1 TableGen Backend Developer’s Guide¶

1.1 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 an output file. These output files are typically .inc files for C++, but may be any type of file that the backend developer needs.

This document is a guide to writing a backend for TableGen. It is not a complete reference manual, but rather a guide to using the facilities provided by TableGen for the backends. For a complete reference to the various data structures and functions involved, see the primary TableGen header file (record.h) and/or the Doxygen documentation.

This document assumes that you have read the TableGen Programmer’s Reference, which provides a detailed reference for coding TableGen source files. For a description of the existing backends, see TableGen BackEnds.

1.2 Data Structures ¶

The following sections describe the data structures that contain the classes and records that are collected from the TableGen source files by the TableGen parser. Note that the term class refers to an abstract record class, while the term record refers to a concrete record.

Unless otherwise noted, functions associated with classes are instance functions.

1.2.1 `RecordKeeper`¶

An instance of the RecordKeeper class acts as the container for all the classes and records parsed and collected by TableGen. The RecordKeeper instance is passed to the backend when it is invoked by TableGen. This class is usually abbreviated RK.

There are two maps in the recordkeeper, one for classes and one for records (the latter often referred to as defs). Each map maps the class or record name to an instance of the Record class (see Record), which contains all the information about that class or record.

In addition to the two maps, the RecordKeeper instance contains:

A map that maps the names of global variables to their values. Global variables are defined in TableGen files with outer defvar statements.
A counter for naming anonymous records.

The RecordKeeper class provides a few useful functions.

Functions to get the complete class and record maps.
Functions to get a subset of the records based on their parent classes.
Functions to get individual classes, records, and globals, by name.

A RecordKeeper instance can be printed to an output stream with the << operator.

1.2.2 `Record`¶

Each class or record built by TableGen is represented by an instance of the Record class. The RecordKeeper instance contains one map for the classes and one for the records. The primary data members of a record are the record name, the vector of field names and their values, and the vector of superclasses of the record.

The record name is stored as a pointer to an Init (see Init), which is a class whose instances hold TableGen values (sometimes referred to as initializers). The field names and values are stored in a vector of RecordVal instances (see RecordVal), each of which contains both the field name and its value. The superclass vector contains a sequence of pairs, with each pair including the superclass record and its source file location.

In addition to those members, a Record instance contains:

A vector of source file locations that includes the record definition itself, plus the locations of any multiclasses involved in its definition.
For a class record, a vector of the class’s template arguments.
An instance of DefInit (see DefInit) corresponding to this record.
A unique record ID.
A boolean that specifies whether this is a class definition.
A boolean that specifies whether this is an anonymous record.

The Record class provides many useful functions.

Functions to get the record name, fields, source file locations, template arguments, and unique ID.
Functions to get all the record’s superclasses or just its direct superclasses.
Functions to get a particular field value by specifying its name in various forms, and returning its value in various forms (see Getting Record Names and Fields).
Boolean functions to check the various attributes of the record.

A Record instance can be printed to an output stream with the << operator.

1.2.3 `RecordVal`¶

Each field of a record is stored in an instance of the RecordVal class. The Record instance includes a vector of these value instances. A RecordVal instance contains the name of the field, stored in an Init instance. It also contains the value of the field, likewise stored in an Init. (A better name for this class might be RecordField.)

In addition to those primary members, the RecordVal has other data members.

The source file location of the field definition.
The type of the field, stored as an instance of the RecTy class (see RecTy).

The RecordVal class provides some useful functions.

Functions to get the name of the field in various forms.
A function to get the type of the field.
A function to get the value of the field.
A function to get the source file location.

Note that field values are more easily obtained directly from the Record instance (see Record).

A RecordVal instance can be printed to an output stream with the << operator.

1.2.4 `RecTy`¶

The RecTy class is used to represent the types of field values. It is the base class for a series of subclasses, one for each of the available field types. The RecTy class has one data member that is an enumerated type specifying the specific type of field value. (A better name for this class might be FieldTy.)

The RecTy class provides a few useful functions.

