Archives September 2023

Since parsing and creation of records are done through an LLVM library, you only need to care about the backend implementation, which consists mostly of generating C++ source code fragments based on the information in the records. First, you need to be clear about what source code to generate before you can put it into the backend.
Sketching the source code to be generated
The output of the TableGen tool is a single file containing C++ fragments. The fragments are guarded by macros. The goal is to replace the TokenKinds.def database file. Based on the information in the TableGen file, you can generate the following:

1. The enumeration members used to define flags. The developer is free to name the type; however, it should be based on the unsigned type. If the generated file is named TokenKinds.inc, then the intended use is this:

enum Flags : unsigned {
define GET_TOKEN_FLAGS
include “TokenKinds.inc”
}

2. The TokenKind enumeration, and the prototypes and definitions of the getTokenName(), getPunctuatorSpelling(), and getKeywordSpelling() functions. This code replaces the TokenKinds.def database file, most of the TokenKinds.h include file and the TokenKinds.cpp. source file.
3. A new lookupKeyword() function that can be used instead of the current implementation using the llvm::StringMap. type. This is the function you want to optimize.
   Knowing what you want to generate, you can now turn to implementing the backend.
   Creating a new TableGen tool
   A simple structure for your new tool is to have a driver that evaluates the command-line options and calls the generation functions and the actual generator functions in a different file. Let’s call the driver file TableGen.cpp and the file containing the generator TokenEmitter.cpp. You also need a TableGenBackends.h header file. Let’s begin the implementation with the generation of the C++ code in the TokenEmitter.cpp file:
4. As usual, the file begins with including the required headers. The most important one is llvm/TableGen/Record.h, which defines a Record class, used to hold records generated by parsing the .td file:

include “TableGenBackends.h”
include “llvm/Support/Format.h”
include “llvm/TableGen/Record.h”
include “llvm/TableGen/TableGenBackend.h”
include

2. To simplify coding, the llvm namespace is imported:

using namespace llvm;

3. The TokenAndKeywordFilterEmitter class is responsible for generating the C++ source code. The emitFlagsFragment(), emitTokenKind(), and emitKeywordFilter() methods emit the source code, as described in the previous section, Sketching the source code to be generated. The only public method, run(), calls all the code-emitting methods. The records are held in an instance of RecordKeeper, which is passed as a parameter to the constructor. The class is inside an anonymous namespace:

namespace {
class TokenAndKeywordFilterEmitter {
RecordKeeper &Records;
public:
explicit TokenAndKeywordFilterEmitter(RecordKeeper &R)
: Records(R) {}
void run(raw_ostream &OS);
private:
void emitFlagsFragment(raw_ostream &OS);
void emitTokenKind(raw_ostream &OS);
void emitKeywordFilter(raw_ostream &OS);
};
} // End anonymous namespace