16. The computer program product of claim 13 , wherein the cache directory comprises two or more synonym slots for each cache entry, and wherein a position of each synonym within the synonym slots is based on recent use relative to one or more other synonyms in the synonym slots.
13. A computer program product for implementing a least recently used replacement policy for synonyms in a cache directory of a shared cache, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to perform a method comprising: receiving a request to access a cache entry in a shared cache, the request referencing a synonym for the cache entry, a cache directory of the shared cache comprising, for each cache entry of the shared cache, a first-ranked synonym slot for storing a most recently used synonym for the cache entry and a second-ranked synonym slot for storing a second most recently used synonym for the cache entry; and based on receiving the request: writing contents of the first-ranked synonym slot for the cache entry to the second-ranked synonym slot for the cache entry; and writing the synonym referenced in the request to the first-ranked synonym slot for the cache entry.