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- LLVM BackEnds
- Clang BackEnds
- How to write a back-end
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.
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 TbleGen files.
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>CodeEmitter.cpp, and with option -mc-emitter to be included in <Target>MCCodeEmitter.cpp.
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.
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 register 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.
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.
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.
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.
Purpose: Generate pseudo instruction lowering.
Output: Implements ARMAsmPrinter::emitPseudoExpansionLowering().
Usage: Included directly into <Target>AsmPrinter.cpp.
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 sellection function.
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.
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.
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.
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).
Purpose: Generate (target) intrinsic information.
Purpose: Print enum values for a class.
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.
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.
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.
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.
Purpose: Creates AttrPCHRead.inc, which is used to deserialize attributes in the ASTReader::ReadAttributes function.
Purpose: Creates AttrPCHWrite.inc, which is used to serialize attributes in the ASTWriter::WriteAttributes function.
Purpose: Creates AttrSpellings.inc, which is used to implement the __has_attribute feature test macro.
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.
Purpose: Creates AttrVisitor.inc, which is used when implementing recursive AST visitors.
Purpose: Creates AttrTemplateInstantiate.inc, which implements the instantiateTemplateAttribute function, used when instantiating a template that requires an attribute to be cloned.
Purpose: Creates AttrParsedAttrList.inc, which is used to generate the AttributeList::Kind parsed attribute enumeration.
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.
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.
Purpose: Creates AttrDump.inc, which dumps information about an attribute. It is used to implement ASTDumper::dumpAttr.
Generate Clang diagnostics definitions.
Generate Clang diagnostic groups.
Generate Clang diagnostic name index.
Generate Clang AST comment nodes.
Generate Clang AST declaration nodes.
Generate Clang AST statement nodes.
Generate Clang Static Analyzer checkers.
Generate function to translate named character references to UTF-8 sequences.
Generate command properties for commands that are used in documentation comments.
Generate list of commands that are used in documentation comments.
Generate arm_neon.h for clang.
Generate ARM NEON sema support for clang.
Generate ARM NEON tests for clang.
Purpose: Creates AttributeReference.rst from AttrDocs.td, and is used for documenting user-facing attributes.
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.