TableGen BackEnds — LLVM 16.0.6 documentation

# TableGen BackEnds

## Introduction

TableGen backends are at the core of TableGen’s functionality. The source files provide the classes and records that are parsed and end up as a collection of record instances, but it’s up to the backend to interpret and print the records in a way that is meaningful to the user (normally a C++ include file or a textual list of warnings, options, and error messages).

TableGen is used by both LLVM, Clang, and MLIR 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 makes it is easy for preprocessor tricks to be used. 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.

MLIR uses TableGen to define operations, operation dialects, and operation traits.

See the TableGen Programmer’s Reference for an in-depth description of TableGen, and the TableGen Backend Developer’s Guide for a guide to writing a new backend.

## LLVM BackEnds

Warning

This portion is incomplete. Each section below needs three subsections: 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.

See SearchableTables Reference for a detailed description.

### 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

### Print Records

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. See the TableGen Backend Developer’s Guide for more information.

### Print Detailed Records

The TableGen command option `--print-detailed-records` invokes a backend that prints all the global variables, classes, and records defined in the source files, with more detail than the default record printer. See the TableGen Backend Developer’s Guide for more information.

### JSON Reference

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.

### SearchableTables Reference

A TableGen include file, `SearchableTable.td`, provides classes for generating C++ searchable tables. These tables are described in the following sections. To generate the C++ code, run `llvm-tblgen` with the `--gen-searchable-tables` option, which invokes the backend that generates the tables from the records you provide.

Each of the data structures generated for searchable tables is guarded by an `#ifdef`. This allows you to include the generated `.inc` file and select only certain data structures for inclusion. The examples below show the macro names used in these guards.

#### Generic Enumerated Types

The `GenericEnum` class makes it easy to define a C++ enumerated type and the enumerated elements of that type. To define the type, define a record whose parent class is `GenericEnum` and whose name is the desired enum type. This class provides three fields, which you can set in the record using the `let` statement.

- `string FilterClass`. The enum type will have one element for each record that derives from this class. These records are collected to assemble the complete set of elements.

- `string NameField`. The name of a field in the collected records that specifies the name of the element. If a record has no such field, the record’s name will be used.

- `string ValueField`. The name of a field in the collected records that specifies the numerical value of the element. If a record has no such field, it will be assigned an integer value. Values are assigned in alphabetical order starting with 0.

Here is an example where the values of the elements are specified explicitly, as a template argument to the `BEntry` class. The resulting C++ code is shown.

    def BValues : GenericEnum {
      let FilterClass = "BEntry";
      let NameField = "Name";
      let ValueField = "Encoding";
    }

    class BEntry<bits<16> enc> {
      string Name = NAME;
      bits<16> Encoding = enc;
    }

    def BFoo   : BEntry<0xac>;
    def BBar   : BEntry<0x14>;
    def BZoo   : BEntry<0x80>;
    def BSnork : BEntry<0x4c>;

    #ifdef GET_BValues_DECL
    enum BValues {
      BBar = 20,
      BFoo = 172,
      BSnork = 76,
      BZoo = 128,
    };
    #endif

In the following example, the values of the elements are assigned automatically. Note that values are assigned from 0, in alphabetical order by element name.

    def CEnum : GenericEnum {
      let FilterClass = "CEnum";
    }

    class CEnum;

    def CFoo : CEnum;
    def CBar : CEnum;
    def CBaz : CEnum;

    #ifdef GET_CEnum_DECL
    enum CEnum {
      CBar = 0,
      CBaz = 1,
      CFoo = 2,
    };
    #endif

#### Generic Tables

The `GenericTable` class is used to define a searchable generic table. TableGen produces C++ code to define the table entries and also produces the declaration and definition of a function to search the table based on a primary key. To define the table, define a record whose parent class is `GenericTable` and whose name is the name of the global table of entries. This class provides six fields.

- `string FilterClass`. The table will have one entry for each record that derives from this class.

- `string CppTypeName`. The name of the C++ struct/class type of the table that holds the entries. If unspecified, the `FilterClass` name is used.

- `list<string> Fields`. A list of the names of the fields in the collected records that contain the data for the table entries. The order of this list determines the order of the values in the C++ initializers. See below for information about the types of these fields.

- `list<string> PrimaryKey`. The list of fields that make up the primary key.

