New TableGen Programmer's Reference document

[Paul-C-Anagnostopoulos]

Summary

This new TableGen Programmer's Reference document replaces the current Language Introduction and Language Reference documents. It brings all the TableGen reference information into one document.

As an experiment, I numbered the sections in the document. See what you think about that.

[lattner]

This is really fantastic work, thank you for doing this!

llvm/docs/TableGen/ProgRef.rst

14

"complete detail" might be a stretch :-). People will always want more, and this will replace the existing documentation - you don't need to reference the old thing.

The first paragraph of the introduction should be an introduction to tablegen, something closer to your second paragraph.

63–64

I'd phrase it slightly differently maybe something like:

It is typical for abstract records to have many missing fields which are then filled in when they are instantiated into a concrete record, but even a concrete record may have fields with the value of ?.

66

I'd replace "In a complex program such as LLVM," with "In a complex use case like the LLVM code generator,"

93

101

I'd mention the preprocessor in this section

216

I'd mention #ifndef/define as well since this is an overview

310

It would be nice to deprecate and remove this at some point, .. is much nicer :-)

[Paul-C-Anagnostopoulos]

Thanks! I quite enjoyed writing it. I will incorporate your comments and submit a new patch.

I did not yet attempt to find all the references to the existing two TableGen documents and change those references. I figure I would do that as a separate patch later. Does that make sense?

Could you comment on the '..' patch? https://reviews.llvm.org/D85585

[lattner]

Yep, as long as you're committed to doing it, I'm perfectly fine with doing that in a followup patch. You might want to land this at one of the old URLs to make that easier though.

Thank you again for driving this!

[madhur13490]

llvm/docs/TableGen/ProgRef.rst

263

I think this is the most complex type. May be its worthwhile to get some more info on this document? In general I would like if we can capture its syntax, explanations on keywords like ops, ins, outs and some examples on this.

[Paul-C-Anagnostopoulos]

In D85838#2217330, @lattner wrote:

Yep, as long as you're committed to doing it, I'm perfectly fine with doing that in a followup patch. You might want to land this at one of the old URLs to make that easier though.

I think to do this, the file would have to be named the same as one of the old files. I gave the document a new title to avoid any conflicts with the old names. I don't mind hunting up all the existing references and changing them to point to the new document.

[Paul-C-Anagnostopoulos]

I will add a section on the dag type. However, I will do that as a future update, since it will take awhile to write the new section. In particular, I have to see how much is written elsewhere and include references as appropriate.

[Paul-C-Anagnostopoulos]

Here is the final document with Chris Lattner's suggestions. I have also included index.rst, which now refers to this document rather than the two old ones.

Could someone commit this for me, since I do not have commit privilege?

[Paul-C-Anagnostopoulos]

Sorry, I noticed one typo just after I submitted the previous revised patch. It is corrected in this patch.

[lattner]

Hi Paul,

Please take a look at the LLVM Developer Policy and follow the instructions on commit access. :-)

[Paul-C-Anagnostopoulos]

In D85838#2219529, @lattner wrote:

Hi Paul,

Please take a look at the LLVM Developer Policy and follow the instructions on commit access. :-)

I appreciate the offer, but, as Paul Robinson will attest, I need more practice with Git. I'm reading a book on it for the third time, which I hope will do the trick.

I feel comfortable messing around with my own repository, but I'd appreciate someone else committing my next couple of patches.

[johnwbyrd]

I'm a little disappointed that this document does not describe the *intention* of TableGen, what exactly its output files are, their specific purposes, or where they are generally used in LLVM. It's possible that this information is ill suited for a reference manual, and belongs in a guide instead. Regardless, despite this draft's limitations, it's an improvement over the current state of things, and with a few improvements I'll happily sign off on it.

llvm/docs/TableGen/ProgRef.rst

14–16

This doesn't distinguish the purpose of TableGen from any other program that processes input to output. Why does TableGen need to exist? Who should know how to work with it? What are its areas of responsibility?

30–31

Why just a few of the things? Why not all of the things? This is a reference document.

51–52

Differentiate between TableGen classes and C++ classes, please.

64

What kind of output files? This is way too vague.

