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

Performance profiling with XRay

If your application seems to run slow, then you might want to know where all the time is spent in the code. In this case, instrumenting the code with XRay helps you. Basically, at each function entry and exit, a special call into the runtime library is inserted. This allows counting how often a function is called, and also how much time is spent in the function. You find the implementation for the instrumentation pass in the llvm/lib/XRay/ directory. The runtime portion is part of compiler-rt.

In the following example source, real work is simulated by calling the usleep() function. The func1() function sleeps for 10 µs. The func2() function either calls func1() or sleeps for 100 µs, depending on whether the n parameter is odd or even. Inside the main() function, both functions are called inside a loop. This is already enough to get interesting information. You'll need to save the following source code in the xraydemo.c file: