useful, a concrete record (or another class)
+can request a class as a superclass and pass *template arguments* to it.
+These template arguments can be used in the fields of the superclass to
+initialize them in a custom manner. That is, record or class ``A`` can
+request superclass ``S`` with one set of template arguments, while record or class
+``B`` can request ``S`` with a different set of arguments. Without template
+arguments, many more classes would be required, one for each combination of
+the template arguments.
+
+Both classes and concrete records can include fields that are uninitialized.
+The uninitialized "value" is represented by a question mark (``?``). Classes
+often have uninitialized fields that are expected to be filled in when those
+classes are inherited by concrete records. Even so, some fields of concrete
+records may remain uninitialized.
+
+TableGen provides *multiclasses* to collect a group of record definitions in
+one place. A multiclass is a sort of macro that can be "invoked" to define
+multiple concrete records all at once. A multiclass can inherit from other
+multiclasses, which means that the multiclass inherits all the definitions
+from its parent multiclasses.
+
+`Appendix B: Sample Record`_ illustrates a complex record in the Intel X86
+target and the simple way in which it is defined.
+
+Source Files
+============
+
+TableGen source files are plain ASCII text files. The files can contain
+statements, comments, and blank lines (see `Lexical Analysis`_). The standard file
+extension for TableGen files is ``.td``.
+
+TableGen files can grow quite large, so there is an include mechanism that
+allows one file to include the content of another file (see `Include
+Files`_). This allows large files to be broken up into smaller ones, and
+also provides a simple library mechanism where multiple source files can
+include the same library file.
+
+TableGen supports a simple preprocessor that can be used to conditionalize
+portions of ``.td`` files. See `Preprocessing Facilities`_ for more
+information.
+
+Lexical Analysis
+================
+
+The lexical and syntax notation used here is intended to imitate
+`Python's`_ notation. 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
+
+TableGen supports BCPL-style comments (``// ...``) and nestable C-style
+comments (``/* ... */``).
+TableGen also provides simple `Preprocessing Facilities`_.
+
+Formfeed characters may be used freely in files to produce page breaks when
+the file is printed for review.
+
+The following are the basic punctuation tokens::
+
+   - + [ ] { } ( ) < > : ; .  = ? #
+
+Literals
+--------
+
+Numeric literals take one of the following forms:
+
+.. productionlist::
+   TokInteger: `DecimalInteger` | `HexInteger` | `BinInteger`
+   DecimalInteger: ["+" | "-"] ("0"..."9")+
+   HexInteger: "0x" ("0"..."9" | "a"..."f" | "A"..."F")+
+   BinInteger: "0b" ("0" | "1")+
+
+Observe that the :token:`DecimalInteger` token includes the optional ``+``
+or ``-`` sign, unlike most languages where the sign would be treated as a
+unary operator.
+
+TableGen has two kinds of string literals:
+
+.. productionlist::
+   TokString: '"' (non-'"' characters and escapes) '"'
+   TokCodeFragment: "[{" (shortest text not containing "}]") "}]"
+
+A :token:`TokCodeFragment` is nothing more than a multi-line string literal
+delimited by ``[{`` and ``}]``. It can break across lines.
+
+The current implementation accepts the following escape sequences::
+
+   \\ \' \" \t \n
+
+Identifiers
+-----------
+
+TableGen has name- and identifier-like tokens, which are case-sensitive.
+
+.. 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 an integer. In case of ambiguity, a token is interpreted as a
+numeric literal rather than an identifier.
+
+TableGen has the following reserved words, which cannot be used as
+identifiers::
+
+   bit        bits          class         code          dag
+   def        else          foreach       defm          defset
+   defvar     field         if            in            include
+   int        let           list          multiclass    string
+   then
+
+.. warning::
+  The ``field`` reserved word is deprecated.
+
+Bang operators
+--------------
+
+TableGen provides "bang operators" that have a wide variety of uses:
+
+.. productionlist::
+   BangOperator: one of
+               : !add    !and         !cast         !con         !dag
+               : !empty  !eq          !foldl        !foreach     !ge
+               : !getop  !gt          !head         !if          !isa
+               : !le     !listconcat  !listsplat    !lt          !mul
+               : !ne     !or          !setop        !shl         !size
+               : !sra    !srl         !strconcat    !subst       !tail
+
+The ``!cond`` operator has a slightly different
+syntax compared to other bang operators, so it is defined separately:
+
+.. productionlist::
+   CondOperator: !cond
+
+See `Appendix A: Bang Operators`_ for a description of each bang operator.
+
+Include files
+-------------
+
+TableGen has an include mechanism. The content of the included file
+lexically replaces the ``include`` directive and is then parsed as if it was
+originally in the main file.
+
+.. productionlist::
+   IncludeDirective: "include" `TokString`
+
+Portions of the main file and included files can be conditionalized using
+preprocessor directives.
+
+.. productionlist::
+   PreprocessorDirective: "#define" | "#ifdef" | "#ifndef"
+
+Types
+=====
+
+The TableGen language is statically typed, using a simple but complete type
+system. Types are used to check for errors, to perform implicit conversions,
+and to help interface designers constrain the allowed input. Every value is
+required to have an associated type.
+
+TableGen supports a mixture of low-level types (e.g., ``bit``) and
+high-level types (e.g., ``dag``). This flexibility allows you to describe a
+wide range of records conveniently and compactly.
+
+.. productionlist::
+   Type: "bit" | "int" | "string" | "code" | "dag"
+       :| "bits" "<" `TokInteger` ">"
+       :| "list" "<" `Type` ">"
+       :| `ClassID`
+   ClassID: `TokIdentifier`
+
+``bit``
+    A ``bit`` is a boolean value that can be 0 or 1.
+
+``int``
+    The ``int`` type represents a simple 64-bit integer value, such as 5 or
+    -42.
+
+``string``
+    The ``string`` type represents an ordered sequence of characters of arbitrary
+    length.
+
+``code``
+    The ``code`` type represents a code fragment. The values are the same as
+    those for the ``string`` type; the ``code`` type is provided just to indicate
+    the programmer's intention.
+
+``bits<``\ *n*\ ``>``
+    The ``bits`` type is a fixed-size integer of arbitrary length *n* that
+    is treated as separate bits. These bits can be accessed individually.
+    A field of this type is useful for representing an instruction operation
+    code, register number, or address mode/register/displacement.  The bits of
+    the field can be set individually or as subfields. For example, in an
+    instruction address, the addressing mode, base register number, and
+    displacement can be set separately.
+
+``list<``\ *type*\ ``>``
+    This type represents a list whose elements are of the *type* specified in
+    angle brackets. The element type is arbitrary; it can even be another
+    list type. List elements are indexed from 0.
+
+``dag``
+    This type represents a nestable directed acyclic graph (DAG) of nodes.
+    Each node has an operator and one or more operands. A operand can be
+    another ``dag`` object, allowing an arbitrary tree of nodes and edges.
+    As an example, DAGs are used to represent code and patterns for use by
+    the code generator instruction selection algorithms.
+
+:token:`ClassID`
+    Specifying a class name in a type context indicates
+    that the type of 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).
+    The :token:`ClassID` must name a class that has been previously
+    declared or defined.
+
+
+Values and Expressions
+======================
+
+There are many contexts in TableGen statements where a value is required. A
+common example is in the definition of a record, where each field is
+specified by a name and an optional value. TableGen allows for a reasonable
+number of different forms when building up values. These forms allow the
+TableGen file to be written in a syntax that is natural for the application.