113

Reformat to 80 columns.

265–267

This doesn't describe why "bits" fields are useful, how they are stored, or generally what their purpose is in code.

275

Why are DAGs important to TableGen?

277

Yes, and please don't submit this rev without adding some.

289–290

Not needed as part of this reference.

534

Example, not "interlude."

590

Redundant. What, specifically, does it mean to "define" a record?

630–634

"Interlude"? Unnecessarily pretentious. This is an example.

749–751

This concept is good, but it doesn't follow the parallel structure of headings in the rest of the document. The heading is the keyword. Use text to define the concept.

839

Example, not an interlude. Also, the example should go here, not after defm.

845–846

The records are not "specified" in the multiclasses. Multiclasses permit instantiation of multiple similar TableGen definitions, with a single "defm" statement. Not the same thing -- please reword.

856–857

The records are not defined in the multiclasses. They are defined by the defm statement itself.

888–889

Use the term example, please. There is no other place in the LLVM documentation that mentions "interludes."

896–913

I am seeing now that the use of "interlude" or "example" is inconsistent. Some code is marked as an example and some is not, without clear logic as to which is which. I suggest removing the numbering from all interludes/examples, and linking to them from the body text if the example does not immediately follow the code that describes it.

950–951

Defm doesn't allow you to "gain multiple levels." Reword to be more technically accurate please.

1146

Unnecessary -- your other examples merely follow without being introduced.

1212–1213

This section looks new. Useful info.

1220

What does "left to right" mean in this context? The order in which the superclasses were instantiated? Or from child to parent order? Word more precisely please.

1284–1285

This section looks new too. Good job.

1563

Not really relevant. All generated instruction records tend to look more or less this complex on all platforms.

[Paul-C-Anagnostopoulos]

In D85838#2219986, @johnwbyrd wrote:

I'm a little disappointed that this document does not describe the *intention* of TableGen, what exactly its output files are, their specific purposes, or where they are generally used in LLVM. It's possible that this information is ill suited for a reference manual, and belongs in a guide instead. Regardless, despite this draft's limitations, it's an improvement over the current state of things, and with a few improvements I'll happily sign off on it.

I will attempt to be more specific about the purpose of TableGen. I don't think I can list "all" the uses of it, however, since it is a general-purpose tool.

I will address your other points and resubmit in the next few days. Thanks for the critique.

[johnwbyrd]

I will attempt to be more specific about the purpose of TableGen. I don't think I can list "all" the uses of it, however, since it is a general-purpose tool.

The uses of TableGen are finite and enumerable. An effort has been made to list all the uses of TableGen -- see llvm/docs/TableGen/BackEnds.rst . I am not convinced these descriptions are canonical.

[Paul-C-Anagnostopoulos]

In D85838#2220067, @johnwbyrd wrote:

I will attempt to be more specific about the purpose of TableGen. I don't think I can list "all" the uses of it, however, since it is a general-purpose tool.

The uses of TableGen are finite and enumerable. An effort has been made to list all the uses of TableGen -- see llvm/docs/TableGen/BackEnds.rst . I am not convinced these descriptions are canonical.

The uses are finite within the LLVM and Clang projects. But nothing stops someone outside these projects from using it.

https://www.embecosm.com/2015/04/14/utilizing-tablegen-for-non-compiling-processes/

[johnwbyrd]

The uses are finite within the LLVM and Clang projects. But nothing stops someone outside these projects from using it.
https://www.embecosm.com/2015/04/14/utilizing-tablegen-for-non-compiling-processes/

That project does not seem to be part of LLVM, so it's not clear to me why the reference guide should document it. Anyway, as you prefer.

[Paul-C-Anagnostopoulos]

In D85838#2220106, @johnwbyrd wrote:

The uses are finite within the LLVM and Clang projects. But nothing stops someone outside these projects from using it.
https://www.embecosm.com/2015/04/14/utilizing-tablegen-for-non-compiling-processes/

That project does not seem to be part of LLVM, so it's not clear to me why the reference guide should document it. Anyway, as you prefer.

Oh, I don't think that the reference should document it. I was just pointing that an exhaustive list of TableGen uses is not possible.

