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

Chapter 7. Generating Code for Target Architecture

The code generated by the compiler finally has to execute on the target machines. The abstract form of the LLVM IR helps to generate code for various architectures. The target machine can be anything – CPU, GPU, DSP's, and so on. The target machine has some defining aspects such as the register sets, the instruction set, the calling convention of the function, and the instruction pipeline. These aspects or properties are generated using the tablegen tool so that they can be used easily while programming code generation for the machine.

LLVM has a pipeline structure for the backend, where instructions travel through phases—from the LLVM IR to SelectionDAG, then to MachineDAG, then to MachineInstr, and finally to MCInst. The IR is converted to SelectionDAG. SelectionDAG then goes through legalization and optimizations. After this stage, the DAG nodes are mapped to target instructions (instruction selection). The DAG then goes through instruction...