TableGen is a widely used domain-specific language (DSL) within the LLVM compiler infrastructure. It offers a simple and organized method for specifying advanced data structures including but not limited to instruction sets, registers, and code generation and optimization patterns. The .td files are the most significant part of this language since they enable developers to write less or no boilerplate code, thus simplifying and cleansing the code base.

In this OpenGenus article, we cover some of the basic concepts of .td files and the functionalities and components of TableGen and its importance in the LLVM project. We will also review several use cases and benefits of using TableGen for code generation and optimization.

Table of Contents

1. What is LLVM?
2. What is TableGen?
3. Stracture of a .td File
4. Getting Started with TableGen
5. Use Cases of TableGen
6. Advantages of Using TableGen
7. TableGen to Source Code: The Workflow
8. Test Your Knowledge
9. Key Takeaways

What is LLVM?

Low-Level Virtual Machine (LLVM) is a cross-platform open-source compiler infrastructure that offers capabilities for composing different kinds of compiler tools and technology. It contains a set of components such as code optimization, code analysis, code generation, etc. which can be put to use in combination to create custom compiler backends. Following are the basic components of LLVM-

Intermediate Representation (IR): A simple programming language employed by the LLVM which aims to make the representation of code architecture-neutral.

LLVM Backend: This is the component that is responsible for the integration of the Machine Code from the IR and the specific target architecture defining the codes using components like TableGen to define instructions, registers and code patterns.

Code Generation: The final phase where LLVM converts IR into target-specific machine code, using the model specified in .td files.

These fundamental principles assist in placing TableGen and .td files within the context of the purpose for the provision of backend in LLVM.

What is TableGen?

TableGen is a language that helps define different components of a compiler’s backend such as instruction set architecture, calling conventions, or hardware characteristics. It has a simple and readable syntax and can produce a substantial amount of the C++ code that is a part of the LLVM backend. TableGen is mainly used for the purposes of specifying compiler dependent information, register assignment, and instruction bit patterns.

The main advantage of TableGen is that it focuses on eliminating repetitive writing by allowing a one-time definition of intricate relationships among data and then generates all the necessary code from the explanation.

Stracture of a .td File

A .td file comprises numerous TableGen definitions pertaining to different data structures. The elements that form the essence of a .td file are:

Classes: These provide the structure for designing records and holds different variables.

Records: These are the specific instances of classes, which can be any objects like instructions or registers.

Multiclasses: These are constructs used to create and fill in numerous records in a single go.

Fields: These are the components, or characteristics, of classes and records.

In most cases, .td files contain class declarations and definitions at the beginning, then instances of the classes which are records, and finally the application of multiclasses. The file extension .td shows that it contains a code written using TableGen.

Getting Started with TableGen

Here is an example code is given that explains the structure and elements of a .td file with classes, records and multiclasses.

// Define a base class for instructions
class Instruction {
  string Name;       // Instruction name
  bit IsPseudo = 0;  // Flag to indicate if it’s a pseudo-instruction
}

// Create a multiclass to define multiple instructions
multiclass ALUInstr<Instruction OpCode, int OpID> {
  def : OpCode {     // Create a record for each instruction
    Name = OpCode.Name;
    let OpcodeID = OpID;
  }
}

// Define specific instructions using records
def ADD : Instruction { Name = "ADD"; }
def SUB : Instruction { Name = "SUB"; }

// Instantiate the multiclass to define instructions with unique IDs

ALUInstr<ADD, 1>;
ALUInstr<SUB, 2>;

This example demonstrates the definition of a simple instruction set using classes, records and a multiclass to facilitate the process of instruction generation and code generation. For additional information, please see the complete LLVM TableGen Programmer’s Reference.

Use Cases of TableGen

Let’s start with what TableGen is used for in the context of LLVM backends:

Describing the Instruction Set
Instruction set description is one of the purposes of TableGen and is targeted to many architectures. This means specifying the operand, name and formats of opcodes.

Describing Registers and Operands
Registers, classes of registers, their constraints and limitations, are usually included in the designs made with TableGen. This is to make sure that the allocation of registers in the back-end compiler will be according to the defined limits.

Declaring Optimisation via Pattern Matching
In this case, TableGen allows the notation of so-called instruction selection and optimization patterns. Such patterns may also address transformations that allow implementation of complex instructions with a set of simpler or more efficient instructions.

Advantages of Using TableGen

Lessens Code Repetition: A Description and data definition as being composed once is reflected in the code that is generated devoid of inconsistencies and errors.

Structured and Modular Approach: TableGen files are easy to read and modify. Changes in .td files automatically propagate to the generated source code.

Offers Ease with Maintaining: Placing all relevant descriptions within .td files means there is less code to maintain and updates are much easier to do.

Generates Code Consistently: TableGen guarantees that code is generated in a consistent form therefore eliminating chances of human error.

TableGen to Source Code: The Workflow

The llvm-tblgen utility processes the TableGen files (.td) and produces C++ source files. These source files are subsequently built along with the other components of the LLVM project. The overall undertaking consists of several orderly stages, including:

1. Create the TableGen declarations within a .td file.
2. Execute llvm-tblgen to create the required Header/Source files.
3. Build the generated files with the LLVM source code.

Test Your Knowledge