llvm-project/llvm/docs/TableGen/index.rst

=================
TableGen Overview
=================

.. contents::
   :local:

.. toctree::
   :hidden:

   BackEnds
   BackGuide
   ProgRef

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 TableGen front end 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. See :doc:`xxx-tblgen - Target Description to C++
Code <../CommandGuide/tblgen>` for details on the various
``xxx-tblgen`` commands that invoke TableGen.

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:


The TableGen program
====================

TableGen files are interpreted by the TableGen program: `llvm-tblgen` available
on your build directory under `bin`. It is not installed in the system (or where
your sysroot is set to), since it has no use beyond LLVM's build process.

Running TableGen
----------------

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

To be useful, one of the `backends`_ must be used.  These backends are
selectable on the command line (type '``llvm-tblgen -help``' for a list).  For
example, to get a list of all of the definitions that subclass a particular type
(which can be useful for building up an enum list of these records), use the
``-print-enums`` option:

.. code-block:: bash

  $ llvm-tblgen X86.td -print-enums -class=Register
  AH, AL, AX, BH, BL, BP, BPL, BX, CH, CL, CX, DH, DI, DIL, DL, DX, EAX, EBP, EBX,
  ECX, EDI, EDX, EFLAGS, EIP, ESI, ESP, FP0, FP1, FP2, FP3, FP4, FP5, FP6, IP,
  MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7, R10, R10B, R10D, R10W, R11, R11B, R11D,
  R11W, R12, R12B, R12D, R12W, R13, R13B, R13D, R13W, R14, R14B, R14D, R14W, R15,
  R15B, R15D, R15W, R8, R8B, R8D, R8W, R9, R9B, R9D, R9W, RAX, RBP, RBX, RCX, RDI,
  RDX, RIP, RSI, RSP, SI, SIL, SP, SPL, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7,
  XMM0, XMM1, XMM10, XMM11, XMM12, XMM13, XMM14, XMM15, XMM2, XMM3, XMM4, XMM5,
  XMM6, XMM7, XMM8, XMM9,

  $ llvm-tblgen X86.td -print-enums -class=Instruction 
  ABS_F, ABS_Fp32, ABS_Fp64, ABS_Fp80, ADC32mi, ADC32mi8, ADC32mr, ADC32ri,
  ADC32ri8, ADC32rm, ADC32rr, ADC64mi32, ADC64mi8, ADC64mr, ADC64ri32, ADC64ri8,
  ADC64rm, ADC64rr, ADD16mi, ADD16mi8, ADD16mr, ADD16ri, ADD16ri8, ADD16rm,
  ADD16rr, ADD32mi, ADD32mi8, ADD32mr, ADD32ri, ADD32ri8, ADD32rm, ADD32rr,
  ADD64mi32, ADD64mi8, ADD64mr, ADD64ri32, ...

The default backend prints out all of the records. There is also a general
backend which outputs all the records as a JSON data structure, enabled using
the `-dump-json` option.

If you plan to use TableGen, you will most likely have to write a `backend`_
that extracts the information specific to what you need and formats it in the
appropriate way. You can do this by extending TableGen itself in C++, or by
writing a script in any language that can consume the JSON output.

Example
-------

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

.. code-block:: text

  ...
  def ADD32rr {   // Instruction X86Inst I
    string Namespace = "X86";
    dag OutOperandList = (outs GR32:$dst);
    dag InOperandList = (ins GR32:$src1, GR32:$src2);
    string AsmString = "add{l}\t{$src2, $dst|$dst, $src2}";
    list<dag> Pattern = [(set GR32:$dst, (add GR32:$src1, GR32:$src2))];
    list<Register> Uses = [];
    list<Register> Defs = [EFLAGS];
    list<Predicate> Predicates = [];
    int CodeSize = 3;
    int AddedComplexity = 0;
    bit isReturn = 0;
    bit isBranch = 0;
    bit isIndirectBranch = 0;
    bit isBarrier = 0;
    bit isCall = 0;
    bit canFoldAsLoad = 0;
    bit mayLoad = 0;
    bit mayStore = 0;
    bit isImplicitDef = 0;
    bit isConvertibleToThreeAddress = 1;
    bit isCommutable = 1;
    bit isTerminator = 0;
    bit isReMaterializable = 0;
    bit isPredicable = 0;
    bit hasDelaySlot = 0;
    bit usesCustomInserter = 0;
    bit hasCtrlDep = 0;
    bit isNotDuplicable = 0;
    bit hasSideEffects = 0;
    InstrItinClass Itinerary = NoItinerary;
    string Constraints = "";
    string DisableEncoding = "";
    bits<8> Opcode = { 0, 0, 0, 0, 0, 0, 0, 1 };
    Format Form = MRMDestReg;
    bits<6> FormBits = { 0, 0, 0, 0, 1, 1 };
    ImmType ImmT = NoImm;
    bits<3> ImmTypeBits = { 0, 0, 0 };
    bit hasOpSizePrefix = 0;
    bit hasAdSizePrefix = 0;
    bits<4> Prefix = { 0, 0, 0, 0 };
    bit hasREX_WPrefix = 0;
    FPFormat FPForm = ?;
    bits<3> FPFormBits = { 0, 0, 0 };
  }
  ...