+
+Note that all of the values have rules for converting them from one type to
+another. For example, these rules allow you to assign a value like ``7``
+to an entity of type ``bits<4>``.
+
+.. productionlist::
+   Value: `SimpleValue` `ValueSuffix`*
+   ValueSuffix: "{" `RangeList` "}"
+              :| "[" `RangeList` "]"
+              :| "." `TokIdentifier`
+   RangeList: `RangePiece` ("," `RangePiece`)*
+   RangePiece: `TokInteger`
+             :| `TokInteger` ".." `TokInteger`
+             :| `TokInteger` "-" `TokInteger`
+             :| `TokInteger` `TokInteger`
+
+.. warning::
+  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 tokens, with values ``1`` and ``-5``,
+  instead of "1", "-", and "5".
+
+Simple values
+-------------
+
+The :token:`SimpleValue` has a number of forms.
+
+.. productionlist::
+   SimpleValue: `TokInteger` | `TokString`+ | `TokCodeFragment`
+
+A value can be an integer literal, a string literal, or a code fragment
+literal. Multiple adjacent string literals are concatenated as in C/C++; the
+simple value is the concatenation of the strings. Code fragments become
+strings and then are indistinguishable from them.
+
+.. productionlist::
+   SimpleValue2: "?"
+
+A question mark represents an uninitialized value.
+
+.. productionlist::
+   SimpleValue3: "{" [`ValueList`] "}"
+   ValueList: `ValueListNE`
+   ValueListNE: `Value` ("," `Value`)*
+
+This value represents a sequence of bits, which can be used to initialize a
+``bits<``\ *n*\ ``>`` field (note the braces). When doing so, the values
+must represent a total of *n* bits.
+
+.. productionlist::
+   SimpleValue4: "[" `ValueList` "]" ["<" `Type` ">"]
+
+This value is a list initializer (note the brackets). The values in brackets
+are the elements of the list. The optional :token:`Type` can be used to
+indicate a specific element type; otherwise the element type is inferred
+from the given values. TableGen can usually infer the type, although
+sometimes not when the value is the empty list (``[]``).
+
+.. productionlist::
+   SimpleValue5: "(" `DagArg` [`DagArgList`] ")"
+   DagArgList: `DagArg` ("," `DagArg`)*
+   DagArg: `Value` [":" `TokVarName`] | `TokVarName`
+
+This represents a DAG initializer (note the parentheses).  The first
+:token:`DagArg` is called the "operator" of the DAG and must be a record.
+
+.. productionlist::
+   SimpleValue6: `TokIdentifier`
+
+The resulting value is the value of the entity named by the identifier. The
+possible identifiers are described here, but the descriptions will make more
+sense after reading the remainder of this guide.
+
+.. The code for this is exceptionally abstruse. These examples are a
+   best-effort attempt.
+
+* A template argument of a ``class``, such as the use of ``Bar`` in::
+
+     class Foo <int Bar> {
+       int Baz = Bar;
+     }
+
+* The implicit template argument ``NAME`` in a ``class`` or ``multiclass``
+  definition (see `NAME`_).
+
+* A field local to a ``class``, such as the use of ``Bar`` in::
+
+     class Foo {
+       int Bar = 5;
+       int Baz = Bar;
+     }
+
+* The name of a record definition, such as the use of ``Bar`` in the
+  definition of ``Foo``::
+
+     def Bar : SomeClass {
+       int X = 5;
+     }
+
+     def Foo {
+       SomeClass Baz = Bar;
+     }
+
+* A field local to a record definition, such as the use of ``Bar`` in::
+
+     def Foo {
+       int Bar = 5;
+       int Baz = Bar;
+     }
+
+  Fields inherited from the record's parent classes can be accessed the same way.
+
+* A template argument of a ``multiclass``, such as the use of ``Bar`` in::
+
+     multiclass Foo <int Bar> {
+       def : SomeClass<Bar>;
+     }
+
+* A variable defined with the ``defvar`` or ``defset`` statements.
+
+* The iteration variable of a ``foreach``, such as the use of ``i`` in::
+
+     foreach i = 0..5 in
+       def Foo#i;
+
+.. productionlist::
+   SimpleValue7: `ClassID` "<" `ValueListNE` ">"
+
+This form creates a new anonymous record definition (as would be created by an
+unnamed ``def`` inheriting from the given class with the given template
+arguments; see `def`_) and the value is that record. (A field of the record can be
+obtained using a suffix; see `Suffixed Values`_.)
+
+.. productionlist::
+   SimpleValue8: `BangOperator` ["<" `Type` ">"] "(" `ValueListNE` ")"
+              :| `CondOperator` "(" `CondClause` ("," `CondClause`)* ")"
+   CondClause: `Value` ":" `Value`
+
+The bang operators provide functions that are not available with the other
+simple values. Except in the case of ``!cond``, a bang
+operator takes a list of arguments enclosed in parentheses and performs some
+function on those arguments, producing a value for that
+bang operator. The ``!cond`` operator takes a list of pairs of arguments
+separated by colons. See `Appendix A: Bang Operators`_ for a description of
+each bang operator.
+
+
+Suffixed values
+---------------
+
+The :token:`SimpleValue` values described above can be specified with
+certain suffixes. The purpose of a suffix is to obtain a subvalue of the
+primary value. Here are the possible suffixes for some primary *value*.
+
+*value*\ ``{17}``
+    The final value is bit 17 of the integer *value* (note the braces).
+
+*value*\ ``{8..15}``
+    The final value is bits 8--15 of the integer *value*. The order of the
+    bits can be reversed by specifying ``{15..8}``.
+
+*value*\ ``[4..7,17,2..3,4]``
+    The final value is a new list that is a slice of the list *value* (note
+    the brackets). The
+    new list contains elements 4, 5, 6, 7, 17, 2, 3, and 4. Elements may be
+    included multiple times and in any order.
+
+*value*\ ``.`` *field*
+    The final value is the value of the specified *field* in the specified
+    record *value*.
+
+Statements
+==========
+
+The following statements may appear at the top level of TableGen source
+files.
+
+.. productionlist::
+   TableGenFile: `Statement`*
+   Statement: `Class` | `Def` | `Defm` | `Defset` | `Defvar` | `Foreach`
+            :| `If` | `Let` | `MultiClass`
+
+The following sections describe each of these top-level statements. 
+
+
+``class`` --- define an abstract record class
+---------------------------------------------
+
+A ``class`` statement defines an abstract record class from which other
+classes and records can inherit. 
+
+.. productionlist::
+   Class: "class" `ClassID` [`TemplateArgList`] `RecordBody`
+   TemplateArgList: "<" `TemplateArgDecl` ("," `TemplateArgDecl`)* ">"
+   TemplateArgDecl: `Type` `TokIdentifier` ["=" `Value`]
+
+A class can be parameterized by a list of "template arguments," whose values
+can be used in the class's record body.  These template arguments are
+specified each time the class is inherited by another class or record.
+
+If a template argument is not assigned a default value with ``=``, it is
+uninitialized (has the "value" ``?``) and must be specified in the template
+argument list when the class is inherited. If an argument is assigned a
+default value, then it need not be specified in the argument list. The
+template argument default values are evaluated from left to right.
+
+The :token:`RecordBody` is defined below. It can include a list of
+superclasses from which the current class inherits, along with field definitions
+and other statements. When a class ``C`` inherits from another class ``D``,
+the fields of ``D`` are effectively merged into the fields of ``C``.
