LLVM Code Generation

“If you're working in or interested in LLVM and code generation, this book should definitely be on your list. Highly recommended!” -Yacob (Kobi) Cohen Arazi, Software Engineer at NVIDIA.

“If you're passionate about compilers and eager to stay ahead in this dynamic field, I highly recommend giving this book a read.” -Arya Mazaheri, Co-founder of PanocularAI

“I think that this book will lower the access barrier to LLVM significantly and very quickly be on the desks of compiler engineers and students.” -Sebastian Hac, Professor (Compilers) at Saarland University

Key Features

• Understand the steps involved in generating assembly code from LLVM IR
• Learn the key constructs needed to leverage LLVM for your hardware or backend
• Strengthen your understanding with targeted exercises and practical examples in every chapter

Book Description

The LLVM infrastructure is a popular compiler ecosystem widely used in the tech industry and academia. This technology is crucial for both experienced and aspiring compiler developers looking to make an impact in the field. Written by Quentin Colombet, a veteran LLVM contributor and architect of the GlobalISel framework, this book provides a primer on the main aspects of LLVM, with an emphasis on its backend infrastructure; that is, everything needed to transform the intermediate representation (IR) produced by frontends like Clang into assembly code and object files.

You’ll learn how to write an optimizing code generator for a toy backend in LLVM. The chapters will guide you step by step through building this backend while exploring key concepts, such as the ABI, cost model, and register allocation. You’ll also find out how to express these concepts using LLVM's existing infrastructure and how established backends address these challenges.

By the end of this book, you’ll have gained a deeper understanding of LLVM. The concepts presented are expected to remain stable across different LLVM versions, making this book a reliable quick reference guide for understanding LLVM.

What you will learn

• Understand essential compiler concepts, such as SSA, dominance, and ABI
• Build and extend LLVM backends for creating custom compiler features
• Optimize code by manipulating LLVM's Intermediate Representation
• Contribute effectively to LLVM open-source projects and development
• Develop debugging skills for LLVM optimizations and passes

Who this book is for

This book is for both beginners to LLVM and experienced LLVM developers. If you’re new to LLVM, it offers a clear, approachable guide to compiler backends, starting with foundational concepts. For seasoned LLVM developers, it dives into less-documented areas such as TableGen, MachineIR, and MC, enabling you to solve complex problems and expand your expertise. Whether you’re starting out or looking to deepen your knowledge, this book has something for you.

Table of Contents

1. Building LLVM and Understanding the Directory Structure
2. Contributing to LLVM
3. Compiler Basics and How They Map to LLVM APIs
4. Writing Your First Optimization
5. Dealing with Pass Managers
6. TableGen – LLVM Swiss Army Knife for Modeling
7. Understanding LLVM IR
8. Survey of the Existing Passes
9. Introducing Target-Specific Constructs
10. Hands-On Debugging LLVM IR Passes
11. Legacy Instruction Selection Framework - SelectionDAG
12. Getting Started with the Machine Code Layer
13. The Machine Pass Pipeline
14. Getting Started with Instruction Selection
15. Instruction Selection: The IR Building Phase
16. Instruction Selection: The Legalization Phase

Quentin Colombet is a veteran LLVM contributor specializing in compiler backends. He is the architect of the new instruction selection framework (GlobalISel) and code owner of the LLVM register allocators. With over two decades of experience, he has worked on compiler backends for a variety of architectures, including GPU, CPU, microcontrollers, DSP, and ASICs. Quentin joined Apple in 2012 and has contributed to x86, Aarch64, and Apple GPU backends. He is passionate about helping newcomers onboard the LLVM infrastructure, having mentored interns and new hires over the years.