 caches for
            shader memory accesses.  Implementations are not required to keep these
            caches coherent with shader memory writes.  Stores issued by one
            invocation may not be immediately observable by other pipeline stages or
            other shader invocations because the value stored may remain in a cache
            local to the processor executing the store, or because data overwritten by
            the store is still in a cache elsewhere in the system.  When <code clas