+
+A given class can only be defined once. A ``class`` statement is
+considered to define the class if *any* of the following are true (the
+:token:`RecordBody` elements are described below).
+
+* The :token:`TemplateArgList` is present, or
+* The :token:`ParentClassList` in the :token:`RecordBody` is present, or
+* The :token:`Body` in the :token:`RecordBody` is present and not empty.
+
+You can declare an empty class by specifying an empty :token:`TemplateArgList`
+and an empty :token:`RecordBody`. This can serve as a restricted form of
+forward declaration. Note that records derived from a forward-declared
+class will inherit no fields from it, because those records are built when
+their declarations are parsed, and thus before the class is finally defined.
+
+.. _NAME:
+
+Every class has an implicit template argument named ``NAME`` (uppercse),
+which is bound to the name of the :token:`Def` or :token:`Defm` inheriting
+the class. The value of ``NAME`` is undefined if the class is inherited by
+an anonymous record.
+
+See `Examples: classes and records`_ for examples.
+
+Record Bodies
+`````````````
+
+Record bodies appear in both class and record definitions. A record body can
+include a parent class list, which specifies the classes from which the
+current class or record inherits fields. Such classes are called the
+superclasses or parent classes of the class or record. The record body also
+includes the main body of the definition, which contains the specification
+of the fields of the class or record.
+
+.. productionlist::
+   RecordBody: `ParentClassList` `Body`
+   ParentClassList: [":" `ParentClassListNE`]
+   ParentClassListNE: `ClassRef` ("," `ClassRef`)*
+   ClassRef: (`ClassID` | `MultiClassID`) ["<" `ValueList` ">"]
+
+A :token:`ParentClassList` containing a :token:`MultiClassID` is valid only
+in the class list of a ``defm`` statement. In that case, the ID must be the
+name of a multiclass.
+
+.. productionlist::
+   Body: ";" | "{" `BodyItem`* "}"
+   BodyItem: `Type` `TokIdentifier` ["=" `Value`] ";"
+           :| "let" `TokIdentifier` ["{" `RangeList` "}"] "=" `Value` ";"
+           :| "defvar" `TokIdentifier` "=" `Value` ";"
+
+A field definition in the body specifies a field to be included in the class
+or record. If no initial value is specified, then the field's value is
+uninitialized. The type must be specified; TableGen will not infer it from
+the value.
+
+The ``let`` form is used to reset a field to a new value. This can be done
+for fields defined directly in the body or fields inherited from
+superclasses.  A :token:`RangeList` can be specified to reset certain bits
+in a ``bit<n>`` field.
+
+The ``defvar`` form defines a variable whose value can be used in other
+value expressions within the body. The variable is not a field: it does not
+become a field of the class or record being defined. Variables are provided
+to hold temporary values while processing the body. See `Defvar in Record
+Body`_ for more details.
+
+When class ``C2`` inherits from class ``C1``, it acquires all the field
+definitions of ``C1``. As those definitions are merged into class ``C2``, any
+template arguments passed to ``C1`` by ``C2`` are substituted into the
+definitions. In other words, the abstract record fields defined by ``C1`` are
+expanded with the template arguments before being merged into ``C2``.
+
+
+.. _def:
+
+``def`` --- define a concrete record
+------------------------------------
+
+A ``def`` statement defines a new concrete record.
+
+.. productionlist::
+   Def: "def" [`NameValue`] `RecordBody`
+   NameValue: `Value`
+
+The name value is optional. If specified, it is parsed in a special mode
+where undefined (unrecognized) identifiers are interpreted as literal
+strings.  In particular, global identifiers are considered unrecognized.
+These include global variables defined by ``defvar`` and ``defset``.
+
+If no name value is given, the record is *anonymous*. The final name of an
+anonymous record is unspecified but globally unique.
+
+Special handling occurs if a ``def`` appears inside a ``multiclass``
+statement. See the ``multiclass`` section below for details.
+
+A record can inherit from one or more classes by specifying the
+:token:`ParentClassList` clause at the beginning of its record body. All of
+the fields in the parent classes are added to the record. If two or more
+parent classes provide the same field, the record ends up with the field value
+of the last parent class.
+
+As a special case, the name of a record can be passed in a template argument
+to that record's superclasses. For example:
+
+.. code-block:: text
+
+  class A <dag d> {
+    dag the_dag = d;
+  }
+
+  def rec1 : A<(ops rec1)>
+
+The DAG ``(ops rec1)`` is passed as a template argument to class ``A``. Notice
+that the DAG includes ``rec1``, the record being defined.
+
+The steps taken to create a new record are somewhat complex. See `How
+records are built`_.
+
+See `Examples: classes and records`_ for examples.
+
+
+Examples: classes and records
+-----------------------------
+
+Here is a simple TableGen file with one class and two record definitions.
+
+.. code-block:: text
+
+  class C {
+    bit V = 1;
+  }
+
+  def X : C;
+  def Y : C {
+    let V = 0;
+    string Greeting = "Hello!";
+  }
+
+First, the abstract class ``C`` is defined. It has one field named ``V``
+that is a bit initialized to 1.
+
+Next, two records are defined, derived from class ``C``; that is, with ``C``
+as their superclass. Thus they both inherit the ``V`` field. Record ``Y``
+also defines another string field, ``Greeting``, which is initialized to
+``"Hello!"``. In addition, ``Y`` overrides the inherited ``V`` field,
+setting it to 0.
+
+A class is useful for isolating the common features of multiple records in
+one place. A class can initialize common fields to default values, but
+records inheriting from that class can override the defaults.
+
+TableGen supports the definition of parameterized classes as well as
+nonparameterized ones. Parameterized classes specify a list of variable
+declarations, which may optionally have defaults, that are bound when the
+class is specified as a superclass of another class or record.
+
+.. code-block:: text
+
+  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>;
+
+The purpose of the ``FPFormat`` class is to act as a sort of enumerated
+type. It provides a single field, ``Value``, which holds a 3-bit number. Its
+template argument, ``val``, is used to set the ``Value`` field.
+Each of the eight records is defined with ``FPFormat`` as its superclass. The
+enumeration value is passed in angle brackets as the template argument. Each
+record will inherent the ``Value`` field with the appropriate enumeration
+value.
+
+Here is a more complex example of classes with template arguments. First, we
+define a class similar to the ``FPFormat`` class above. It takes a template
+argument and uses it to initialize a field named ``Value``. Then we define
+four records that inherit the ``Value`` field with its four different
+integer values.
+
+.. code-block:: text
+
+  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>;
+
+This is somewhat contrived, but let's say we would like to examine the two
+bits of the ``Value`` field independently. We can define a class that
+accepts a ``ModRefVal`` record as a template argument and splits up its
+value into two fields, one bit each. Then we can define records that inherit from
+``ModRefBits`` and so acquire two fields from it, one for each bit in the
+``ModRefVal`` record passed as the template argument.
+
+.. code-block:: text
+
+  class ModRefBits <ModRefVal mrv> {
+    // Break the value up into its bits, which can provide a nice
+    // interface to the ModRefVal values.
+    bit isMod = mrv.Value{0};
+    bit isRef = mrv.Value{1};
+  }
+
+  // Example uses.
+  def foo   : ModRefBits<Mod>;
+  def bar   : ModRefBits<Ref>;
+  def snork : ModRefBits<ModRef>;
+
+This illustrates how one class can be defined to reorganize the
+fields in another class, thus hiding the internal representation of that
+other class.
