Chapter 18 - Understanding Your C Compiler: How to Minimize Code Size

Abstract

A C compiler is a basic tool for most embedded systems programmers. It is the tool by which the ideas and algorithms in your application (expressed as C source code) are transformed into machine code executable by your target processor. To a large extent, the C compiler determines how large the executable code for the application will be. A compiler performs many transformations on a program in order to generate the best possible code. In order to write code that is compiler friendly, programmers need to have a working understanding of compilers. Some simple changes to a program, like changing the data type of a frequently-accessed variable, can have a big impact on code size while other changes have no effect at all. Having an idea of what a compiler can and cannot do makes such optimization work much easier. A common transformation on the target code level is to find common sequences of instructions from several functions, and break them out into subroutines. This transformation can be very effective at shrinking the executable code of a program, at the cost of performing more jumps. Moreover, writing things in a straightforward way helps both humans and compilers understand the code, thereby giving better results.