This definition corresponds to the 32-bit register-register ``add`` instruction
of the x86 architecture.  ``def ADD32rr`` defines a record named
``ADD32rr``, and the comment at the end of the line indicates the superclasses
of the definition.  The body of the record contains all of the data that
TableGen assembled for the record, indicating that the instruction is part of
the "X86" namespace, the pattern indicating how the instruction is selected by
the code generator, that it is a two-address instruction, has a particular
encoding, etc.  The contents and semantics of the information in the record are
specific to the needs of the X86 backend, and are only shown as an example.

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

.. code-block:: text

  let Defs = [EFLAGS],
      isCommutable = 1,                  // X = ADD Y,Z --> X = ADD Z,Y
      isConvertibleToThreeAddress = 1 in // Can transform into LEA.
  def ADD32rr  : I<0x01, MRMDestReg, (outs GR32:$dst),
                                     (ins GR32:$src1, GR32:$src2),
                   "add{l}\t{$src2, $dst|$dst, $src2}",
                   [(set GR32:$dst, (add GR32:$src1, GR32:$src2))]>;

This definition makes use of the custom class ``I`` (extended from the custom
class ``X86Inst``), which is defined in the X86-specific TableGen file, to
factor out the common features that instructions of its class share.  A key
feature of TableGen is that it allows the end-user to define the abstractions
they prefer to use when describing their information.

Syntax
======

TableGen has a syntax that is loosely based on C++ templates, with built-in
types and specification. In addition, TableGen's syntax introduces some
automation concepts like multiclass, foreach, let, etc.

Basic concepts
--------------

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

**TableGen records** have a unique name, a list of values, and a list of
superclasses.  The list of values is the main data that TableGen builds for each
record; it is this that holds the domain specific information for the
application.  The interpretation of this data is left to a specific `backend`_,
but the structure and format rules are taken care of and are fixed by
TableGen.

**TableGen definitions** are the concrete form of 'records'.  These generally do
not have any undefined values, and are marked with the '``def``' keyword.

.. code-block:: text

  def FeatureFPARMv8 : SubtargetFeature<"fp-armv8", "HasFPARMv8", "true",
                                        "Enable ARMv8 FP">;

In this example, FeatureFPARMv8 is ``SubtargetFeature`` record initialised
with some values. The names of the classes are defined via the
keyword `class` either on the same file or some other included. Most target
TableGen files include the generic ones in ``include/llvm/Target``.

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

.. code-block:: text

 class ProcNoItin<string Name, list<SubtargetFeature> Features>
       : Processor<Name, NoItineraries, Features>;

Here, the class ProcNoItin, receiving parameters `Name` of type `string` and
a list of target features is specializing the class Processor by passing the
arguments down as well as hard-coding NoItineraries.

**TableGen multiclasses** are groups of abstract records that are instantiated
all at once.  Each instantiation can result in multiple TableGen definitions.
If a multiclass inherits from another multiclass, the definitions in the
sub-multiclass become part of the current multiclass, as if they were declared
in the current multiclass.

.. code-block:: text

  multiclass ro_signed_pats<string T, string Rm, dag Base, dag Offset, dag Extend,
                          dag address, ValueType sty> {
  def : Pat<(i32 (!cast<SDNode>("sextload" # sty) address)),
            (!cast<Instruction>("LDRS" # T # "w_" # Rm # "_RegOffset")
              Base, Offset, Extend)>;

  def : Pat<(i64 (!cast<SDNode>("sextload" # sty) address)),
            (!cast<Instruction>("LDRS" # T # "x_" # Rm # "_RegOffset")
              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.


.. _backend:
.. _backends:

TableGen backends
=================

TableGen files have no real meaning without a backend. The default operation
when running ``xxx-tblgen`` is to print the information in a textual format, but
that's only useful for debugging the TableGen files themselves. The power
in TableGen is, however, to interpret the source files into an internal 
representation that can be generated into anything you want.

Current usage of TableGen is to create huge include files with tables that you
can either include directly (if the output is in the language you're coding),
or be used in pre-processing via macros surrounding the include of the file.

Direct output can be used if the backend already prints a table in C format
or if the output is just a list of strings (for error and warning messages).
Pre-processed output should be used if the same information needs to be used
in different contexts (like Instruction names), so your backend should print
a meta-information list that can be shaped into different compile-time formats.

See :doc:`TableGen BackEnds <./BackEnds>` for a list of available
backends, and see the :doc:`TableGen Backend Developer's Guide <./BackGuide>`
for information on how to write and debug a new backend.

TableGen Deficiencies
=====================

Despite being very generic, TableGen has some deficiencies that have been
pointed out numerous times. The common theme is that, while TableGen allows
you to build domain specific languages, the final languages that you create
lack the power of other DSLs, which in turn increase considerably the size
and complexity of TableGen files.

At the same time, TableGen allows you to create virtually any meaning of
the basic concepts via custom-made backends, which can pervert the original
design and make it very hard for newcomers to understand the evil TableGen
file.

There are some in favor of extending the semantics even more, but making sure
backends adhere to strict rules. Others are suggesting we should move to less,
more powerful DSLs designed with specific purposes, or even reusing existing
DSLs.