A programmer might have come across compilers at some or the other point when programming. Simply speaking, a compiler converts a human-readable, high-level language into machine-executable code. But have you ever wondered what goes on under the hood? A compiler does lot of processing before emitting optimized machine code. Lots of complex algorithms are involved in writing a good compiler.
This book travels through all the phases of compilation: frontend processing, code optimization, code emission, and so on. And to make this journey easy, LLVM is the simplest compiler infrastructure to study. It's a modular, layered compiler infrastructure where every phase is dished out as a separate recipe. Written in object-oriented C++, LLVM gives programmers a simple interface and lots of APIs to write their own compiler.
As authors, we maintain that simple solutions frequently work better than complex solutions; throughout this book, we'll look at a variety of recipes that will help develop your skills, make you consider all the compiling options, and understand that there is more to simply compiling code than meets the eye.
We also believe that programmers who are not involved in compiler development will benefit from this book, as knowledge of compiler implementation will help them code optimally next time they write code.
We hope you will find the recipes in this book delicious, and after tasting all the recipes, you will be able to prepare your own dish of compilers. Feeling hungry? Let's jump into the recipes!
Chapter 1, LLVM Design and Use, introduces the modular world of LLVM infrastructure, where you learn how to download and install LLVM and Clang. In this chapter, we play with some examples to get accustomed to the workings of LLVM. We also see some examples of various frontends.
Chapter 2, Steps in Writing a Frontend, explains the steps to write a frontend for a language. We will write a bare-metal toy compiler frontend for a basic toy language. We will also see how a frontend language can be converted into the LLVM intermediate representation (IR).
Chapter 3, Extending the Frontend and Adding JIT Support, explores the more advanced features of the toy language and the addition of JIT support to the frontend. We implement some powerful features of a language that are found in most modern programming languages.
Chapter 4, Preparing Optimizations, takes a look at the pass infrastructure of the LLVM IR. We explore various optimization levels, and the optimization techniques kicking at each level. We also see a step-by-step approach to writing our own LLVM pass.
Chapter 5, Implementing Optimizations, demonstrates how we can implement various common optimization passes on LLVM IR. We also explore some vectorization techniques that are not yet present in the LLVM open source code.
Chapter 6, Target-independent Code Generator, takes us on a journey through the abstract infrastructure of a target-independent code generator. We explore how LLVM IR is converted to Selection DAGs, which are further processed to emit target machine code.
Chapter 7, Optimizing the Machine Code, examines how Selection DAGs are optimized and how target registers are allocated to variables. This chapter also describes various optimization techniques on Selection DAGs as well as various register allocation techniques.
Chapter 8, Writing an LLVM Backend, takes us on a journey of describing a target architecture. This chapter covers how to describe registers, instruction sets, calling conventions, encoding, subtarget features, and so on.
Chapter 9, Using LLVM for Various Useful Projects, explores various other projects where LLVM IR infrastructure can be used. Remember that LLVM is not just a compiler; it is a compiler infrastructure. This chapter explores various projects that can be applied to a code snippet to get useful information from it.
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 for comparison of two files; it works well on the Linux platform.
The book is for compiler programmers who are familiar with concepts of compilers and want to indulge in understanding, exploring, and using LLVM infrastructure in a meaningful way in their work.
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.
In this book, you will find several headings that appear frequently (Getting ready, How to do it, How it works, There's more, and See also).
To give clear instructions on how to complete a recipe, we use these sections.
This section tells you what to expect in the recipe, and describes how to set up any software or any preliminary settings required for the recipe.
This section usually consists of a detailed explanation of what happened in the previous section.
This section consists of additional information about the recipe in order to make you more knowledgeable about the recipe.
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.
Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "We can include other contexts through the use of the include directive."
A block of code is set as follows:
primary := identifier_expr :=numeric_expr :=paran_expr
When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
primary := identifier_expr
:=numeric_expr
:=paran_exprAny command-line input or output is written as follows:
$ cat testfile.ll
New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Clicking on the Next button moves you to the next screen."
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