When you compile your sketch, the IDE will tell you how big the program image is. If you have reached or exceeded the space available, some of these optimizations may help get you back under the limit.
This is not meant to be a definitive treatise on how to optimize your code - there are libraries full of books on the subject. What is presented here are just some simple tips to help harvest the low-hanging fruit.
Remove Dead Code
If your project is a mash-up of code from several sources, chances are there are parts that are not getting used and can be eliminated to save space.
Unused Libraries - Are all the #include libraries actually used?
Unused Functions - Are all the functions actually being called?
Unused Variables - Are all the variables actually being used?
Unreachable Code - Are there conditional expressions which will never be true?
Hint - If you are not sure about an #include, a function or a variable. Comment it out. If the program still compiles, that code is not being used.
Consolidate Repeated Code
If you have the same sequence of code statements in two or more places, consider making a function out of them.
This guide was first published on Aug 02, 2013. It was last
updated on Aug 02, 2013.
This page (Optimizing Program Memory) was last updated on Mar 11, 2018.
Eliminate the Bootloader
If space is really-really tight, you might consider eliminating the bootloader. This can save as much as 2K or 4K of Flash - depending on which bootloader you are currently using.
The downside of this is that you will need to load your code using an ISP programmer instead of via a standard USB cable.