+
+Running ``llvm-tblgen`` on the example prints the following definitions:
+
+.. code-block:: text
+
+  def bar {      // Value
+    bit isMod = 0;
+    bit isRef = 1;
+  }
+  def foo {      // Value
+    bit isMod = 1;
+    bit isRef = 0;
+  }
+  def snork {      // Value
+    bit isMod = 1;
+    bit isRef = 1;
+  }
+
+``let`` --- override fields in classes or records
+-------------------------------------------------
+
+A ``let`` statement collects a set of field values (sometimes called
+*bindings*) and applies them to all the classes and records defined by
+statements within the scope of the ``let``.
+
+.. productionlist::
+   Let:  "let" `LetList` "in" "{" `Statement`* "}"
+      :| "let" `LetList` "in" `Statement`
+   LetList: `LetItem` ("," `LetItem`)*
+   LetItem: `TokIdentifier` ["<" `RangeList` ">"] "=" `Value`
+
+The ``let`` statement establishes a scope, which is a sequence of statements
+in braces or a single statement with no braces. The bindings in the
+:token:`LetList` apply to the statements in that scope.
+
+The field names in the :token:`LetList` must name fields in classes inherited by
+the classes and records defined in the statements. The field values are
+applied to the classes and records *after* the records inherit all the fields from
+their superclasses. So the ``let`` acts to override inherited field
+values. A ``let`` cannot override the value of a template argument.
+
+Top-level ``let`` statements are often useful when a few fields need to be
+overriden in several records. Here are two examples. Note that ``let``
+statements can be nested.
+
+.. code-block:: text
+
+  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", []>;
+    }
+
+Note that a top-level ``let`` will not override fields defined in the classes or records
+themselves.
+
+
+``multiclass`` --- define multiple records
+------------------------------------------
+
+While classes with template arguments are a good way to factor out commonality
+between multiple records, multiclasses allow a convenient method for
+defining multiple records at once. For example, consider a 3-address
+instruction architecture whose instructions come in two formats: ``reg = reg
+op reg`` and ``reg = reg op imm`` (e.g., SPARC). We would like to specify in
+one place that these two common formats exist, then in a separate place
+specify what all the operations are. The ``multiclass`` and ``defm``
+statements accomplish this goal. You can think of a multiclass as a macro or
+template that expands into multiple records.
+
+.. productionlist::
+   MultiClass: "multiclass" `TokIdentifier` [`TemplateArgList`]
+             : [":" `ParentMultiClassList`]
+             : "{" `Statement`+ "}"
+   ParentMultiClassList: `MultiClassID` ("," `MultiClassID`)*
+   MultiClassID: `TokIdentifier`
+
+As with regular classes, the multiclass has a name and can accept template
+arguments. The body of the multiclass contains a series of statements that
+define records, using :token:`Def` and :token:`Defm`. In addition,
+:token:`Defvar`, :token:`Foreach`, and :token:`Let`
+statements can be used to factor out even more common elements.
+
+Also as with regular classes, the multiclass has the implicit template
+argument ``NAME`` (see NAME_). When a named (non-anonymous) record is
+defined in a multiclass and the record's name does not contain a use of the
+template argument ``NAME``, such a use is automatically prepended
+to the name.  That is, the following are equivalent inside a multiclass::
+
+    def Foo ...
+    def NAME#Foo ...
+
+The records defined in a multiclass are instantiated when the multiclass is
+"invoked" by a ``defm`` statement outside the multiclass definition. Each
+``def`` statement produces a record. As with top-level ``def`` statements,
+these definitions can inherit from multiple superclasses.
+
+See `Examples: multiclasses and defms`_ for examples.
+
+
+``defm`` --- invoke multiclasses to define multiple records
+-----------------------------------------------------------
+
+Once multiclasses have been defined, you use the ``defm`` statement to
+"invoke" multiclasses and process the multiple record definitions in those
+multiclasses. Those record definitions are specified by ``def`` 
+statements in the multiclasses, and indirectly by ``defm`` statements.
+
+.. productionlist::
+   Defm: "defm" [`NameValue`] `ParentClassList` ";"
+
+The optional :token:`NameValue` is formed in the same way as the name of a
+``def``. The :token:`ParentClassList` is a colon followed by a list of at least one
+multiclass and any number of regular classes. The multiclasses must
+precede the regular classes. Note that the ``defm`` does not have a body.
+
+This statement instantiates all the records defined in all the specified
+multiclasses, either directly by ``def`` statements or indirectly by
+``defm`` statements. These records also receive the fields defined in any
+regular classes included in the parent class list. This is useful for adding
+a common set of fields to all the records created by the ``defm``.
+
+The name is parsed in the same special mode used by ``def``. If the name is
+not included, a globally unique name is provided. That is, the following
+examples end up with different names::
+
+    defm    : SomeMultiClass<...>;   // A globally unique name.
+    defm "" : SomeMultiClass<...>;   // An empty name.
+
+The ``defm`` statement can be used in a multiclass body. When this occurs,
+the second variant is equivalent to::
+
+  defm NAME : SomeMultiClass<...>;
+
+More generally, when ``defm`` occurs in a multiclass and its name does not
+include a use of the implicit template argument ``NAME``, then ``NAME`` will
+be prepended automatically. That is, the following are equivalent inside a
+multiclass::
+
+    defm Foo      : SomeMultiClass<...>;
+    defm NAME#Foo : SomeMultiClass<...>;
+
+See `Examples: multiclasses and defms`_ for examples.
+
+Examples: multiclasses and defms
+--------------------------------
+
+Here is a simple example using ``multiclass`` and ``defm``.  Consider a
+3-address instruction architecture whose instructions come in two formats:
+``reg = reg op reg`` and ``reg = reg op imm`` (immediate). The SPARC is an
+example of such an architecture.
+
+.. code-block:: text
+
+  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)>;
+  }
+
+  // Define records for each instruction in the RR and RI formats.
+  defm ADD : ri_inst<0b111, "add">;
+  defm SUB : ri_inst<0b101, "sub">;
+  defm MUL : ri_inst<0b100, "mul">;
+
+Each use of the ``ri_inst`` multiclass defines two records, one with the
+``_rr`` suffix and one with ``_ri``. Recall that the name of the ``defm``
+that uses a multiclass is prepended to the names of the records defined in
+that multiclass. So the resulting definitions are named::
+
+  ADD_rr, ADD_ri
+  SUB_rr, SUB_ri
+  MUL_rr, MUL_ri
+
+Without the ``multiclass`` feature, the instructions would have to be
+defined as follows.
+
+.. code-block:: text
+
+  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)>;
+
+  // Define records for each instruction in the RR and RI formats.
+  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 be used in a multiclass to "invoke" other multiclasses and
+create the records defined in those multiclasses in addition to the records
+defined in the current multiclass. In the following example, the ``basic_s``
+and ``basic_p`` multiclasses contain ``defm`` statements that refer to the
+``basic_r`` multiclass. The ``basic_r`` multiclass contains only ``def``
+statements.
+
+.. code-block:: text
+
+  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>;
+
+The final ``defm`` creates the following records, five from the ``basic_s``
+multiclass and five from the ``basic_p`` multiclass::
+
+  ADDSSrr, ADDSSrm
+  ADDSDrr, ADDSDrm
+  ADDX
+  ADDPSrr, ADDPSrm
+  ADDPDrr, ADDPDrm
+  ADDY
+
+A ``defm`` statement, both at top level and in a multiclass, can inherit
+from regular classes in addition to multiclasses. The rule is that the
+regular classes must be listed after the multiclasses, and there must be at least
+one multiclass.
