Using redundancies to find errors

Programmers generally attempt to perform useful work. If they performed an action, it was because they believed it served some purpose. Redundant operations violate this belief. However, in the past, redundant operations have been typically regarded as minor cosmetic problems rather than serious errors. This paper demonstrates that, in fact, many redundancies are as serious as traditional hard errors (such as race conditions or null pointer dereferences). We experimentally test this idea by writing and applying five redundancy checkers to a number of large open source projects, finding many errors. We then show that, even when redundancies are harmless, they strongly correlate with the presence of traditional hard errors. Finally, we show how flagging redundant operations gives a way to detect mistakes and omissions in specifications. For example, a locking specification that binds shared variables to their protecting locks can use redundancies to detect missing bindings by flagging critical sections that include no shared state.