- `string PrimaryKeyName`. The name of the generated C++ function that performs a lookup on the primary key.

- `bit PrimaryKeyEarlyOut`. See the third example below.

TableGen attempts to deduce the type of each of the table fields so that it can format the C++ initializers in the emitted table. It can deduce `bit`, `bits<n>`, `string`, `Intrinsic`, and `Instruction`. These can be used in the primary key. Any other field types must be specified explicitly; this is done as shown in the second example below. Such fields cannot be used in the primary key.

One special case of the field type has to do with code. Arbitrary code is represented by a string, but has to be emitted as a C++ initializer without quotes. If the code field was defined using a code literal (`[{...}]`), then TableGen will know to emit it without quotes. However, if it was defined using a string literal or complex string expression, then TableGen will not know. In this case, you can force TableGen to treat the field as code by including the following line in the `GenericTable` record, where xxx is the code field name.

    string TypeOf_xxx = "code";

Here is an example where TableGen can deduce the field types. Note that the table entry records are anonymous; the names of entry records are irrelevant.

    def ATable : GenericTable {
      let FilterClass = "AEntry";
      let Fields = ["Str", "Val1", "Val2"];
      let PrimaryKey = ["Val1", "Val2"];
      let PrimaryKeyName = "lookupATableByValues";
    }

    class AEntry<string str, int val1, int val2> {
      string Str = str;
      bits<8> Val1 = val1;
      bits<10> Val2 = val2;
    }

    def : AEntry<"Bob",   5, 3>;
    def : AEntry<"Carol", 2, 6>;
    def : AEntry<"Ted",   4, 4>;
    def : AEntry<"Alice", 4, 5>;
    def : AEntry<"Costa", 2, 1>;

Here is the generated C++ code. The declaration of `lookupATableByValues` is guarded by `GET_ATable_DECL`, while the definitions are guarded by `GET_ATable_IMPL`.

    #ifdef GET_ATable_DECL
    const AEntry *lookupATableByValues(uint8_t Val1, uint16_t Val2);
    #endif

    #ifdef GET_ATable_IMPL
    constexpr AEntry ATable[] = {
      { "Costa", 0x2, 0x1 }, // 0
      { "Carol", 0x2, 0x6 }, // 1
      { "Ted", 0x4, 0x4 }, // 2
      { "Alice", 0x4, 0x5 }, // 3
      { "Bob", 0x5, 0x3 }, // 4
    };

    const AEntry *lookupATableByValues(uint8_t Val1, uint16_t Val2) {
      struct KeyType {
        uint8_t Val1;
        uint16_t Val2;
      };
      KeyType Key = { Val1, Val2 };
      auto Table = ArrayRef(ATable);
      auto Idx = std::lower_bound(Table.begin(), Table.end(), Key,
        [](const AEntry &LHS, const KeyType &RHS) {
          if (LHS.Val1 < RHS.Val1)
            return true;
          if (LHS.Val1 > RHS.Val1)
            return false;
          if (LHS.Val2 < RHS.Val2)
            return true;
          if (LHS.Val2 > RHS.Val2)
            return false;
          return false;
        });

      if (Idx == Table.end() ||
          Key.Val1 != Idx->Val1 ||
          Key.Val2 != Idx->Val2)
        return nullptr;
      return &*Idx;
    }
    #endif

The table entries in `ATable` are sorted in order by `Val1`, and within each of those values, by `Val2`. This allows a binary search of the table, which is performed in the lookup function by `std::lower_bound`. The lookup function returns a reference to the found table entry, or the null pointer if no entry is found.

This example includes a field whose type TableGen cannot deduce. The `Kind` field uses the enumerated type `CEnum` defined above. To inform TableGen of the type, the record derived from `GenericTable` must include a string field named `TypeOf_`field, where field is the name of the field whose type is required.

    def CTable : GenericTable {
      let FilterClass = "CEntry";
      let Fields = ["Name", "Kind", "Encoding"];
      string TypeOf_Kind = "CEnum";
      let PrimaryKey = ["Encoding"];
      let PrimaryKeyName = "lookupCEntryByEncoding";
    }

    class CEntry<string name, CEnum kind, int enc> {
      string Name = name;
      CEnum Kind = kind;
      bits<16> Encoding = enc;
    }

    def : CEntry<"Apple", CFoo, 10>;
    def : CEntry<"Pear",  CBaz, 15>;
    def : CEntry<"Apple", CBar, 13>;

Here is the generated C++ code.