+
+.. code-block:: text
+
+  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;    // First multiclass R, then regular class XD.
+    defm SD : R, XS;
+  }
+
+  defm Instr : Y;
+
+This example will create four records, shown here in alphabetical order with
+their fields.
+
+.. code-block:: text
+
+  def InstrSDrm {
+    bits<4> opcode = { 0, 0, 1, 0 };
+    bits<4> Prefix = { 1, 1, 0, 0 };
+  }
+
+  def InstrSDrr {
+    bits<4> opcode = { 0, 1, 0, 0 };
+    bits<4> Prefix = { 1, 1, 0, 0 };
+  }
+
+  def InstrSSrm {
+    bits<4> opcode = { 0, 0, 1, 0 };
+    bits<4> Prefix = { 1, 0, 1, 1 };
+  }
+
+  def InstrSSrr {
+    bits<4> opcode = { 0, 1, 0, 0 };
+    bits<4> Prefix = { 1, 0, 1, 1 };
+  }
+
+It's also possible to use ``let`` statements inside multiclasses, providing
+another way to factor out commonality from the records, especially when
+using several levels of multiclass instantiations.
+
+.. code-block:: text
+
+  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>;
+
+
+``defset`` --- create a definition set
+--------------------------------------
+
+The ``defset`` statement is used to collect a set of records into a global
+list of records.
+
+.. productionlist::
+   Defset: "defset" `Type` `TokIdentifier` "=" "{" `Statement`* "}"
+
+All records defined inside the braces via ``def`` and ``defm`` are defined
+as usual, and they are also collected in a global list of the given name
+(:token:`TokIdentifier`).
+
+The specified type must be ``list<``\ *class*\ ``>``, where *class* is some
+record class.  The ``defset`` statement establishes a scope for its
+statements. It is an error to define a record in the scope of the
+``defset`` that is not of type *class*.
+
+The ``defset`` statement can be nested. The inner ``defset`` adds the
+records to its own set, and all those records are also added to the outer
+set.
+
+Anonymous records created inside initialization expressions using the
+``ClassID<...>`` syntax are not collected in the set.
+
+
+``defvar`` --- define a variable
+--------------------------------
+
+A ``defvar`` statement defines a global variable. Its value can be used
+throughout the statements that follow the definition.
+
+.. productionlist::
+   Defvar: "defvar" `TokIdentifier` "=" `Value` ";"
+
+The identifier on the left of the ``=`` is defined to be a global variable
+whose value is given by the value expression on the right of the ``=``. The
+type of the variable is automatically inferred.
+
+Once a variable has been defined, it cannot be set to another value.
+
+Variables defined in a top-level ``foreach`` go out of scope at the end of
+each loop iteration, so their value in one iteration is not available in
+the next iteration.  The following ``defvar`` will not work::
+
+  defvar i = !add(i, 1)
+
+Variables can also be defined with ``defvar`` in a record body. See
+`Defvar in Record Body`_ for more details.
+
+``foreach`` --- iterate over a sequence
+---------------------------------------
+
+The ``foreach`` statement iterates over a series of statements, varying a
+variable over a sequence of values.
+
+.. productionlist::
+   Foreach: "foreach" `ForeachIterator` "in" "{" `Statement`* "}"
+          :| "foreach" `ForeachIterator` "in" `Statement`
+   ForeachIterator: `TokIdentifier` "=" ("{" `RangeList` "}" | `RangePiece` | `Value`)
+
+The body of the ``foreach`` is a series of statements in braces or a
+single statement with no braces. The statements are re-evaluated once for
+each value in the range list, range piece, or single value. On each
+iteration, the :token:`TokIdentifier` variable is set to the value and can
+be used in the statements.
+
+The statement list establishes an inner scope. Variables local to a
+``foreach`` go out of scope at the end of each loop iteration, so their
+values do not carry over from one iteration to the next. Foreach loops may
+be nested.
+
+The ``foreach`` statement can also be used in a record :token:`Body`.
+
+.. Note that the productions involving RangeList and RangePiece have precedence
+   over the more generic value parsing based on the first token.
+
+.. code-block:: text
+
+  foreach i = [0, 1, 2, 3] in {
+    def R#i : Register<...>;
+    def F#i : Register<...>;
+  }
+
+This loop defines records named ``R0``, ``R1``, ``R2``, and ``R3``, along
+with ``F0``, ``F1``, ``F2``, and ``F3``.
+
+
+``if`` --- select statements based on a test
+--------------------------------------------
+
+The ``if`` statement allows one of two statement groups to be selected based
+on the value of an expression.
+
+.. productionlist::
+   If: "if" `Value` "then" `IfBody`
+     :| "if" `Value` "then" `IfBody` "else" `IfBody`
+   IfBody: "{" `Statement`* "}" | `Statement`
+
+The value expression is evaluated. If it evaluates to true (in the same
+sense used by the bang operators), then the statements following the
+``then`` reserved word are processed. Otherwise, if there is an ``else``
+reserved word, the statements following the ``else`` are processed. If the
+value is false and there is no ``else`` arm, no statements are processed.
+
+Because the braces around the ``then`` statements are optional, this grammar rule
+has the usual ambiguity with "dangling else" clauses, and it is resolved in
+the usual way: in a case like ``if v1 then if v2 then {...} else {...}``, the
+``else`` associates with the inner ``if`` rather than the outer one.
+
+The :token:`IfBody` of the then and else arms of the ``if`` establish an
+inner scope. Any ``defvar`` variables defined in the bodies go out of scope
+when the bodies are finished (see `Defvar in Record Body`_ for more details).
+
+The ``if`` statement can also be used in a record :token:`Body`.
+
+
+Additional Details
+==================
+
+Defvar in record body
+---------------------
+
+In addition to defining global variables, the ``defvar`` statement can
+be used inside the :token:`Body` of a class or record definition to define
+local variables. The scope of the variable extends from the ``defvar``
+statement to the end of the body. It cannot be set to a different value
+within its scope. The ``defvar`` statement can also be used in the statement
+list of a ``foreach``, which establishes a scope.
+
+A variable named ``V`` in an inner scope shadows (hides) any variables ``V``
+in outer scopes. In particular, ``V`` in a record body shadows a global
+``V``, and ``V`` in a ``foreach`` statement list shadows any ``V`` in
+surrounding global or record scopes.
+
+Variables defined in a ``foreach`` go out of scope at the end of
+each loop iteration, so their value in one iteration is not available in
+the next iteration.  The following ``defvar`` will not work::
+
+  defvar i = !add(i, 1)
+
+How records are built
+---------------------
+
+The following steps are taken by TableGen when a record is built. Classes are simply
+abstract records and so go through the same steps.
+
+1. Build the record name (:token:`NameValue`) and create an empty record.
+
+2. Parse the superclasses in the :token:`ParentClassList` from left to
+   right, visiting each superclass's ancestor classes from top to bottom.
+
+  a. Add the fields from the superclass to the record.
+  b. Substitute the template arguments into those fields.
+  c. Add the superclass to the record's list of inherited classes.
+
+3. Apply any top-level ``let`` bindings to the record. Recall that top-level
+   bindings only apply to inherited fields.
+
+4. Parse the body of the record.
+
+  * Add any fields to the record.
+  * Modify the values of fields according to local ``let`` statements.
+  * Define any ``defvar`` variables.
+
+5. Make a pass over all the fields to resolve any inter-field references.
