Data-Flow Prescheduling for Large Instruction Windows in Out-of-Order Processors

The performance of out-of-order processors increases with the instruction window size. In conventional processors, the effective instruction window cannot be larger than the issue buffer. Determining which instructions from the issue buffer can be launched to the execution units is a timecritical operation which complexity increases with the issue buffer size. We propose to relieve the issue stage by reordering instructions before they enter the issue buffer. This study introduces the general principle of data-flow prescheduling. Then we describe a possible implementation. Our preliminary results show that data-flow prescheduling makes it possible to enlarge the effective instruction window while keeping the issue buffer small.