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

Pass and Pass Manager

LLVM's Pass infrastructure is one of the many important features of the LLVM system. There are a number of analysis and optimization passes that can be run using this infrastructure. The starting point for LLVM passes is the Pass class, which is a superclass of all the passes. We need to inherit from some predefined subclasses taking into account what our pass is going to implement.

  • ModulePass: This is the most general superclass. By inheriting this class we allow the entire module to be analyzed at once. The functions within the module may not be referred to in a particular order. To use it, write a subclass that inherits from the ModulePass subclass and overloads the runOnModule function.


    Before going ahead with the discussion of other Pass classes, let's look into the three virtual methods that the Pass classes override:

    • doInitialization: This is meant to do initialization stuff that does not depend on the current function being processed.

    • runOn{Passtype}: This...