+
+6. Add the record to the master record list.
+
+
+Because references between fields are resolved (step 5) after ``let`` bindings are
+applied (step 3), the ``let`` statement has unusual power. For example:
+
+.. code-block:: text
+
+  class C <int x> {
+    int Y = x;
+    int Yplus1 = !add(Y, 1);
+    int xplus1 = !add(x, 1);
+  }
+
+  let Y = 10 in {
+    def rec1 : C<5> {
+    }
+  }
+
+  def rec2 : C<5> {
+    let Y = 10;
+  }
+
+In both cases, one where a top-level ``let`` is used to bind ``Y`` and one
+where a local ``let`` does the same thing, the results are:
+
+.. code-block:: text
+
+  def rec1 {      // C
+    int Y = 10;
+    int Yplus1 = 11;
+    int xplus1 = 6;
+  }
+  def rec2 {      // C
+    int Y = 10;
+    int Yplus1 = 11;
+    int xplus1 = 6;
+  }
+
+``Yplus1`` is 11 because the ``let Y`` is performed before the ``!add(Y,
+1)`` is resolved. Use this power wisely.
+
+
+Preprocessing Facilities
+========================
+
+The preprocessor embedded in TableGen is intended only for simple
+conditional compilation. It supports the following directives, which are
+specified somewhat informally.
+
+.. productionlist::
+   LineBegin: beginning of line
+   LineEnd: newline | return | EOF
+   WhiteSpace: space | tab
+   CComment: "/*" ... "*/"
+   BCPLComment: "//" ... `LineEnd`
+   WhiteSpaceOrCComment: `WhiteSpace` | `CComment`
+   WhiteSpaceOrAnyComment: `WhiteSpace` | `CComment` | `BCPLComment`
+   MacroName: `ualpha` (`ualpha` | "0"..."9")*
+   PreDefine: `LineBegin` (`WhiteSpaceOrCComment`)*
+            : "#define" (`WhiteSpace`)+ `MacroName`
+            : (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PreIfdef: `LineBegin` (`WhiteSpaceOrCComment`)*
+           : ("#ifdef" | "#ifndef") (`WhiteSpace`)+ `MacroName`
+           : (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PreElse: `LineBegin` (`WhiteSpaceOrCComment`)*
+          : "#else" (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PreEndif: `LineBegin` (`WhiteSpaceOrCComment`)*
+           : "#endif" (`WhiteSpaceOrAnyComment`)* `LineEnd`
+
+..
+   PreRegContentException: `PreIfdef` | `PreElse` | `PreEndif` | EOF
+   PreRegion: .* - `PreRegContentException`
+             :| `PreIfdef`
+             :  (`PreRegion`)*
+             :  [`PreElse`]
+             :  (`PreRegion`)*
+             :  `PreEndif`
+
+A :token:`MacroName` can be defined anywhere in a TableGen file. The name has
+no value; it can only be tested to see whether it is defined.
+
+A macro test region begins with an ``#ifdef`` or ``#ifndef`` directive. If
+the macro name is defined (``#ifdef``) or undefined (``#ifndef``), then the
+source code between the directive and the corresponding ``#else`` or
+``#endif`` is processed. If the test fails but there is an ``#else``
+portion, the source code between the ``#else`` and the ``#endif`` is
+processed. If the test fails and there is no ``#else`` portion, then no
+source code in the test region is processed.
+
+Test regions may be nested, but they must be properly nested. A region
+started in a file must end in that file; that is, must have its
+``#endif`` in the same file.
+
+A :token:`MacroName` may be defined externally using the ``-D`` option on the
+``llvm-tblgen`` command line::
+
+  llvm-tblgen self-reference.td -Dmacro1 -Dmacro3
+
+Appendix A: Bang Operators
+==========================
+
+Bang operators act as functions in value expressions. A bang operator takes
+one or more arguments, operates on them, and produces a result. If the
+operator produces a boolean result, the result value will be 1 for true or 0
+for false. When an operator tests a boolean argument, it interprets 0 as false
+and non-0 as true.
+
+``!add(``\ *a*\ ``,`` *b*\ ``, ...)``
+    This operator adds *a*, *b*, etc., and produces the sum.
+
+``!and(``\ *a*\ ``,`` *b*\ ``, ...)``
+    This operator does a bitwise AND on *a*, *b*, etc., and produces the
+    result.
+
+``!cast<``\ *type*\ ``>(``\ *a*\ ``)``
+    This operator performs a cast on *a* and produces the result.
+    If *a* is not a string, then a straightforward cast is performed, say
+    between an ``int`` and a ``bit``, or between record types. This allows
+    casting a record to a class. If a record is cast to ``string``, the
+    record's name is produced.
+
+    If *a* is a string, then it is treated as a record name and looked up in
+    the list of all defined records. The resulting record is expected to be of
+    the specified *type*.
+
+    For example, if ``!cast<``\ *type*\ ``>(``\ *name*\ ``)``
+    appears in a multiclass definition, or in a
+    class instantiated inside a multiclass definition, and the *name* does not
+    reference any template arguments of the multiclass, then a record by
+    that name must have been instantiated earlier
+    in the source file. If *name* does reference
+    a template argument, then the lookup is delayed until ``defm`` statements
+    instantiating the multiclass (or later, if the defm occurs in another
+    multiclass and template arguments of the inner multiclass that are
+    referenced by *name* are substituted by values that themselves contain
+    references to template arguments of the outer multiclass).
+
+    If the type of *a* does not match *type*, TableGen raises an error.
+
+``!con(``\ *a*\ ``,`` *b*\ ``, ...)``
+    This operator concatenates the DAG nodes *a*, *b*, etc. Their operations
+    must equal.
+
+    ``!con((op a1:$name1, a2:$name2), (op b1:$name3))``
+
+    results in the DAG node ``(op a1:$name1, a2:$name2, b1:$name3)``.
+
+``!cond(``\ *cond1* ``:`` *val1*\ ``,`` *cond2* ``:`` *val2*\ ``, ...,`` *condn* ``:`` *valn*\ ``)``
+    This operator tests *cond1* and returns *val1* if the result is true.
+    If false, the operator tests *cond2* and returns *val2* if the result is
+    true. And so forth. An error is reported if no conditions are true.
+
+    This example produces the sign word for an integer::
+
+    !cond(!lt(x, 0) : "negative", !eq(x, 0) : "zero", 1 : "positive")
+
+``!dag(``\ *op*\ ``,`` *children*\ ``,`` *names*\ ``)``
+    This operator creates a DAG node.
+    The *children* and *names* arguments must be lists
+    of equal length or uninitialized (``?``). The *names* argument
+    must be of type ``list<string>``.
+
+    Due to limitations of the type system, *children* must be a list of items
+    of a common type. In practice, this means that they should either have the
+    same type or be records with a common superclass. Mixing ``dag`` and
+    non-``dag`` items is not possible. However, ``?`` can be used.
+
+    Example: ``!dag(op, [a1, a2, ?], ["name1", "name2", "name3"])`` results in
+    ``(op a1:$name1, a2:$name2, ?:$name3)``.
+
+``!empty(``\ *list*\ ``)``
+    This operator produces 1 if the *list* is empty; 0 otherwise.
+
+``!eq(`` *a*\ `,` *b*\ ``)``
+    This operator produces 1 if *a* is equal to *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!foldl(``\ *start*\ ``,`` *list*\ ``,`` *a*\ ``,`` *b*\ ``,`` *expr*\ ``)``
+    This operator performs a left-fold over the items in *list*. The
+    variable *a* acts as the accumulator and is initialized to *start*.
