Optimizing C and C++ Code

                Embedded software often runs on processors with limited computation power, thus optimizing the code becomes a necessity. In this article we will explore the following optimization techniques for C and C++ code developed for Real-time and Embedded Systems. 

1. Adjust structure sizes to power of two
2. Place case labels in narrow range
3. Place frequent case labels first
4. Break big switch statements into nested switches
5. Minimize local variables
6. Declare local variables in the inner most scope
7. Reduce the number of parameters
8. Use references for parameter passing and return value for types bigger than 4 bytes
9. Don't define a return value if not used
10. Consider locality of reference for code and data
11. Prefer int over char and short
12. Define lightweight constructors
13. Prefer initialization over assignment
14. Use constructor initialization lists
15. Do not declare "just in case" virtual functions
16. In-line 1 to 3 line functions

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

C optimisation tutorial

            This document has evolved over time and contains a number of the best ways to hand-optimise your C-code. Compilers are good, but they can't do everything, and here I hope to help you squeeze the best performance out of your code. This is not intended for beginners, but for more experienced programmers. 

Origins of UNIX

 In 1969, Ken Thompson from AT&T’s Bell Telephone Labs wrote the first version of the UNIX operating system, on a DEC PDP-7. Disillusioned with the inefficiency of the Multics (Multiplexed Information and Computing Service) project, Thompson decided to create a programmer-friendly operating system that limited the functions contained within the kernel and allowed greater flexibility in the design and implementation of applications. The PDP-7 was a modest system on which to build a new operating system. it had only an assembler and a loader,and it would allow only a single user login at any one time. It didn't even have a hard disk, the developers were forced to partition physical memory into an operating system segment and a RAM disk segment.

         Thus, the first UNIX file system was emulated entirely in RAM!