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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
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.
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Seitenzahl: 166
Veröffentlichungsjahr: 2015
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Table of Contents
LLVM Essentials
LLVM Essentials
Copyright © 2015 Packt Publishing
All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.
Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the authors, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.
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First published: December 2015
Production reference: 1021215
Published by Packt Publishing Ltd.
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ISBN 978-1-78528-080-1
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Credits
Authors
Suyog Sarda
Mayur Pandey
Reviewer
Renato Golin
Commissioning Editor
Nadeem Bagban
Acquisition Editor
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Content Development Editor
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Technical Editor
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Cover Work
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About the Authors
Suyog Sarda is a professional software engineer and an open source enthusiast. He focuses on compiler development and compiler tools. He is an active contributor to the LLVM open source community. Suyog was also involved in code performance improvements for the ARM and X86 architectures. He has been a part of the compiler team for the Tizen project. His interest in compiler development lies more in code optimization and vectorization.
Previously, he has authored a book on LLVM, titled LLVM Cookbook by Packt Publishing.
Apart from compilers, Suyog is also interested in Linux Kernel Development. He has published a technical paper titled Secure Co-resident Virtualization in Multicore Systems by VM Pinning and Page Coloring at the IEEE Proceedings of the 2012 International Conference on Cloud Computing, Technologies, Applications, and Management at the Birla Institute of Technology, Dubai. He has earned a bachelor's degree in computer technology from the College of Engineering, Pune, India.
I would like to thank my family and friends for encouraging me to write this book. I am thankful to my co-author and reviewers who did a tremendous job of refining the contents. I would also like to thank the LLVM open source community for always being helpful. It has been a great experience to interact with the community. It is amazing to see how fast LLVM has evolved.
Mayur Pandey is a professional software engineer and open source enthusiast focused on compiler development and tools. He is an active contributor to the LLVM open source community. He has been a part of the compiler team for the Tizen project and has hands-on experience of other proprietary compilers.
He has earned a bachelor's degree in Information Technology from Motilal Nehru National Institute of Technology, Allahabad, India. Currently, he lives in Bengaluru, India.
I would like to thank my family and friends who made it possible for me to complete the book by taking care of my other commitments, and who have always being encouraging.
About the Reviewer
Renato Golin has worked with toolchains since 2008, developing debuggers and compilers for multiple languages and platforms, and has also been LLVM Tech Lead at ARM and Linaro, focusing on code generation, correctness, performance, and providing a complete toolchain solution based on LLVM for the diverse ARM platforms.
Before that, he spent a decade moving between web back-ends, databases, distributed systems, big data and bioinformatics, always working on and with open source projects.
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Preface
LLVM is one of the very hot topics in recent times. It is an open source project with an ever-increasing number of contributors. Every programmer comes across a compiler at some point or the other while programming. Simply speaking, a compiler converts a high-level language to machine-executable code. However, what goes on under the hood is a lot of complex algorithms at work. So, to get started with compiler, LLVM will be the simplest infrastructure to study. Written in object-oriented C++, modular in design, and with concepts that are very easy to map to theory, LLVM proves to be attractive for experienced compiler programmers and for novice students who are willing to learn.
As authors, we maintain that simple solutions frequently work better and are easier to grasp than complex solutions. Throughout the book we will look at various topics that will help you enhance your skills and drive you to learn more.
We also believe that this book will be helpful for people not directly involved in compiler development as knowledge of compiler development will help them write code optimally.
What this book covers
Chapter 1, Playing with LLVM, introduces you to the modular design of LLVM and LLVM Intermediate Representation. In this chapter, we also look into some of the tools that LLVM provides.
Chapter 2, Building LLVM IR, introduces you to some basic function calls provided by the LLVM infrastructure to build LLVM IR. This chapter demonstrates building of modules, functions, basic blocks, condition statements, and loops using LLVM APIs.
Chapter 3, Advanced LLVM IR, introduces you to some advanced IR paradigms. This chapter explains advanced IR to the readers and shows how LLVM function calls can be used to emit them in the IR.
Chapter 4, Basic IR Transformations, deals with basic transformation optimizations at the IR level using the LLVM optimizer tool opt and the LLVM Pass infrastructure. You will learn how to use the information of one pass in another and then look into Instruction Simplification and Instruction Combining Passes.
Chapter 5, Advanced IR Block Transformations, deals with optimizations at block level on IR. We will discuss various optimizations such as Loop Optimizations, Scalar Evolution, Vectorization, and so on, followed by the summary of this chapter.
Chapter 6, IR to Selection DAG phase, takes you on a journey through the abstract infrastructure of a target-independent code generator. We explore how LLVM IR is converted to Selection DAG and various phases thereafter. It also introduces you to instruction selection, scheduling, register allocation, and so on.
