Miscompilations often happen because optimization safety checks are inadequate, static analyses are unsound, or transfor- mations are flawed. These bugs are out of reach for current and future automated program-verification tools because the specifica- tions that need to be checked were never written down in a precise way, if they were written down at all. Where verification is imprac- tical, however, other methods for improving compiler quality can succeed. This paper reports our experience in using testing to make C compilers better.