TableGen BackEnds

Introduction

TableGen backends are at the core of TableGen’s functionality. The source files provide the semantics to a generated (in memory) structure, but it’s up to the backend to print this out in a way that is meaningful to the user (normally a C program including a file or a textual list of warnings, options and error messages).

TableGen is used by both LLVM and Clang with very different goals. LLVM uses it as a way to automate the generation of massive amounts of information regarding instructions, schedules, cores and architecture features. Some backends generate output that is consumed by more than one source file, so they need to be created in a way that is easy to use pre-processor tricks. Some backends can also print C code structures, so that they can be directly included as-is.

Clang, on the other hand, uses it mainly for diagnostic messages (errors, warnings, tips) and attributes, so more on the textual end of the scale.

LLVM BackEnds

Warning

This document is raw. Each section below needs three sub-sections: description of its purpose with a list of users, output generated from generic input, and finally why it needed a new backend (in case there’s something similar).

Overall, each backend will take the same TableGen file type and transform into similar output for different targets/uses. There is an implicit contract between the TableGen files, the back-ends and their users.

For instance, a global contract is that each back-end produces macro-guarded sections. Based on whether the file is included by a header or a source file, or even in which context of each file the include is being used, you have todefine a macro just before including it, to get the right output:

#define GET_REGINFO_TARGET_DESC
#include "ARMGenRegisterInfo.inc"

And just part of the generated file would be included. This is useful if you need the same information in multiple formats (instantiation, initialization, getter/setter functions, etc) from the same source TableGen file without having to re-compile the TableGen file multiple times.

Sometimes, multiple macros might be defined before the same include file to output multiple blocks:

#define GET_REGISTER_MATCHER
#define GET_SUBTARGET_FEATURE_NAME
#define GET_MATCHER_IMPLEMENTATION
#include "ARMGenAsmMatcher.inc"

The macros will be undef’d automatically as they’re used, in the include file.

On all LLVM back-ends, the llvm-tblgen binary will be executed on the root TableGen file <Target>.td, which should include all others. This guarantees that all information needed is accessible, and that no duplication is needed in the TableGen files.

CodeEmitter

Purpose: CodeEmitterGen uses the descriptions of instructions and their fields to construct an automated code emitter: a function that, given a MachineInstr, returns the (currently, 32-bit unsigned) value of the instruction.

Output: C++ code, implementing the target’s CodeEmitter class by overriding the virtual functions as <Target>CodeEmitter::function().

Usage: Used to include directly at the end of <Target>MCCodeEmitter.cpp.

RegisterInfo

Purpose: This tablegen backend is responsible for emitting a description of a target register file for a code generator. It uses instances of the Register, RegisterAliases, and RegisterClass classes to gather this information.

Output: C++ code with enums and structures representing the register mappings, properties, masks, etc.

Usage: Both on <Target>BaseRegisterInfo and <Target>MCTargetDesc (headers and source files) with macros defining in which they are for declaration vs. initialization issues.

InstrInfo

Purpose: This tablegen backend is responsible for emitting a description of the target instruction set for the code generator. (what are the differences from CodeEmitter?)

Output: C++ code with enums and structures representing the instruction mappings, properties, masks, etc.

Usage: Both on <Target>BaseInstrInfo and <Target>MCTargetDesc (headers and source files) with macros defining in which they are for declaration vs. initialization issues.

AsmWriter

Purpose: Emits an assembly printer for the current target.

Output: Implementation of <Target>InstPrinter::printInstruction(), among other things.

Usage: Included directly into InstPrinter/<Target>InstPrinter.cpp.

AsmMatcher

Purpose: Emits a target specifier matcher for converting parsed assembly operands in the MCInst structures. It also emits a matcher for custom operand parsing. Extensive documentation is written on the AsmMatcherEmitter.cpp file.

Output: Assembler parsers’ matcher functions, declarations, etc.

Usage: Used in back-ends’ AsmParser/<Target>AsmParser.cpp for building the AsmParser class.

Disassembler

Purpose: Contains disassembler table emitters for various architectures. Extensive documentation is written on the DisassemblerEmitter.cpp file.

Output: Decoding tables, static decoding functions, etc.

Usage: Directly included in Disassembler/<Target>Disassembler.cpp to cater for all default decodings, after all hand-made ones.

PseudoLowering

Purpose: Generate pseudo instruction lowering.

Output: Implements <Target>AsmPrinter::emitPseudoExpansionLowering().

Usage: Included directly into <Target>AsmPrinter.cpp.

CallingConv

Purpose: Responsible for emitting descriptions of the calling conventions supported by this target.

Output: Implement static functions to deal with calling conventions chained by matching styles, returning false on no match.

Usage: Used in ISelLowering and FastIsel as function pointers to implementation returned by a CC selection function.

DAGISel

Purpose: Generate a DAG instruction selector.

Output: Creates huge functions for automating DAG selection.

Usage: Included in <Target>ISelDAGToDAG.cpp inside the target’s implementation of SelectionDAGISel.

