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

Reading from the memory

Now, since we have the address, we are ready to read the data from that address and assign the read value to a variable.

In LLVM the load instruction is used to read from a memory location. This simple instruction or combination of similar instructions may then be mapped to some of the sophisticated memory read instructions in low-level assembly.

A load instruction takes an argument, which is the memory address from which the data should be read. We obtained the address in the previous section by the getelementptr instruction in a1.

The load instruction looks like the following:

%val = load i32, i32* a1

This means that the load will take the data pointed by a1 and save in %val.

To emit this we can use the API provided by LLVM in a function, as shown in the following code:

Value *getLoad(IRBuilder<> &Builder, Value *Address) {
  return Builder.CreateLoad(Address, "load");

Let's also return the loaded value:


The whole code is as follows...