    #ifdef GET_CTable_DECL
    const CEntry *lookupCEntryByEncoding(uint16_t Encoding);
    #endif

    #ifdef GET_CTable_IMPL
    constexpr CEntry CTable[] = {
      { "Apple", CFoo, 0xA }, // 0
      { "Apple", CBar, 0xD }, // 1
      { "Pear", CBaz, 0xF }, // 2
    };

    const CEntry *lookupCEntryByEncoding(uint16_t Encoding) {
      struct KeyType {
        uint16_t Encoding;
      };
      KeyType Key = { Encoding };
      auto Table = ArrayRef(CTable);
      auto Idx = std::lower_bound(Table.begin(), Table.end(), Key,
        [](const CEntry &LHS, const KeyType &RHS) {
          if (LHS.Encoding < RHS.Encoding)
            return true;
          if (LHS.Encoding > RHS.Encoding)
            return false;
          return false;
        });

      if (Idx == Table.end() ||
          Key.Encoding != Idx->Encoding)
        return nullptr;
      return &*Idx;
    }

The `PrimaryKeyEarlyOut` field, when set to 1, modifies the lookup function so that it tests the first field of the primary key to determine whether it is within the range of the collected records’ primary keys. If not, the function returns the null pointer without performing the binary search. This is useful for tables that provide data for only some of the elements of a larger enum-based space. The first field of the primary key must be an integral type; it cannot be a string.

Adding `let PrimaryKeyEarlyOut = 1` to the `ATable` above:

    def ATable : GenericTable {
      let FilterClass = "AEntry";
      let Fields = ["Str", "Val1", "Val2"];
      let PrimaryKey = ["Val1", "Val2"];
      let PrimaryKeyName = "lookupATableByValues";
      let PrimaryKeyEarlyOut = 1;
    }

causes the lookup function to change as follows:

    const AEntry *lookupATableByValues(uint8_t Val1, uint16_t Val2) {
      if ((Val1 < 0x2) ||
          (Val1 > 0x5))
        return nullptr;

      struct KeyType {
      ...

#### Search Indexes

The `SearchIndex` class is used to define additional lookup functions for generic tables. To define an additional function, define a record whose parent class is `SearchIndex` and whose name is the name of the desired lookup function. This class provides three fields.

- `GenericTable Table`. The name of the table that is to receive another lookup function.

- `list<string> Key`. The list of fields that make up the secondary key.

- `bit EarlyOut`. See the third example in Generic Tables.

Here is an example of a secondary key added to the `CTable` above. The generated function looks up entries based on the `Name` and `Kind` fields.

    def lookupCEntry : SearchIndex {
      let Table = CTable;
      let Key = ["Name", "Kind"];
    }

This use of `SearchIndex` generates the following additional C++ code.

    const CEntry *lookupCEntry(StringRef Name, unsigned Kind);

    ...

    const CEntry *lookupCEntryByName(StringRef Name, unsigned Kind) {
      struct IndexType {
        const char * Name;
        unsigned Kind;
        unsigned _index;
      };
      static const struct IndexType Index[] = {
        { "APPLE", CBar, 1 },
        { "APPLE", CFoo, 0 },
        { "PEAR", CBaz, 2 },
      };

      struct KeyType {
        std::string Name;
        unsigned Kind;
      };
      KeyType Key = { Name.upper(), Kind };
      auto Table = ArrayRef(Index);
      auto Idx = std::lower_bound(Table.begin(), Table.end(), Key,
        [](const IndexType &LHS, const KeyType &RHS) {
          int CmpName = StringRef(LHS.Name).compare(RHS.Name);
          if (CmpName < 0) return true;
          if (CmpName > 0) return false;
          if ((unsigned)LHS.Kind < (unsigned)RHS.Kind)
            return true;
          if ((unsigned)LHS.Kind > (unsigned)RHS.Kind)
            return false;
          return false;
        });

      if (Idx == Table.end() ||
          Key.Name != Idx->Name ||
          Key.Kind != Idx->Kind)
        return nullptr;
      return &CTable[Idx->_index];
    }