DFAPacketizer

Purpose: This class parses the Schedule.td file and produces an API that can be used to reason about whether an instruction can be added to a packet on a VLIW architecture. The class internally generates a deterministic finite automaton (DFA) that models all possible mappings of machine instructions to functional units as instructions are added to a packet.

Output: Scheduling tables for GPU back-ends (Hexagon, AMD).

Usage: Included directly on <Target>InstrInfo.cpp.

FastISel

Purpose: This tablegen backend emits code for use by the “fast” instruction selection algorithm. See the comments at the top of lib/CodeGen/SelectionDAG/FastISel.cpp for background. This file scans through the target’s tablegen instruction-info files and extracts instructions with obvious-looking patterns, and it emits code to look up these instructions by type and operator.

Output: Generates Predicate and FastEmit methods.

Usage: Implements private methods of the targets’ implementation of FastISel class.

Subtarget

Purpose: Generate subtarget enumerations.

Output: Enums, globals, local tables for sub-target information.

Usage: Populates <Target>Subtarget and MCTargetDesc/<Target>MCTargetDesc files (both headers and source).

Intrinsic

Purpose: Generate (target) intrinsic information.

OptParserDefs

Purpose: Print enum values for a class.

SearchableTables

Purpose: Generate custom searchable tables.

Output: Enums, global tables and lookup helper functions.

Usage: This backend allows generating free-form, target-specific tables from TableGen records. The ARM and AArch64 targets use this backend to generate tables of system registers; the AMDGPU target uses it to generate meta-data about complex image and memory buffer instructions.

More documentation is available in include/llvm/TableGen/SearchableTable.td, which also contains the definitions of TableGen classes which must be instantiated in order to define the enums and tables emitted by this backend.

CTags

Purpose: This tablegen backend emits an index of definitions in ctags(1) format. A helper script, utils/TableGen/tdtags, provides an easier-to-use interface; run ‘tdtags -H’ for documentation.

X86EVEX2VEX

Purpose: This X86 specific tablegen backend emits tables that map EVEX encoded instructions to their VEX encoded identical instruction.

Clang BackEnds

ClangAttrClasses

Purpose: Creates Attrs.inc, which contains semantic attribute class declarations for any attribute in Attr.td that has not set ASTNode = 0. This file is included as part of Attr.h.

ClangAttrParserStringSwitches

Purpose: Creates AttrParserStringSwitches.inc, which contains StringSwitch::Case statements for parser-related string switches. Each switch is given its own macro (such as CLANG_ATTR_ARG_CONTEXT_LIST, or CLANG_ATTR_IDENTIFIER_ARG_LIST), which is expected to be defined before including AttrParserStringSwitches.inc, and undefined after.

ClangAttrImpl

Purpose: Creates AttrImpl.inc, which contains semantic attribute class definitions for any attribute in Attr.td that has not set ASTNode = 0. This file is included as part of AttrImpl.cpp.

ClangAttrList

Purpose: Creates AttrList.inc, which is used when a list of semantic attribute identifiers is required. For instance, AttrKinds.h includes this file to generate the list of attr::Kind enumeration values. This list is separated out into multiple categories: attributes, inheritable attributes, and inheritable parameter attributes. This categorization happens automatically based on information in Attr.td and is used to implement the classof functionality required for dyn_cast and similar APIs.

ClangAttrPCHRead

Purpose: Creates AttrPCHRead.inc, which is used to deserialize attributes in the ASTReader::ReadAttributes function.

ClangAttrPCHWrite

Purpose: Creates AttrPCHWrite.inc, which is used to serialize attributes in the ASTWriter::WriteAttributes function.

ClangAttrSpellings

Purpose: Creates AttrSpellings.inc, which is used to implement the __has_attribute feature test macro.

ClangAttrSpellingListIndex

Purpose: Creates AttrSpellingListIndex.inc, which is used to map parsed attribute spellings (including which syntax or scope was used) to an attribute spelling list index. These spelling list index values are internal implementation details exposed via AttributeList::getAttributeSpellingListIndex.

ClangAttrVisitor

Purpose: Creates AttrVisitor.inc, which is used when implementing recursive AST visitors.

ClangAttrTemplateInstantiate

Purpose: Creates AttrTemplateInstantiate.inc, which implements the instantiateTemplateAttribute function, used when instantiating a template that requires an attribute to be cloned.

ClangAttrParsedAttrList

Purpose: Creates AttrParsedAttrList.inc, which is used to generate the AttributeList::Kind parsed attribute enumeration.

ClangAttrParsedAttrImpl

Purpose: Creates AttrParsedAttrImpl.inc, which is used by AttributeList.cpp to implement several functions on the AttributeList class. This functionality is implemented via the AttrInfoMap ParsedAttrInfo array, which contains one element per parsed attribute object.

ClangAttrParsedAttrKinds

Purpose: Creates AttrParsedAttrKinds.inc, which is used to implement the AttributeList::getKind function, mapping a string (and syntax) to a parsed attribute AttributeList::Kind enumeration.

ClangAttrDump

Purpose: Creates AttrDump.inc, which dumps information about an attribute. It is used to implement ASTDumper::dumpAttr.

ClangDiagsDefs

Generate Clang diagnostics definitions.

