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.

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LLVM Essentials
Mayur Pandey | Suyog Sarda | David Farago
Dec, 2015 | 166 pages
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Table of Contents (14 chapters)
LLVM Essentials
LLVM Essentials
Credits
Credits
About the Authors
About the Authors
About the Reviewer
About the Reviewer
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
Playing with LLVM
Playing with LLVM
Modular design and collection of libraries
Getting familiar with LLVM IR
LLVM tools and using them in the command line
Summary
2
Building LLVM IR
Building LLVM IR
Creating an LLVM module
Emitting a function in a module
Adding a block to a function
Emitting a global variable
Emitting a return statement
Emitting function arguments
Emitting a simple arithmetic statement in a basic block
Emitting if-else condition IR
Emitting LLVM IR for loop
Summary
3
Advanced LLVM IR
Advanced LLVM IR
Memory access operations
Getting the address of an element
Reading from the memory
Writing into a memory location
Inserting a scalar into a vector
Extracting a scalar from a vector
Summary
4
Basic IR Transformations
Basic IR Transformations
Opt Tool
Pass and Pass Manager
Using other Pass info in current Pass
Instruction simplification example
Instruction Combining
Summary
5
Advanced IR Block Transformations
Advanced IR Block Transformations
Loop processing
Scalar evolution
LLVM intrinsics
Vectorization
Summary
6
IR to Selection DAG phase
IR to Selection DAG phase
Converting IR to selectionDAG
Legalizing SelectionDAG
Optimizing SelectionDAG
Instruction Selection
Scheduling and emitting machine instructions
Register allocation
Code Emission
Summary
7
Generating Code for Target Architecture
Generating Code for Target Architecture
Sample backend
Implementing frame lowering
Lowering instructions
Printing an instruction
Summary
Index
Index

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:

   builder.CreateRet(val);

The whole code is as follows...

Previous Section
End of Section 4
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