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

Code Emission

We started from LLVM IR in the first section and converted it to SelectioDAG and then to MachineInstr. Now, we need to emit this code. Currently, we have LLVM JIT and MC to do so. LLVM JIT is the traditional way of generating the object code for a target on the go directly in the memory. What we are more interested in is the LLVM MC layer.

The MC layer is responsible for generation of assembly file/object file from the MachineInstr passed on to it from the previous step. In the MC Layer, the instructions are represented as MCInst, which are lightweight, as in they don't carry much information about the program as MachineInstr.

The code emission starts with the AsmPrinter class, which is overloaded by the target specific AsmPrinter class. This class deals with general lowering process by converting the MachineFunction functions into MC label constructs by making use of the target specific MCInstLowering interface(for x86 it is X86MCInstLower class in the lib/Target/x86/X86MCInstLower...

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