1. A method of migrating data to a register file cache, the method comprising: storing in a table an entry for each of a plurality of physical registers, each entry comprising information indicating each functional unit of a plurality of functional units that has recently accessed a physical register, wherein a functional unit has recently accessed the physical register if a predetermined amount of time has not yet elapsed since the functional unit last accessed the physical register; receiving at a migration unit a register read operation to be executed for a particular functional unit, the register read operation specifying a particular physical register to be read; reviewing the entries in the table at the migration unit to determine whether the particular functional unit has recently accessed the particular physical register; and in response to determining the particular functional unit has not recently accessed the particular physical register, migrating data to a register file cache associated with the particular functional unit.