ClangDiagGroups

Generate Clang diagnostic groups.

ClangDiagsIndexName

Generate Clang diagnostic name index.

ClangCommentNodes

Generate Clang AST comment nodes.

ClangDeclNodes

Generate Clang AST declaration nodes.

ClangStmtNodes

Generate Clang AST statement nodes.

ClangSACheckers

Generate Clang Static Analyzer checkers.

ClangCommentHTMLTags

Generate efficient matchers for HTML tag names that are used in documentation comments.

ClangCommentHTMLTagsProperties

Generate efficient matchers for HTML tag properties.

ClangCommentHTMLNamedCharacterReferences

Generate function to translate named character references to UTF-8 sequences.

ClangCommentCommandInfo

Generate command properties for commands that are used in documentation comments.

ClangCommentCommandList

Generate list of commands that are used in documentation comments.

ArmNeon

Generate arm_neon.h for clang.

ArmNeonSema

Generate ARM NEON sema support for clang.

ArmNeonTest

Generate ARM NEON tests for clang.

AttrDocs

Purpose: Creates AttributeReference.rst from AttrDocs.td, and is used for documenting user-facing attributes.

General BackEnds

JSON

Purpose: Output all the values in every def, as a JSON data structure that can be easily parsed by a variety of languages. Useful for writing custom backends without having to modify TableGen itself, or for performing auxiliary analysis on the same TableGen data passed to a built-in backend.

Output:

The root of the output file is a JSON object (i.e. dictionary), containing the following fixed keys:

• !tablegen_json_version: a numeric version field that will increase if an incompatible change is ever made to the structure of this data. The format described here corresponds to version 1.
• !instanceof: a dictionary whose keys are the class names defined in the TableGen input. For each key, the corresponding value is an array of strings giving the names of def records that derive from that class. So root["!instanceof"]["Instruction"], for example, would list the names of all the records deriving from the class Instruction.

For each def record, the root object also has a key for the record name. The corresponding value is a subsidiary object containing the following fixed keys:

• !superclasses: an array of strings giving the names of all the classes that this record derives from.
• !fields: an array of strings giving the names of all the variables in this record that were defined with the field keyword.
• !name: a string giving the name of the record. This is always identical to the key in the JSON root object corresponding to this record’s dictionary. (If the record is anonymous, the name is arbitrary.)
• !anonymous: a boolean indicating whether the record’s name was specified by the TableGen input (if it is false), or invented by TableGen itself (if true).

For each variable defined in a record, the def object for that record also has a key for the variable name. The corresponding value is a translation into JSON of the variable’s value, using the conventions described below.

Some TableGen data types are translated directly into the corresponding JSON type:

• A completely undefined value (e.g. for a variable declared without initializer in some superclass of this record, and never initialized by the record itself or any other superclass) is emitted as the JSON null value.
• int and bit values are emitted as numbers. Note that TableGen int values are capable of holding integers too large to be exactly representable in IEEE double precision. The integer literal in the JSON output will show the full exact integer value. So if you need to retrieve large integers with full precision, you should use a JSON reader capable of translating such literals back into 64-bit integers without losing precision, such as Python’s standard json module.
• string and code values are emitted as JSON strings.
• list<T> values, for any element type T, are emitted as JSON arrays. Each element of the array is represented in turn using these same conventions.
• bits values are also emitted as arrays. A bits array is ordered from least-significant bit to most-significant. So the element with index i corresponds to the bit described as x{i} in TableGen source. However, note that this means that scripting languages are likely to display the array in the opposite order from the way it appears in the TableGen source or in the diagnostic -print-records output.

All other TableGen value types are emitted as a JSON object, containing two standard fields: kind is a discriminator describing which kind of value the object represents, and printable is a string giving the same representation of the value that would appear in -print-records.

• A reference to a def object has kind=="def", and has an extra field def giving the name of the object referred to.
• A reference to another variable in the same record has kind=="var", and has an extra field var giving the name of the variable referred to.
• A reference to a specific bit of a bits-typed variable in the same record has kind=="varbit", and has two extra fields: var gives the name of the variable referred to, and index gives the index of the bit.
• A value of type dag has kind=="dag", and has two extra fields. operator gives the initial value after the opening parenthesis of the dag initializer; args is an array giving the following arguments. The elements of args are arrays of length 2, giving the value of each argument followed by its colon-suffixed name (if any). For example, in the JSON representation of the dag value (Op 22, "hello":$foo) (assuming that Op is the name of a record defined elsewhere with a def statement):
  • operator will be an object in which kind=="def" and def=="Op"
  • args will be the array [[22, null], ["hello", "foo"]].
• If any other kind of value or complicated expression appears in the output, it will have kind=="complex", and no additional fields. These values are not expected to be needed by backends. The standard printable field can be used to extract a representation of them in TableGen source syntax if necessary.

How to write a back-end

TODO.

Until we get a step-by-step HowTo for writing TableGen backends, you can at least grab the boilerplate (build system, new files, etc.) from Clang’s r173931.

TODO: How they work, how to write one. This section should not contain details about any particular backend, except maybe -print-enums as an example. This should highlight the APIs in TableGen/Record.h.