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

Instruction Selection

The SelectionDAG at this phase is optimized and legalized. However, the instructions are still not in machine code form. These instructions need to be mapped to architecture-specific instructions in the SelectionDAG itself. The TableGen class helps select target-specific instructions.

The CodeGenAndEmitDAG() function calls the DoInstructionSelection() function that visits each DAG node and calls the Select() function for each node. The Select() function is the main hook targets implement to select a node. The Select() function is a virtual method to be implemented by the targets.

For consideration, assume our target architecture is X86. The X86DAGToDAGISel::Select() function intercepts some nodes for manual matching, but delegates the bulk of the work to the X86DAGToDAGISel::SelectCode() function. The X86DAGToDAGISel::SelectCode() function is auto generated by TableGen. It contains the matcher table, followed by a call to the generic SelectionDAGISel::SelectCodeCommon...