A virtual function to get the type name as a string.
A virtual function to check whether all the values of this type can be converted to another given type.
A virtual function to check whether this type is a subtype of another given type.
A function to get the corresponding list type for lists with elements of this type. For example, the function returns the list<int> type when called with the int type.

The subclasses that inherit from RecTy are BitRecTy, BitsRecTy, CodeRecTy, DagRecTy, IntRecTy, ListRecTy, RecordRecTy, and StringRecTy. Some of these classes have additional members that are described in the following subsections.

All of the classes derived from RecTy provide the get() function. It returns an instance of Recty corresponding to the derived class. Some of the get() functions require an argument to specify which particular variant of the type is desired. These arguments are described in the following subsections.

A RecTy instance can be printed to an output stream with the << operator.

Warning

It is not specified whether there is a single RecTy instance of a particular type or multiple instances.

1.2.4.1 `BitsRecTy`¶

This class includes a data member with the size of the bits value and a function to get that size.

The get() function takes the length of the sequence, n, and returns the BitsRecTy type corresponding to bits<n>.

1.2.4.2 `ListRecTy`¶

This class includes a data member that specifies the type of the list’s elements and a function to get that type.

The get() function takes the RecTy type of the list members and returns the ListRecTy type corresponding to list<type>.

1.2.4.3 `RecordRecTy`¶

This class includes data members that contain the list of parent classes of this record. It also provides a function to obtain the array of classes and two functions to get the iterator begin() and end() values. The class defines a type for the return values of the latter two functions.

using const_record_iterator = Record * const *;

The get() function takes an ArrayRef of pointers to the Record instances of the direct superclasses of the record and returns the RecordRecTy corresponding to the record inheriting from those superclasses.

1.2.5 `Init`¶

The Init class is used to represent TableGen values. The name derives from initialization value. This class should not be confused with the RecordVal class, which represents record fields, both their names and values. The Init class is the base class for a series of subclasses, one for each of the available value types. The primary data member of Init is an enumerated type that represents the specific type of the value.

The Init class provides a few useful functions.

A function to get the type enumerator.
A boolean virtual function to determine whether a value is completely specified; that is, has no uninitialized subvalues.
Virtual functions to get the value as a string.
Virtual functions to cast the value to other types, implement the bit range feature of TableGen, and implement the list slice feature.
A virtual function to get a particular bit of the value.

The subclasses that inherit directly from Init are UnsetInit and TypedInit.

An Init instance can be printed to an output stream with the << operator.

Warning

It is not specified whether two separate initialization values with the same underlying type and value (e.g., two strings with the value “Hello”) are represented by two Inits or share the same Init.

1.2.5.1 `UnsetInit`¶

This class, a subclass of Init, represents the unset (uninitialized) value. The static function get() can be used to obtain the singleton Init of this type.

1.2.5.2 `TypedInit`¶

This class, a subclass of Init, acts as the parent class of the classes that represent specific value types (except for the unset value). These classes include BitInit, BitsInit, DagInit, DefInit, IntInit, ListInit, and StringInit. (There are additional derived types used by the TableGen parser.)

This class includes a data member that specifies the RecTy type of the value. It provides a function to get that RecTy type.

1.2.5.3 `BitInit`¶

The BitInit class is a subclass of TypedInit. Its instances represent the possible values of a bit: 0 or 1. It includes a data member that contains the bit.

All of the classes derived from TypedInit provide the following functions.

A static function named get() that returns an Init representing the specified value(s). In the case of BitInit, get(true) returns an instance of BitInit representing true, while get(false) returns an instance representing false. As noted above, it is not specified whether there is exactly one or more than one BitInit representing true (or false).
A function named GetValue() that returns the value of the instance in a more direct form, in this case as a bool.

1.2.5.4 `BitsInit`¶

The BitsInit class is a subclass of TypedInit. Its instances represent sequences of bits, from high-order to low-order. It includes a data member with the length of the sequence and a vector of pointers to Init instances, one per bit.

The class provides the usual get() function. It does not provide the getValue() function.

The class provides the following additional functions.

A function to get the number of bits in the sequence.
A function that gets a bit specified by an integer index.