Chapter 7, Generating Code for Target Architecture, introduces the readers to the tablegen concept. It shows how target architecture specifications such as register sets, instruction sets, calling conventions, and so on can be represented using tablegen, and how the output of tablegen can be used to emit code for a given architecture. This chapter can be used by readers as a reference for bootstrapping a target machine code generator.
What you need for this book
All you need to work through most of the examples covered in this book is a Linux machine, preferably Ubuntu. You will also need a simple text or code editor, Internet access, and a browser. We recommend installing the meld tool to compare two files; it works well on the Linux platform.
Who this book is for
This book is intended for those who already know some of the concepts concerning compilers and want to quickly become familiar with LLVM's infrastructure and the rich set of libraries that it provides. Compiler programmers, who are familiar with concepts of compilers and want to indulge in understanding, exploring, and using the LLVM infrastructure in a meaningful way in their work, will find this book useful.
This book is also for programmers who are not directly involved in compiler projects but are often involved in development phases where they write thousands of lines of code. With knowledge of how compilers work, they will be able to code in an optimal way and improve performance with clean code.
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Chapter 1. Playing with LLVM
The LLVM Compiler infrastructure project, started in 2000 in University of Illinois, was originally a research project to provide modern, SSA based compilation technique for arbitrary static and dynamic programming languages. Now it has grown to be an umbrella project with many sub projects within it, providing a set of reusable libraries having well defined interfaces.
LLVM is implemented in C++ and the main crux of it is the LLVM core libraries it provides. These libraries provide us with opt tool, the target independent optimizer, and code generation support for various target architectures. There are other tools which make use of core libraries, but our main focus in the book will be related to the three mentioned above. These are built around LLVM Intermediate Representation (LLVM IR), which can almost map all the high-level languages. So basically, to use LLVM's optimizer and code generation technique for code written in a certain programming language, all we need to do is write a frontend for a language that takes the high level language and generates LLVM IR. There are already many frontends available for languages such as C, C++, Go, Python, and so on. We will cover the following topics in this chapter:
Modular design and collection of libraries
The most important thing about LLVM is that it is designed as a collection of libraries. Let's understand these by taking the example of LLVM optimizer opt. There are many different optimization passes that the optimizer can run. Each of these passes is written as a C++ class derived from the Pass class of LLVM. Each of the written passes can be compiled into a .o file and subsequently they are archived into a .a library. This library will contain all the passes for opt tool. All the passes in this library are loosely coupled, that is they mention explicitly the dependencies on other passes.
When the optimizer is ran, the LLVM PassManager uses the explicitly mentioned dependency information and runs the passes in optimal way. The library based design allows the implementer to choose the order in which passes will execute and also choose which passes are to be executed based on the requirements. Only the passes that are required are linked to the final application, not the entire optimizer.
The following figure demonstrates how each pass can be linked to a specific object file within a specific library. In the following figure, the PassA references LLVMPasses.a for PassA.o, whereas the custom pass refers to a different library MyPasses.a for the MyPass.o object file.
The code generator also makes use of this modular design like the Optimizer, for splitting the code generation into individual passes, namely, instruction selection, register allocation, scheduling, code layout optimization, and assembly emission.
In each of the following phases mentioned there are some common things for almost every target, such as an algorithm for assigning physical registers available to virtual registers even though the set of registers for different targets vary. So, the compiler writer can modify each of the passes mentioned above and create custom target-specific passes. The use of the tablegen tool helps in achieving this using table description .td files for specific architectures. We will discuss how this happens later in the book.
Another capability that arises out of this is the ability to easily pinpoint a bug to a particular pass in the optimizer. A tool name Bugpointmakes use of this capability to automatically reduce the test case and pinpoint the pass that is causing the bug.
Getting familiar with LLVM IR
LLVM Intermediate Representation (IR) is the heart of the LLVM project. In general every compiler produces an intermediate representation on which it runs most of its optimizations. For a compiler targeting multiple-source languages and different architectures the important decision while selecting an IR is that it should neither be of very high-level, as in very closely attached to the source language, nor it should be very low-level, as in close to the target machine instructions. LLVM IR aims to be a universal IR of a kind, by being at a low enough level that high-level ideas may be cleanly mapped to it. Ideally the LLVM IR should have been target-independent, but it is not so because of the inherent target dependence in some of the programming languages itself. For example, when using standard C headers in a Linux system, the header files itself are target dependent, which may specify a particular type to an entity so that it matches the system calls of the particular target architecture.
Most of the LLVM tools revolve around this Intermediate Representation. The frontends of different languages generate this IR from the high-level source language. The optimizer tool of LLVM runs on this generated IR to optimize the code for better performance and the code generator makes use of this IR for target specific code generation. This IR has three equivalent forms:
