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

Instruction simplification example

In this section, we will see how we fold instructions into simpler forms in LLVM. Here, the creation of new instructions will not take place. Instruction simplification does constant folding:

sub i32 2, 1 -> 1

That is, it simplifies the sub instruction to a constant value 1.

It can handle non-constant operands as well:

or i32 %x, 0 -> %x

It returns a value of variable %x

and i32 %x %x -> %x

In this case, it returns an already existing value.

The implementations for the methods that simplify instructions are located in lib/Analysis/InstructionSimplify.cpp.

Some of the important methods of dealing with the simplification of instructions are:

  • SimplifyBinOp method: This is used to simplify binary operations such as addition, subtraction, and multiplication, and so on. It has the function signature as follows:

    static Value *SimplifyBinOp(unsigned Opcode, Value *LHS, 
    Value *RHS, const Query &Q, unsigned MaxRecurse)

Here, by Opcode, we mean the operator...