1.2.5.5 `DagInit`¶

The DagInit class is a subclass of TypedInit. Its instances represent the possible direct acyclic graphs (dag).

The class includes a pointer to an Init for the DAG operator and a pointer to a StringInit for the operator name. It includes the count of DAG operands and the count of operand names. Finally, it includes a vector of pointers to Init instances for the operands and another to StringInit instances for the operand names. (The DAG operands are also referred to as arguments.)

The class provides two forms of the usual get() function. It does not provide the usual getValue() function.

The class provides many additional functions:

Functions to get the operator in various forms and to get the operator name in various forms.
Functions to determine whether there are any operands and to get the number of operands.
Functions to the get the operands, both individually and together.
Functions to determine whether there are any names and to get the number of names
Functions to the get the names, both individually and together.
Functions to get the operand iterator begin() and end() values.
Functions to get the name iterator begin() and end() values.

The class defines two types for the return values of the operand and name iterators.

using const_arg_iterator = SmallVectorImpl<Init*>::const_iterator;
using const_name_iterator = SmallVectorImpl<StringInit*>::const_iterator;

1.2.5.6 `DefInit`¶

The DefInit class is a subclass of TypedInit. Its instances represent the records that were collected by TableGen. It includes a data member that is a pointer to the record’s Record instance.

The class provides the usual get() function. It does not provide getValue(). Instead, it provides getDef(), which returns the Record instance.

1.2.5.7 `IntInit`¶

The IntInit class is a subclass of TypedInit. Its instances represent the possible values of a 64-bit integer. It includes a data member that contains the integer.

The class provides the usual get() and getValue() functions. The latter function returns the integer as an int64_t.

The class also provides a function, getBit(), to obtain a specified bit of the integer value.

1.2.5.8 `ListInit`¶

The ListInit class is a subclass of TypedInit. Its instances represent lists of elements of some type. It includes a data member with the length of the list and a vector of pointers to Init instances, one per element.

The class provides the usual get() and getValues() functions. The latter function returns an ArrayRef of the vector of pointers to Init instances.

The class provides these additional functions.

A function to get the element type.
Functions to get the length of the vector and to determine whether it is empty.
Functions to get an element specified by an integer index and return it in various forms.
Functions to get the iterator begin() and end() values. The class defines a type for the return type of these two functions.

using const_iterator = Init *const *;

1.2.5.9 `StringInit`¶

The StringInit class is a subclass of TypedInit. Its instances represent arbitrary-length strings. It includes a data member that contains a StringRef of the value.

The class provides the usual get() and getValue() functions. The latter function returns the StringRef.

1.3 Creating a New Backend ¶

The following steps are required to create a new backend for TableGen.

Invent a name for your backend C++ file, say GenAddressModes.
Write the new backend, using the file TableGenBackendSkeleton.cpp as a starting point.
Determine which instance of TableGen requires the new backend. There is one instance for Clang and another for LLVM. Or you may be building your own instance.
Modify the selected tablegen.cpp to include your new backend.

Add the name to the enumerated type ActionType.

Add a keyword to the ActionType command option using the clEnumValN() function.

Add a case to the switch statement in the xxxTableGenMain() function. It should invoke the “main function” of your backend, which in this case, according to convention, is named EmitAddressModes.

Add a declaration of your “main function” to the corresponding TableGenBackends.h header file.
Add your backend C++ file to the appropriate CMakeLists.txt file so that it will be built.
Add your C++ file to the system.

1.4 The Backend Skeleton ¶

The file TableGenBackendSkeleton.cpp provides a skeleton C++ translation unit for writing a new TableGen backend. Here are a few notes on the file.

