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 Combining

Instruction combining is a LLVM Pass and compiler technique in which we replace a sequence of instructions with instructions that are more effective and give the same result on execution in a smaller number of machine cycles. Instruction combining does not alter the CFG of the program and is mainly used for algebraic simplification. The major difference between instruction combining and instruction simplification is that in instruction simplification we cannot generate new instructions, which is possible in instruction combining. This pass is run by specifying the –instcombine argument to the opt tool and is implemented in the lib/transforms/instcombine folder. The instcombine Pass combines

%Y = add i32 %X, 1
%Z = add i32 %Y, 1
%Z = add i32 %X, 2

It has removed one redundant add instruction and hence combined the two add instructions to one.

The LLVM page states that this pass guarantees that the following canonicalizations are performed on the program:

  • Constant operand...