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

Opt Tool

Opt is the LLVM Optimizer and analyzer tool that is run on LLVM IR to optimize the IR or produce an analysis about it. We saw in the first chapter a very basic introduction to the opt tool, and how to use it to run analysis and transformation passes. In this section, we will see what else the opt tool does. We must note that opt is a developer tool and all the optimizations that it provides can be invoked from the frontend as well.

With the opt tool, we can specify the level of optimization that we need, which means we can specify the optimization levels from O0, O1, O2, to O3(O0 being the least optimized code and O3 being the most optimized code). Apart from these, there is also an optimization level Os or Oz, which deals with space optimization. The syntax to invoke one of these optimizations is:

$ opt -Ox -S input.ll

Here, x represents the optimization level, which can have a value from 0 to 3 or s or z. These optimization levels are similar to what Clang frontend specifies....