[Paul-C-Anagnostopoulos]

In D85838#2219986, @johnwbyrd wrote:

This concept is good, but it doesn't follow the parallel structure of headings in the rest of the document. The heading is the keyword. Use text to define the concept.

John: that is what you said about the heading for the let statement. I'm not sure I understand your objection. Can you elaborate?

[johnwbyrd]

John: that is what you said about the heading for the let statement. I'm not sure I understand your objection. Can you elaborate?

After reviewing the other headings, I see that my comment is in error. Please mark the comment as done.

[Paul-C-Anagnostopoulos]

I have incorporated John's comments. If this is now acceptable, I would appreciate someone committing it for me.

[nhaehnle]

First of all, thank you for doing this. I haven't gone through it fully yet. The main thing that is missing in my opinion is to remove stuff from the existing files. You include here a listing of TableGen syntax -- that's fine, but it's now redundant with what's in TableGen/LangRef.rst, and this redundancy is a very bad place for us to be in.

llvm/docs/TableGen/ProgRef.rst

58

This is only partially true. Anonymous defs are a thing. They end up having a standing name like anonymous_NNNN, but that doesn't tend to be very useful.

[nhaehnle]

llvm/docs/TableGen/ProgRef.rst

308–309

... and an optional value.

603

Missing a colon?

[Paul-C-Anagnostopoulos]

In D85838#2222000, @nhaehnle wrote:

First of all, thank you for doing this. I haven't gone through it fully yet. The main thing that is missing in my opinion is to remove stuff from the existing files. You include here a listing of TableGen syntax -- that's fine, but it's now redundant with what's in TableGen/LangRef.rst, and this redundancy is a very bad place for us to be in.

You are most welcome. My plan was to get this committed first. Note that this includes an update to the TableGen Overview to refer to this document rather than the two old ones. A search turns up no other references to the two old documents in the Clang or LLVM documentation. Then once this is committed, we can delete the two old files. (Is that what we do, delete them?)

Shall I wait for additional review comments from you?

[nhaehnle]

In D85838#2222073, @Paul-C-Anagnostopoulos wrote:

In D85838#2222000, @nhaehnle wrote:

First of all, thank you for doing this. I haven't gone through it fully yet. The main thing that is missing in my opinion is to remove stuff from the existing files. You include here a listing of TableGen syntax -- that's fine, but it's now redundant with what's in TableGen/LangRef.rst, and this redundancy is a very bad place for us to be in.

You are most welcome. My plan was to get this committed first. Note that this includes an update to the TableGen Overview to refer to this document rather than the two old ones. A search turns up no other references to the two old documents in the Clang or LLVM documentation. Then once this is committed, we can delete the two old files. (Is that what we do, delete them?)

Why not just delete them at the same time? (them = LangIntro.rst and LangRef.rst, I assume) Making the change atomically like that feels more "right" to me. And after actually scanning those two docs again, yes, I agree that we can just delete them.

Shall I wait for additional review comments from you?

I've sent a few more a few minutes ago, but that should cover it as far as I'm concerned.

[Paul-C-Anagnostopoulos]

In D85838#2222091, @nhaehnle wrote:

In D85838#2222073, @Paul-C-Anagnostopoulos wrote:

In D85838#2222000, @nhaehnle wrote:

First of all, thank you for doing this. I haven't gone through it fully yet. The main thing that is missing in my opinion is to remove stuff from the existing files. You include here a listing of TableGen syntax -- that's fine, but it's now redundant with what's in TableGen/LangRef.rst, and this redundancy is a very bad place for us to be in.

You are most welcome. My plan was to get this committed first. Note that this includes an update to the TableGen Overview to refer to this document rather than the two old ones. A search turns up no other references to the two old documents in the Clang or LLVM documentation. Then once this is committed, we can delete the two old files. (Is that what we do, delete them?)

Why not just delete them at the same time? (them = LangIntro.rst and LangRef.rst, I assume) Making the change atomically like that feels more "right" to me. And after actually scanning those two docs again, yes, I agree that we can just delete them.

Shall I wait for additional review comments from you?

