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

Using other Pass info in current Pass

For the Pass Manager to work optimally it needs to know the dependencies between the Passes. Each of the passes can itself declare its dependencies: the analysis passes that need to be executed before this pass is executed and the passes that will get invalidated after the current pass is run. To specify these dependencies, a pass needs to implement the getAnalysisUsage method.

virtual void getAnalysisUsage(AnalysisUsage &Info) const;

Using this method the current pass can specify the required and invalidated sets by filling in the details in the AnalysisUsage object. To fill in the information the Pass needs to call any of the following methods:

AnalysisUsage::addRequired<> method

This method arranges for the execution of a Pass prior to the current Pass. One example of this is: for memory copy optimization it needs the results of an alias analysis:

void getAnalysisUsage(AnalysisUsage &AU) const override {