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

Emitting a global variable

Global variables have visibility of all the functions within a given module. LLVM provides the GlobalVariable class to create global variables and set its properties such as linkage type, alignment, and so on. The Module class has the method getOrInsertGlobal() to create a global variable. It takes two arguments—the first is the name of the variable and the second is the data type of the variable.

As global variables are part of a module, we create global variables after creating the module. Insert the following code just after creating the module in toy.cpp:

GlobalVariable *createGlob(IRBuilder<> &Builder, std::string Name) {
  ModuleOb->getOrInsertGlobal(Name, Builder.getInt32Ty());
  GlobalVariable *gVar = ModuleOb->getNamedGlobal(Name);
  return gVar;

Linkage is what determines if multiple declarations of the same object refer to the same object, or to separate ones...