A method and corresponding tool, the method comprising: generating a lower-level control flow structure representing a portion of an executable program, the lower-level control flow structure comprising a plurality of lower-level nodes representing operations occurring within the program and a plurality of directional edges representing program flow between nodes; generating a higher-level control flow structure by matching a plurality of the lower-level nodes and edges to higher-level structure nodes representing internal structure, each higher-level structure node representing a group of one or more lower-level nodes and one or more associated edges; and using the higher-level control flow structure to estimate a timing property relating to execution of the program on a processor. The higher-level structure nodes are selected exclusively from a predetermined set of structure node patterns, each pattern in the set having at most one entry point and at most one exit point.