+    The variable *b* is bound to each element in the *list*. The *expr*
+    expression is evaluated for each element and presumably uses *a* and *b*
+    to calculate the accumulated value, which ``!foldl`` stores in *a*. The
+    type of *a* is the same as *start*; the type of *b* is the same as the
+    elements of *list*; *expr* must have the same type as *start*.
+
+    The following example computes the total of the ``Number`` field in the
+    list of records in ``RecList``::
+
+      int x = !foldl(0, RecList, total, rec, !add(total, rec.Number));
+
+``!foreach(``\ *var*\ ``,`` *seq*\ ``,`` *form*\ ``)``
+    This operator creates a new ``list``/``dag`` in which each element is a
+    function of the corresponding element in the *seq* ``list``/``dag``. To
+    perform the function, TableGen binds the variable *var* to an element and
+    then evaluates the *form* expression. The form presumably refers to the
+    variable *var* and calculates the result value.
+
+``!ge(``\ *a*\ `,` *b*\ ``)``
+    This operator produces 1 if *a* is greater than or equal to *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!getop(``\ *dag*\ ``)`` --or-- ``!getop<``\ *type*\ ``>(``\ *dag*\ ``)``
+    This operator produces the operator of the given *dag* node.
+    Example: ``!getop((foo 1, 2))`` results in ``foo``.
+
+    The result of ``!getop`` can be used directly in a context where
+    any record value at all is acceptable (typically placing it into
+    another dag value). But in other contexts, it must be explicitly
+    cast to a particular class type. The ``<``\ *type*\ ``>`` syntax is
+    provided to make this easy.
+
+    For example, to assign the result to a value of type ``BaseClass``, you
+    could write either of these::
+
+      BaseClass b = !getop<BaseClass>(someDag);
+      BaseClass b = !cast<BaseClass>(!getop(someDag));
+
+    But to create a new DAG node that reuses the operator from another, no
+    cast is necessary::
+
+      dag d = !dag(!getop(someDag), args, names);
+
+``!gt(``\ *a*\ `,` *b*\ ``)``
+    This operator produces 1 if *a* is greater than *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!head(``\ *a*\ ``)``
+    This operator produces the zeroth element of the list *a*.
+    (See also ``!tail``.)
+
+``!if(``\ *test*\ ``,`` *then*\ ``,`` *else*\ ``)``
+  This operator evaluates the *test*, which must produce a ``bit`` or
+  ``int``. If the result is not 0, the *then* expression is produced; otherwise
+  the *else* expression is produced.
+
+``!isa<``\ *type*\ ``>(``\ *a*\ ``)``
+    This operator produces 1 if the type of *a* is a subtype of the given *type*; 0
+    otherwise.
+
+``!le(``\ *a*\ ``,`` *b*\ ``)``
+    This operator produces 1 if *a* is less than or equal to *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!listconcat(``\ *list1*\ ``,`` *list2*\ ``, ...)``
+    This operator concatenates the list arguments *list1*, *list2*, etc., and
+    produces the resulting list. The lists must have the same element type.
+
+``!listsplat(``\ *value*\ ``,`` *count*\ ``)``
+    This operator produces a list of length *count* whose elements are all
+    equal to the *value*. For example, ``!listsplat(42, 3)`` results in
+    ``[42, 42, 42]``.
+
+``!lt(``\ *a*\ `,` *b*\ ``)``
+    This operator produces 1 if *a* is less than *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!mul(``\ *a*\ ``,`` *b*\ ``, ...)``
+    This operator multiplies *a*, *b*, etc., and produces the product.
+
+``!ne(``\ *a*\ `,` *b*\ ``)``
+    This operator produces 1 if *a* is not equal to *b*; 0 otherwise.
+    The arguments must be ``bit``, ``int``, or ``string`` values.
+    Use ``!cast<string>`` to compare other types of objects.
+
+``!or(``\ *a*\ ``,`` *b*\ ``, ...)``
+    This operator does a bitwise OR on *a*, *b*, etc., and produces the
+    result.
+
+``!setop(``\ *dag*\ ``,`` *op*\ ``)``
+    This operator produces a DAG node with the same arguments as *dag*, but with its
+    operator replaced with *op*.
+
+    Example: ``!setop((foo 1, 2), bar)`` results in ``(bar 1, 2)``.
+
+``!shl(``\ *a*\ ``,`` *count*\ ``)``
+    This operator shifts *a* left logically by *count* bits and produces the resulting
+    value. The operation is performed on a 64-bit integer; the result
+    is undefined for shift counts outside 0..63.
+
+``!size(``\ *a*\ ``)``
+    This operator produces the number of elements in the list *a*.
+
+``!sra(``\ *a*\ ``,`` *count*\ ``)``
+    This operator shifts *a* right arithmetically by *count* bits and produces the resulting
+    value. The operation is performed on a 64-bit integer; the result
+    is undefined for shift counts outside 0..63.
+
+``!srl(``\ *a*\ ``,`` *count*\ ``)``
+    This operator shifts *a* right logically by *count* bits and produces the resulting
+    value. The operation is performed on a 64-bit integer; the result
+    is undefined for shift counts outside 0..63.
+
+``!strconcat(``\ *str1*\ ``,`` *str2*\ ``, ...)``
+    This operator concatenates the string arguments *str1*, *str2*, etc., and
+    produces the resulting string.
+
+*str1*\ ``#``\ *str2*
+    The paste operator (``#``) is a shorthand for
+    ``!strconcat`` with two arguments.  It can be used to concatenate operands that
+    are not strings, in which
+    case an implicit ``!cast<string>`` is done on those operands.
+
+``!subst(``\ *target*\ ``,`` *repl*\ ``,`` *value*\ ``)``
+    This operator replaces all occurrences of the *target* in the *value* with
+    the *repl* and produces the resulting value. For strings, this is straightforward.
+
+    If the arguments are record names, the function produces the *repl*
+    record if the *target* record name equals the *value* record name; otherwise it
+    produces the *value*.
+
+``!tail(``\ *a*\ ``)``
+    This operator produces a new list with all the elements
+    of the list *a* except for the zeroth one. (See also ``!head``.)
+
+
+Appendix B: Sample Record
+=========================
+
+One target machine supported by LLVM is the Intel x86. The following output
+from TableGen shows the record that is created to represent the 32-bit
+register-to-register ADD instruction.