The list of includes is the minimal list required by most backends.
As with all LLVM C++ files, it has a using namespace llvm; statement. It also has an anonymous namespace that contains all the file-specific data structure definitions, along with the class embodying the emitter data members and functions. Continuing with the GenAddressModes example, this class is named AddressModesEmitter.
The constructor for the emitter class accepts a RecordKeeper reference, typically named RK. The RecordKeeper reference is saved in a data member so that records can be obtained from it. This data member is usually named Records.
One function is named run. It is invoked by the backend’s “main function” to collect records and emit the output file. It accepts an instance of the raw_ostream class, typically named OS. The output file is emitted by writing to this stream.
The run function should use the emitSourceFileHeader helper function to include a standard header in the emitted file.
The only function in the llvm namespace is the backend “main function.” In this example, it is named EmitAddressModes. It creates an instance of the AddressModesEmitter class, passing the RecordKeeper instance, then invokes the run function, passing the raw_ostream instance.

All the examples in the remainder of this document will assume the naming conventions used in the skeleton file.

1.5 Getting Classes ¶

The RecordKeeper class provides two functions for getting the Record instances for classes defined in the TableGen files.

getClasses() returns a RecordMap reference for all the classes.
getClass(name) returns a Record reference for the named class.

If you need to iterate over all the class records:

for (auto ClassPair : Records.getClasses()) {
  Record *ClassRec = ClassPair.second.get();
  ...
}

ClassPair.second gets the class’s unique_ptr, then .get() gets the class Record itself.

1.6 Getting Records ¶

The RecordKeeper class provides four functions for getting the Record instances for concrete records defined in the TableGen files.

getDefs() returns a RecordMap reference for all the concrete records.
getDef(name) returns a Record reference for the named concrete record.
getAllDerivedDefinitions(classname) returns a vector of Record references for the concrete records that derive from the given class.
getAllDerivedDefinitions(classnames) returns a vector of Record references for the concrete records that derive from all of the given classes.

This statement obtains all the records that derive from the Attribute class and iterates over them.

auto AttrRecords = Records.getAllDerivedDefinitions("Attribute");
for (Record *AttrRec : AttrRecords) {
  ...
}

1.7 Getting Record Names and Fields ¶

As described above (see Record), there are multiple functions that return the name of a record. One particularly useful one is getNameInitAsString(), which returns the name as a std::string.

There are also multiple functions that return the fields of a record. To obtain and iterate over all the fields:

for (const RecordVal &Field : SomeRec->getValues()) {
  ...
}

You will recall that RecordVal is the class whose instances contain information about the fields in records.

The getValue() function returns the RecordVal instance for a field specified by name. There are multiple overloaded functions, some taking a StringRef and others taking a const Init *. Some functions return a RecordVal * and others return a const RecordVal *. If the field does not exist, a fatal error message is printed.

More often than not, you are interested in the value of the field, not all the information in the RecordVal. There is a large set of functions that take a field name in some form and return its value. One function, getValueInit, returns the value as an Init *. Another function, isValueUnset, returns a boolean specifying whether the value is unset (uninitialized).

Most of the functions return the value in some more useful form. For example:

std::vector<int64_t> RegCosts =
    SomeRec->getValueAsListOfInts("RegCosts");

The field RegCosts is assumed to be a list of integers. That list is returned as a std::vector of 64-bit integers. If the field is not a list of integers, a fatal error message is printed.

Here is a function that returns a field value as a Record, but returns null if the field does not exist.

if (Record *BaseRec = SomeRec->getValueAsOptionalDef(BaseFieldName)) {
  ...
}

The field is assumed to have another record as its value. That record is returned as a pointer to a Record. If the field does not exist or is unset, the functions returns null.

1.8 Getting Record Superclasses ¶

The Record class provides a function to obtain the superclasses of a record. It is named getSuperClasses and returns an ArrayRef of an array of std::pair pairs. The superclasses are in post-order: the order in which the superclasses were visited while copying their fields into the record. Each pair consists of a pointer to the Record instance for a superclass record and an instance of the SMRange class. The range indicates the source file locations of the beginning and end of the class definition.

This example obtains the superclasses of the Prototype record and then iterates over the pairs in the returned array.

ArrayRef<std::pair<Record *, SMRange>>
    Superclasses = Prototype->getSuperClasses();
for (const auto &SuperPair : Superclasses) {
  ...
}

The Record class also provides a function, getDirectSuperClasses, to append the direct superclasses of a record to a given vector of type SmallVectorImpl<Record *>.

