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

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...