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 LLVM IR for loop

Similar to the if-else statement, loops can also be emitted using LLVM API's, with slight modification of the code. For example, we want to have LLVM IR for the following Loops:

for(i=1; i< b; i++)  {body}

The loop has induction variable i, which has some initial value that updates after each iteration. The induction variable is updated after each iteration by a step value that is 1 in the preceding example. Then there is a loop ending condition. In the preceding example, 'i=1' is the initial value, 'i<b' is the end condition of the loop, and 'i++' is the step value by which the induction variable 'i' is incremented after every iteration of the loop.

Before writing a function to create a loop, some Value and BasicBlock need to be pushed into a list, as follows:

Function::arg_iterator AI = fooFunc->arg_begin();
  Value *Arg1 = AI++;
  Value *Arg2 = AI;
  Value *constant = Builder.getInt32(16);
  Value *val = createArith(Builder, Arg1, constant);
  ValList VL...