Writing an optimizing back end is expensive, in part because it requires mastery of both a target machine and a compiler's internals. We separate these concerns by isolating target-machine knowledge in declarative machine descriptions. We then analyze these descriptions to automatically generate machine-specific components of the back end. In this work, we generate a recognizer; this component, which identifies register transfers that correspond to target-machine instructions, plays a key role in instruction selection in such compilers as vpo, gcc and Quick C–. We present analyses and transformations that address the major challenge in generating a recognizer: accounting for compile-time abstractions not present in a machine description, including variables, pseudo-registers, stack slots, and labels.