Book Image

Learn LLVM 12

By : Kai Nacke
Book Image

Learn LLVM 12

By: Kai Nacke

Overview of this book

LLVM was built to bridge the gap between compiler textbooks and actual compiler development. It provides a modular codebase and advanced tools which help developers to build compilers easily. This book provides a practical introduction to LLVM, gradually helping you navigate through complex scenarios with ease when it comes to building and working with compilers. You’ll start by configuring, building, and installing LLVM libraries, tools, and external projects. Next, the book will introduce you to LLVM design and how it works in practice during each LLVM compiler stage: frontend, optimizer, and backend. Using a subset of a real programming language as an example, you will then learn how to develop a frontend and generate LLVM IR, hand it over to the optimization pipeline, and generate machine code from it. Later chapters will show you how to extend LLVM with a new pass and how instruction selection in LLVM works. You’ll also focus on Just-in-Time compilation issues and the current state of JIT-compilation support that LLVM provides, before finally going on to understand how to develop a new backend for LLVM. By the end of this LLVM book, you will have gained real-world experience in working with the LLVM compiler development framework with the help of hands-on examples and source code snippets.
Table of Contents (17 chapters)
Section 1 – The Basics of Compiler Construction with LLVM
Section 2 – From Source to Machine Code Generation
Section 3 –Taking LLVM to the Next Level

Adding support for disassembling

The definition of the instructions in the target description allows the construction of decoder tables, which are used to disassemble an object file into a textual assembler. The decoder tables and a decoder function are generated by the llvm-tblgen tool. Besides the generated code, we only need to provide the code to register and initialize the M88kDisassembler class and some helper functions to decode registers and operands. We place the implementation in the Disassembler/M88kDisassembler.cpp file.

The getInstruction() method of the M88kDisassembler class does the decoding work. It takes an array of bytes as input and decodes the next instruction into an instance of the MCInst class. The class declaration is as follows:

using DecodeStatus = MCDisassembler::DecodeStatus;
namespace {
class M88kDisassembler : public MCDisassembler {
  M88kDisassembler(const MCSubtargetInfo &STI, MCContext &Ctx)