It means you compiled with e.g. gcc -O3 and the gcc optimiser found that some of your variables were redundant in some way that allowed them to be optimised away. In this particular case you appear to have three variables a, b, c with the same value and presumably they can all be aliassed to a single variable. Compile with optimisation disabled, e.g. gcc -O0, if you want to see such variables (this is generally a good idea for debug builds in any case).