I've sent a few more a few minutes ago, but that should cover it as far as I'm concerned.

I will address your comments and submit a final patch.

@nhaehnle, but please tell me how to delete the two old files. Do I simply do a 'git rm' and then commit that along with the final ProfRef.rst?

[Paul-C-Anagnostopoulos]

I incorporated Nicolai's final suggestions. I also deleted the old Tablegen language intro and reference, since they are replaced by this document.

@nhaehnle: Could you commit this for me?

[foad]

llvm/docs/TableGen/ProgRef.rst

1153

Is this true? This assertion did not appear in the previous documentation, and it doesn't seem to be borne out by the grammar productions: BodyItem only includes let, defvar and assert statements.

@Joe_Nash for awareness.

[kosarev]

llvm/docs/TableGen/ProgRef.rst

197

How do we know that? There are some mentions of deprecated features in the code, but field doesn't seem to be amongst them. Also, the instruction decoder backend looks at the field mark to distinct encoding field definitions from ordinary TableGen class fields which is something we seem to rely on in our instruction definition .td's. If field is deprecated, then what's supposed to be a replacement for this use case?

[Paul-C-Anagnostopoulos]

llvm/docs/TableGen/ProgRef.rst

197

We were hoping that no one would need to use it in the future, which is why it is flagged as deprecated. At some point I am supposed to try to figure out what to do about the current uses.

603

The colon is part of the RecordBody production.

1153

It is not true. Feel free to get rid of that sentence. Good catch.

[foad]

llvm/docs/TableGen/ProgRef.rst

1153

Thanks. 9293539064aead8d1822e181b66fd5590c062a1d.

[kosarev]

llvm/docs/TableGen/ProgRef.rst

197

So if for the current uses there is no alternative and as of today little we can do to avoid using field there, then what is the purpose of officially declaring it deprecated? Wouldn't it save us some concerns and confusion to postpone such a declaration until we have a better solution?

[Paul-C-Anagnostopoulos]

llvm/docs/TableGen/ProgRef.rst

197

I was hoping to prevent new uses. If you want to reword the statement to say that rather than using the term "deprecated," go for it.

[kosarev]

llvm/docs/TableGen/ProgRef.rst

197

I see, thanks. Followed up in https://reviews.llvm.org/D126290.

llvm/docs/TableGen/LangIntro.rst

-==============================
-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.
-

llvm/docs/TableGen/LangRef.rst

-===========================
-TableGen Language Reference
-===========================
-
-.. contents::
-   :local:
-
-.. warning::
-   This document is extremely rough. If you find something lacking, please
-   fix it, file a documentation bug, or ask about it on 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.

llvm/docs/TableGen/ProgRef.rst

+===============================
+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.

llvm/docs/TableGen/index.rst

-========
-TableGen
-========
+=================
+TableGen Overview
+=================
 
 .. contents::
    :local:
 
 .. toctree::
    :hidden:
 
    BackEnds
-   LangRef
-   LangIntro
+   ProgRef
    Deficiencies
 
 Introduction
 ============
 
 TableGen's purpose is to help a human develop and maintain records of
 domain-specific information.  Because there may be a large number of these
 records, it is specifically designed to allow writing flexible descriptions and
 for common features of these records to be factored out.  This reduces the
 amount of duplication in the description, reduces the chance of error, and makes
 it easier to structure domain specific information.
 
 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
 ``llvm/utils/vim`` directories of your LLVM distribution, respectively.
 
 .. _intro:
 
 
def : Pat<(i64 (!cast<SDNode>("sextload" # sty) address)),
               Base, Offset, Extend)>;
   }
 
   defm : ro_signed_pats<"B", Rm, Base, Offset, Extend,
                         !foreach(decls.pattern, address,
                                  !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:
 
 TableGen backends
 =================
 
 TableGen files have no real meaning without a back-end. The default operation
 of running ``llvm-tblgen`` is to print the information in a textual format, but

 file.
 
 There are some in favour of extending the semantics even more, but making sure
 back-ends adhere to strict rules. Others are suggesting we should move to less,
 more powerful DSLs designed with specific purposes, or even re-using existing
 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>`.