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

Loop processing

Before getting started with loop processing and optimization, we must have a little heads up about the concepts of CFG and dominance information. A CFG is the control flow graph of the program that gives a look into how the program may be executed through the various basic blocks. By dominance information, we get to know about the relation between the various basic blocks in the CFG.

In a CFG, we say a node d dominates a node n if every path (from the input towards output) that passes through n must also pass through d. This is denoted by d -> n. The graph G = (V, E), where V is the set of basic blocks and E is the dominance relation defined on V, is called dominator tree.

Let's take an example to show the CFG of a program and the corresponding dominator tree.

Put example code here:

void fun() {
  int iter, a, b;

  for (iter = 0; iter < 10; iter++) {
    a = 5;
    if (iter == a)
      b = 2;
      b = 5;

The CFG for the preceding code looks like the following...