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 metadata for type-based alias analysis

Two pointers may point to the same memory cell, and they then alias each other. Memory is not typed in the LLVM model, which makes it difficult for the optimizer to decide if two pointers alias each other or not. If the compiler can prove that two pointers do not alias each other, then more optimizations are possible. In the next section, we will have a closer look at the problem and investigate how adding additional metadata will help, before we implement this approach.

Understanding the need for additional metadata

To demonstrate the problem, let's look at the following function:

void doSomething(int *p, float *q) {
  *p = 42;
  *q = 3.1425;

The optimizer cannot decide if the p and q pointers point to the same memory cell or not. During optimization this is an important analysis, called an alias analysis. If p and q point to the same memory cell, then they are aliases. If the optimizer can prove...