1. An apparatus, comprising: an execution unit; a register file configured to store operands for instructions to be executed by the execution unit; and an operand cache that includes a plurality of entries configured to store source operands from the register file and result operands of operations by the execution unit, wherein entries of the operand cache include fields that comprise at least: a hint field indicating a retention priority for the operand stored by the entry; and a dirty field indicating whether the operand stored by the entry has been modified; wherein the apparatus is configured to select an entry to evict from the operand cache according to the following priority order: select, at a first priority level, the entry from clean entries associated with a first hint field value that indicates a first retention priority; select, at a second priority level that is lower than the first priority level, the entry from dirty entries associated with a second hint field value that indicates a second retention priority that is higher than the first retention priority; and select, at a third priority level that is lower than the second priority level, the entry from clean entries associated with the second hint field value that indicates the second retention priority.