Book Image

LLVM Essentials

By : Mayur Pandey, Suyog Sarda, David Farago
Book Image

LLVM Essentials

By: Mayur Pandey, Suyog Sarda, David Farago

Overview of this book

LLVM is currently the point of interest for many firms, and has a very active open source community. It provides us with a compiler infrastructure that can be used to write a compiler for a language. It provides us with a set of reusable libraries that can be used to optimize code, and a target-independent code generator to generate code for different backends. It also provides us with a lot of other utility tools that can be easily integrated into compiler projects. This book details how you can use the LLVM compiler infrastructure libraries effectively, and will enable you to design your own custom compiler with LLVM in a snap. We start with the basics, where you’ll get to know all about LLVM. We then cover how you can use LLVM library calls to emit intermediate representation (IR) of simple and complex high-level language paradigms. Moving on, we show you how to implement optimizations at different levels, write an optimization pass, generate code that is independent of a target, and then map the code generated to a backend. The book also walks you through CLANG, IR to IR transformations, advanced IR block transformations, and target machines. By the end of this book, you’ll be able to easily utilize the LLVM libraries in your own projects.
Table of Contents (14 chapters)
LLVM Essentials
About the Authors
About the Reviewer


LLVM is one of the very hot topics in recent times. It is an open source project with an ever-increasing number of contributors. Every programmer comes across a compiler at some point or the other while programming. Simply speaking, a compiler converts a high-level language to machine-executable code. However, what goes on under the hood is a lot of complex algorithms at work. So, to get started with compiler, LLVM will be the simplest infrastructure to study. Written in object-oriented C++, modular in design, and with concepts that are very easy to map to theory, LLVM proves to be attractive for experienced compiler programmers and for novice students who are willing to learn.

As authors, we maintain that simple solutions frequently work better and are easier to grasp than complex solutions. Throughout the book we will look at various topics that will help you enhance your skills and drive you to learn more.

We also believe that this book will be helpful for people not directly involved in compiler development as knowledge of compiler development will help them write code optimally.

What this book covers

Chapter 1, Playing with LLVM, introduces you to the modular design of LLVM and LLVM Intermediate Representation. In this chapter, we also look into some of the tools that LLVM provides.

Chapter 2, Building LLVM IR, introduces you to some basic function calls provided by the LLVM infrastructure to build LLVM IR. This chapter demonstrates building of modules, functions, basic blocks, condition statements, and loops using LLVM APIs.

Chapter 3, Advanced LLVM IR, introduces you to some advanced IR paradigms. This chapter explains advanced IR to the readers and shows how LLVM function calls can be used to emit them in the IR.

Chapter 4, Basic IR Transformations, deals with basic transformation optimizations at the IR level using the LLVM optimizer tool opt and the LLVM Pass infrastructure. You will learn how to use the information of one pass in another and then look into Instruction Simplification and Instruction Combining Passes.

Chapter 5, Advanced IR Block Transformations, deals with optimizations at block level on IR. We will discuss various optimizations such as Loop Optimizations, Scalar Evolution, Vectorization, and so on, followed by the summary of this chapter.

Chapter 6, IR to Selection DAG phase, takes you on a journey through the abstract infrastructure of a target-independent code generator. We explore how LLVM IR is converted to Selection DAG and various phases thereafter. It also introduces you to instruction selection, scheduling, register allocation, and so on.

Chapter 7, Generating Code for Target Architecture, introduces the readers to the tablegen concept. It shows how target architecture specifications such as register sets, instruction sets, calling conventions, and so on can be represented using tablegen, and how the output of tablegen can be used to emit code for a given architecture. This chapter can be used by readers as a reference for bootstrapping a target machine code generator.

What you need for this book

All you need to work through most of the examples covered in this book is a Linux machine, preferably Ubuntu. You will also need a simple text or code editor, Internet access, and a browser. We recommend installing the meld tool to compare two files; it works well on the Linux platform.

Who this book is for

This book is intended for those who already know some of the concepts concerning compilers and want to quickly become familiar with LLVM's infrastructure and the rich set of libraries that it provides. Compiler programmers, who are familiar with concepts of compilers and want to indulge in understanding, exploring, and using the LLVM infrastructure in a meaningful way in their work, will find this book useful.

This book is also for programmers who are not directly involved in compiler projects but are often involved in development phases where they write thousands of lines of code. With knowledge of how compilers work, they will be able to code in an optimal way and improve performance with clean code.


In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "The LLVM Pass Manager uses the explicitly mentioned dependency information."

A block of code is set as follows:

int add(int a) {
return globvar + a;

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

  Value *StartVal = Builder.getInt32(1);
  Value *Res = createLoop(Builder, List, VL, StartVal, Arg2);


Any command-line input or output is written as follows:

$ clang -emit-llvm -c -S add.c
$ cat add.ll

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Clicking the Next button moves you to the next screen."


Warnings or important notes appear in a box like this.


Tips and tricks appear like this.

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