1.9 Emitting Text to the Output Stream ¶

The run function is passed a raw_ostream to which it prints the output file. By convention, this stream is saved in the emitter class member named OS, although some run functions are simple and just use the stream without saving it. The output can be produced by writing values directly to the output stream, or by using the std::format() or llvm::formatv() functions.

OS << "#ifndef " << NodeName << "\n";

OS << format("0x%0*x, ", Digits, Value);

Instances of the following classes can be printed using the << operator: RecordKeeper, Record, RecTy, RecordVal, and Init.

The helper function emitSourceFileHeader() prints the header comment that should be included at the top of every output file. A call to it is included in the skeleton backend file TableGenBackendSkeleton.cpp.

1.10 Printing Error Messages ¶

TableGen records are often derived from multiple classes and also often defined through a sequence of multiclasses. Because of this, it can be difficult for backends to report clear error messages with accurate source file locations. To make error reporting easier, five error reporting functions are provided, each with four overloads.

PrintWarning prints a message tagged as a warning.
PrintError prints a message tagged as an error.
PrintFatalError prints a message tagged as an error and then terminates.
PrintNote prints a note. It is often used after one of the previous functions to provide more information.
PrintFatalNote prints a note and then terminates.

Each of these five functions is overloaded four times.

PrintError(const Twine &Msg): Prints the message with no source file location.
PrintError(ArrayRef<SMLoc> ErrorLoc, const Twine &Msg): Prints the message followed by the specified source line, along with a pointer to the item in error. The array of source file locations is typically taken from a Record instance.
PrintError(const Record *Rec, const Twine &Msg): Prints the message followed by the source line associated with the specified record (see Record).
PrintError(const RecordVal *RecVal, const Twine &Msg): Prints the message followed by the source line associated with the specified record field (see RecordVal).

Using these functions, the goal is to produce the most specific error report possible.

1.11 Debugging Tools ¶

TableGen provides some tools to aid in debugging backends.

1.11.1 The `PrintRecords` Backend ¶

The TableGen command option --print-records invokes a simple backend that prints all the classes and records defined in the source files. This is the default backend option. The format of the output is guaranteed to be constant over time, so that the output can be compared in tests. The output looks like this:

------------- Classes -----------------
...
class XEntry<string XEntry:str = ?, int XEntry:val1 = ?> { // XBase
  string Str = XEntry:str;
  bits<8> Val1 = { !cast<bits<8>>(XEntry:val1){7}, ... };
  bit Val3 = 1;
}
...
------------- Defs -----------------
def ATable {  // GenericTable
  string FilterClass = "AEntry";
  string CppTypeName = "AEntry";
  list<string> Fields = ["Str", "Val1", "Val2"];
  list<string> PrimaryKey = ["Val1", "Val2"];
  string PrimaryKeyName = "lookupATableByValues";
  bit PrimaryKeyEarlyOut = 0;
}
...
def anonymous_0 {     // AEntry
  string Str = "Bob";
  bits<8> Val1 = { 0, 0, 0, 0, 0, 1, 0, 1 };
  bits<10> Val2 = { 0, 0, 0, 0, 0, 0, 0, 0, 1, 1 };
}

Classes are shown with their template arguments, parent classes (following //), and fields. Records are shown with their parent classes and fields. Note that anonymous records are named anonymous_0, anonymous_1, etc.

1.11.2 The `PrintDetailedRecords` Backend ¶

The TableGen command option --print-detailed-records invokes a backend that prints all the global variables, classes, and records defined in the source files. The format of the output is not guaranteed to be constant over time. The output looks like this.

