You're reading from LLVM Code Generation A deep dive into compiler backend development

Product type Paperback

Published in May 2025

Publisher Packt

ISBN-13 9781837637782

Length 620 pages

Edition 1st Edition

Languages

C++

Tools

LLVM

Concepts

Programming Language

Author (1):

Quentin Colombet

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Table of Contents (30) Chapters

Preface

1. Part 1: Getting Started with LLVM

2. Building LLVM and Understanding the Directory Structure FREE CHAPTER

3. Contributing to LLVM

4. Compiler Basics and How They Map to LLVM APIs

5. Writing Your First Optimization

6. Dealing with Pass Managers

7. TableGen – LLVM Swiss Army Knife for Modeling

8. Part 2: Middle-End: LLVM IR to LLVM IR

9. Understanding LLVM IR

10. Survey of the Existing Passes

11. Introducing Target-Specific Constructs

12. Hands-On Debugging LLVM IR Passes

13. Part 3: Introduction to the Backend

14. Getting Started with the Backend

15. Getting Started with the Machine Code Layer

16. The Machine Pass Pipeline

17. Part 4: LLVM IR to Machine IR

18. Getting Started with Instruction Selection

19. Instruction Selection: The IR Building Phase

20. Instruction Selection: The Legalization Phase

21. Instruction Selection: The Selection Phase and Beyond

22. Part 5: Final Lowering and Optimizations

23. Instruction Scheduling

24. Register Allocation

25. Lowering of the Stack Layout

26. Getting Started with the Assembler

27. Unlock Your Book’s Exclusive Benefits

28. Other Books You May Enjoy

29. Index

Optimizations

In GlobalISel and SDISel, it is possible to inject your own optimizations within the instruction selection pipeline. In this section, you will learn how to use these infrastructures to insert your own optimizations.

Let us start with SDISel.

Using the DAGCombiner framework

Since SDISel is a single MachineFunctionPass instance, the optimization opportunities are limited to what SDISel exposes as hooks.

Essentially, the only thing that you can do in SDISel is some pattern rewriting using the DAGCombiner infrastructure. The DAGCombiner infrastructure allows you to run custom pattern rewrites at four predetermined points of the SDISel pipeline, identified by the following CombineLevel enumeration values:

BeforeLegalizeTypes: Before legalization
AfterLegalizeTypes: After the legalization of scalar types but before the legalization of vector types
AfterLegalizeVectorOps: After the legalization of vector types but before the legalization...