To simplify a bit you can think of most static analysis algorithms in terms of graph problems where nodes are statements and functions and edges are flow of control and calls. On large codebases the amount of edges, nodes, and calculated data is just too big to keep in memory. The trick is to break the graph intelligently into parts, calculate some sort of summary information for each of them, distributing between cpus or computers, move up to the supegraph of graphs and perform higher level calculations on it.