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 if-else condition IR

An if-else statement has a condition expression and two code paths to execute, depending on the condition evaluating to true or false. The condition expression is generally a comparison statement. Let's emit a condition statement at the start of the block. For example, let the condition be like a<100.

 Value *val2 = Builder.getInt32(100);
 Value *Compare = Builder.CreateICmpULT(val, val2, "cmptmp"); 

On compilation, we get following output:

; ModuleID = 'my compiler'

@x = common global i32, align 4

define i32 @foo(i32 %a, i32 %b) {
  %multmp = mul i32 %a, 16
  %cmptmp = icmp ult i32 %multmp, 100
  ret i32 %multmp

The next step is to define the then and else block expressions, which will be executed depending on the result of condition expression "booltmp". Here, an important concept of PHI instruction comes into picture. A phi instruction takes various values coming from different basic blocks and decides which value to assign depending on the...