+
+.. code-block:: text
+
+  def ADD32rr {	// InstructionEncoding Instruction X86Inst I ITy Sched BinOpRR BinOpRR_RF
+    int Size = 0;
+    string DecoderNamespace = "";
+    list<Predicate> Predicates = [];
+    string DecoderMethod = "";
+    bit hasCompleteDecoder = 1;
+    string Namespace = "X86";
+    dag OutOperandList = (outs GR32:$dst);
+    dag InOperandList = (ins GR32:$src1, GR32:$src2);
+    string AsmString = "add{l}	{$src2, $src1|$src1, $src2}";
+    EncodingByHwMode EncodingInfos = ?;
+    list<dag> Pattern = [(set GR32:$dst, EFLAGS, (X86add_flag GR32:$src1, GR32:$src2))];
+    list<Register> Uses = [];
+    list<Register> Defs = [EFLAGS];
+    int CodeSize = 3;
+    int AddedComplexity = 0;
+    bit isPreISelOpcode = 0;
+    bit isReturn = 0;
+    bit isBranch = 0;
+    bit isEHScopeReturn = 0;
+    bit isIndirectBranch = 0;
+    bit isCompare = 0;
+    bit isMoveImm = 0;
+    bit isMoveReg = 0;
+    bit isBitcast = 0;
+    bit isSelect = 0;
+    bit isBarrier = 0;
+    bit isCall = 0;
+    bit isAdd = 0;
+    bit isTrap = 0;
+    bit canFoldAsLoad = 0;
+    bit mayLoad = ?;
+    bit mayStore = ?;
+    bit mayRaiseFPException = 0;
+    bit isConvertibleToThreeAddress = 1;
+    bit isCommutable = 1;
+    bit isTerminator = 0;
+    bit isReMaterializable = 0;
+    bit isPredicable = 0;
+    bit isUnpredicable = 0;
+    bit hasDelaySlot = 0;
+    bit usesCustomInserter = 0;
+    bit hasPostISelHook = 0;
+    bit hasCtrlDep = 0;
+    bit isNotDuplicable = 0;
+    bit isConvergent = 0;
+    bit isAuthenticated = 0;
+    bit isAsCheapAsAMove = 0;
+    bit hasExtraSrcRegAllocReq = 0;
+    bit hasExtraDefRegAllocReq = 0;
+    bit isRegSequence = 0;
+    bit isPseudo = 0;
+    bit isExtractSubreg = 0;
+    bit isInsertSubreg = 0;
+    bit variadicOpsAreDefs = 0;
+    bit hasSideEffects = ?;
+    bit isCodeGenOnly = 0;
+    bit isAsmParserOnly = 0;
+    bit hasNoSchedulingInfo = 0;
+    InstrItinClass Itinerary = NoItinerary;
+    list<SchedReadWrite> SchedRW = [WriteALU];
+    string Constraints = "$src1 = $dst";
+    string DisableEncoding = "";
+    string PostEncoderMethod = "";
+    bits<64> TSFlags = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 0 };
+    string AsmMatchConverter = "";
+    string TwoOperandAliasConstraint = "";
+    string AsmVariantName = "";
+    bit UseNamedOperandTable = 0;
+    bit FastISelShouldIgnore = 0;
+    bits<8> Opcode = { 0, 0, 0, 0, 0, 0, 0, 1 };
+    Format Form = MRMDestReg;
+    bits<7> FormBits = { 0, 1, 0, 1, 0, 0, 0 };
+    ImmType ImmT = NoImm;
+    bit ForceDisassemble = 0;
+    OperandSize OpSize = OpSize32;
+    bits<2> OpSizeBits = { 1, 0 };
+    AddressSize AdSize = AdSizeX;
+    bits<2> AdSizeBits = { 0, 0 };
+    Prefix OpPrefix = NoPrfx;
+    bits<3> OpPrefixBits = { 0, 0, 0 };
+    Map OpMap = OB;
+    bits<3> OpMapBits = { 0, 0, 0 };
+    bit hasREX_WPrefix = 0;
+    FPFormat FPForm = NotFP;
+    bit hasLockPrefix = 0;
+    Domain ExeDomain = GenericDomain;
+    bit hasREPPrefix = 0;
+    Encoding OpEnc = EncNormal;
+    bits<2> OpEncBits = { 0, 0 };
+    bit HasVEX_W = 0;
+    bit IgnoresVEX_W = 0;
+    bit EVEX_W1_VEX_W0 = 0;
+    bit hasVEX_4V = 0;
+    bit hasVEX_L = 0;
+    bit ignoresVEX_L = 0;
+    bit hasEVEX_K = 0;
+    bit hasEVEX_Z = 0;
+    bit hasEVEX_L2 = 0;
+    bit hasEVEX_B = 0;
+    bits<3> CD8_Form = { 0, 0, 0 };
+    int CD8_EltSize = 0;
+    bit hasEVEX_RC = 0;
+    bit hasNoTrackPrefix = 0;
+    bits<7> VectSize = { 0, 0, 1, 0, 0, 0, 0 };
+    bits<7> CD8_Scale = { 0, 0, 0, 0, 0, 0, 0 };
+    string FoldGenRegForm = ?;
+    string EVEX2VEXOverride = ?;
+    bit isMemoryFoldable = 1;
+    bit notEVEX2VEXConvertible = 0;
+  }
+
+On the first line of the record, you can see that the ``ADD32rr`` record
+inherited from eight classes. Although the inheritance hierarchy is complex,
+using superclasses is much simpler than specifying the 109 individual fields for each
+instruction.
+
+Here is the code fragment used to define ``ADD32rr`` and multiple other
+``ADD`` instructions:
+
+.. code-block:: text
+
+  defm ADD : ArithBinOp_RF<0x00, 0x02, 0x04, "add", MRM0r, MRM0m,
+                           X86add_flag, add, 1, 1, 1>;
+
+The ``defm`` statement tells TableGen that ``ArithBinOp_RF`` is a
+multiclass, which contains multiple concrete record definitions that inherit
+from ``BinOpRR_RF``. That class, in turn, inherits from ``BinOpRR``, which
+inherits from ``ITy`` and ``Sched``, and so forth. The fields are inherited
+from all the parent classes; for example, ``IsIndirectBranch`` is inherited
+from the ``Instruction`` class.
diff --git a/llvm/docs/TableGen/index.rst b/llvm/docs/TableGen/index.rst
--- a/llvm/docs/TableGen/index.rst
+++ b/llvm/docs/TableGen/index.rst
@@ -1,6 +1,6 @@
-========
-TableGen
-========
+=================
+TableGen Overview
+=================
 
 .. contents::
    :local:
@@ -9,8 +9,7 @@
    :hidden:
 
    BackEnds
-   LangRef
-   LangIntro
+   ProgRef
    Deficiencies
 
 Introduction
@@ -25,10 +24,12 @@
 
 The core part of TableGen parses a file, instantiates the declarations, and
 hands the result off to a domain-specific `backend`_ for processing.
+See the :doc:`TableGen Programmer's Reference <./ProgRef>` for an in-depth
+description of TableGen.
 
-The current major users of TableGen are :doc:`../CodeGenerator`
-and the
-`Clang diagnostics and attributes <https://clang.llvm.org/docs/UsersManual.html#controlling-errors-and-warnings>`_.
+The current major users of TableGen are :doc:`The LLVM Target-Independent
+Code Generator <../CodeGenerator>` and the `Clang diagnostics and attributes
+<https://clang.llvm.org/docs/UsersManual.html#controlling-errors-and-warnings>`_.
 
 Note that if you work on TableGen much, and use emacs or vim, that you can find
 an emacs "TableGen mode" and a vim language file in the ``llvm/utils/emacs`` and
@@ -249,13 +250,10 @@
                                  !subst(SHIFT, imm_eq0, decls.pattern)),
                         i8>;
 
+See the :doc:`TableGen Programmer's Reference <./ProgRef>` for an in-depth
+description of TableGen.
 
 
-See the :doc:`TableGen Language Introduction <LangIntro>` for more generic
-information on the usage of the language, and the
-:doc:`TableGen Language Reference <LangRef>` for more in-depth description
-of the formal language specification.
-
 .. _backend:
 .. _backends:
 
@@ -300,5 +298,4 @@
 DSLs.
 
 Either way, this is a discussion that will likely span across several years,
-if not decades. You can read more in the `TableGen Deficiencies <Deficiencies.html>`_
-document.
+if not decades. You can read more in :doc:`TableGen Deficiencies <./Deficiencies>`.