DETAILED RECORDS for file llvm-project\llvm\lib\target\arc\arc.td

-------------------- Global Variables (5) --------------------

AMDGPUBufferIntrinsics = [int_amdgcn_buffer_load_format, ...
AMDGPUImageDimAtomicIntrinsics = [int_amdgcn_image_atomic_swap_1d, ...
...
-------------------- Classes (758) --------------------

AMDGPUBufferLoad  |IntrinsicsAMDGPU.td:879|
  Template args:
    LLVMType AMDGPUBufferLoad:data_ty = llvm_any_ty  |IntrinsicsAMDGPU.td:879|
  Superclasses: (SDPatternOperator) Intrinsic AMDGPURsrcIntrinsic
  Fields:
    list<SDNodeProperty> Properties = [SDNPMemOperand]  |Intrinsics.td:348|
    string LLVMName = ""  |Intrinsics.td:343|
...
-------------------- Records (12303) --------------------

AMDGPUSample_lz_o  |IntrinsicsAMDGPU.td:560|
  Defm sequence: |IntrinsicsAMDGPU.td:584| |IntrinsicsAMDGPU.td:566|
  Superclasses: AMDGPUSampleVariant
  Fields:
    string UpperCaseMod = "_LZ_O"  |IntrinsicsAMDGPU.td:542|
    string LowerCaseMod = "_lz_o"  |IntrinsicsAMDGPU.td:543|
...

Global variables defined with outer defvar statements are shown with their values.
The classes are shown with their source location, template arguments, superclasses, and fields.
The records are shown with their source location, defm sequence, superclasses, and fields.

Superclasses are shown in the order processed, with indirect superclasses in parentheses. Each field is shown with its value and the source location at which it was set. The defm sequence gives the locations of the defm statements that were involved in generating the record, in the order they were invoked.

1.11.3 Timing TableGen Phases ¶

TableGen provides a phase timing feature that produces a report of the time used by the various phases of parsing the source files and running the selected backend. This feature is enabled with the --time-phases option of the TableGen command.

If the backend is not instrumented for timing, then a report such as the following is produced. This is the timing for the --print-detailed-records backend run on the AMDGPU target.

===-------------------------------------------------------------------------===
                             TableGen Phase Timing
===-------------------------------------------------------------------------===
  Total Execution Time: 101.0106 seconds (102.4819 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  85.5197 ( 84.9%)   0.1560 ( 50.0%)  85.6757 ( 84.8%)  85.7009 ( 83.6%)  Backend overall
  15.1789 ( 15.1%)   0.0000 (  0.0%)  15.1789 ( 15.0%)  15.1829 ( 14.8%)  Parse, build records
   0.0000 (  0.0%)   0.1560 ( 50.0%)   0.1560 (  0.2%)   1.5981 (  1.6%)  Write output
  100.6986 (100.0%)   0.3120 (100.0%)  101.0106 (100.0%)  102.4819 (100.0%)  Total

Note that all the time for the backend is lumped under “Backend overall”.

If the backend is instrumented for timing, then its processing is divided into phases and each one timed separately. This is the timing for the --emit-dag-isel backend run on the AMDGPU target.

===-------------------------------------------------------------------------===
                             TableGen Phase Timing
===-------------------------------------------------------------------------===
  Total Execution Time: 746.3868 seconds (747.1447 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  657.7938 ( 88.1%)   0.1404 ( 90.0%)  657.9342 ( 88.1%)  658.6497 ( 88.2%)  Emit matcher table
  70.2317 (  9.4%)   0.0000 (  0.0%)  70.2317 (  9.4%)  70.2700 (  9.4%)  Convert to matchers
  14.8825 (  2.0%)   0.0156 ( 10.0%)  14.8981 (  2.0%)  14.9009 (  2.0%)  Parse, build records
   2.1840 (  0.3%)   0.0000 (  0.0%)   2.1840 (  0.3%)   2.1791 (  0.3%)  Sort patterns
   1.1388 (  0.2%)   0.0000 (  0.0%)   1.1388 (  0.2%)   1.1401 (  0.2%)  Optimize matchers
   0.0000 (  0.0%)   0.0000 (  0.0%)   0.0000 (  0.0%)   0.0050 (  0.0%)  Write output
  746.2308 (100.0%)   0.1560 (100.0%)  746.3868 (100.0%)  747.1447 (100.0%)  Total

The backend has been divided into four phases and timed separately.

If you want to instrument a backend, refer to the backend DAGISelEmitter.cpp and search for Records.startTimer.

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