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

Generating code with the LLVM backend

The backend's task is to create optimized machine code from an IR of a module. The IR is the interface to the backend and can be created using a C++ interface or in textual form. Again, the IR is generated from the AST.

Textual representation of the LLVM IR

Before trying to generate the LLVM IR, we need to understand what we want to generate. For the example expression language, the high-level plan is as follows:

  1. Ask the user for the value of each variable.
  2. Calculate the value of the expression.
  3. Print the result.

To ask the user to provide a value for a variable and to print the result, two library functions, calc_read() and calc_write(), are used. For the with a: 3*a expression, the generated IR is as follows:

  1. The library functions must be declared, like in C. The syntax also resembles C. The type before the function name is the return type. The type names surrounded by parenthesis are the argument types...