Code Optimization Using the GNU C Compiler

This article describes some of the code optimization techniques used by the GNU C Compiler, in order to give the reader a feel of what code optimization is and how it can increase the efficiency of the generated object code.

Contents

1. Introduction
2. Assembly Language Code for a C Program
3. Constant Folding
4. Common Subexpression Elimination
5. Dead Code Elimination
6. Strength Reduction using Induction Variable
7. Conclusion
8. Acknowledgments
9. References

1. Introduction

As we all know, a compiler is a program that reads the source program in a high-level language and translates it into (typically) machine language. This is a complicated process involving a number of stages. If the compiler is an optimizing compiler, one of these stages "optimizes" the machine language code so that it either takes less time to run or occupies less memory or sometimes both. Of course, whatever optimizations the compiler does, it must not affect the logic of the program i.e. the optimization must preserve the meaning of the program. One might wonder what type of optimizations the compiler uses to produce efficient machine code? Since in no case the meaning of the program being compiled should be changed, the compiler must inspect the program very thoroughly and find out the suitable optimizations that can be applied. As we may wonder, such a thorough analysis of the program and then finding and applying suitable optimizations is a complex and time consuming process, the details of which are beyond the scope of this article.

What I am going to describe in this article are a few type of code optimizations that the GNU C Compiler uses so that we can understand how the code is optimized and appreciate the complexity of the optimization process. The GNU C Compiler is a sophisticated optimizing C compiler that uses a large array of optimization techniques to produce efficient code. It may not be possible to describe all of them, so I have chosen some that are interesting and also easy to understand. A complete list of the optimization techniques that are used by the GNU C compiler is available at http://www.redhat.com/products/support/gnupro/gnupro_gcc.html. Instead of simply describing what a particular optimization technique does, I will describe them using the assembly language code that is generated by the GNU C Compiler. This method will also enable the readers to further explore code optimization carried out by the compiler and also the advanced optimization techniques, in case they are interested.