Reduced register-dependency checking for paired-instruction dispatch in

Abstract

The dispatch unit of a superscalar processor checks for register dependencies among instructions to be issued together as a group. The first instruction's destination register is compared to the following instructions' sources, but the destinations of following instructions are not checked with the first instruction's destination. Instead, instructions with destination-destination dependencies are dispatched together as a group. These instructions flow down the pipelines. At the end of the pipelines the destinations are compared. If the destinations match then the results are merged together and written to the register. When instructions write to only a portion of the register, merging ensures that the correct portions of the register are written by the appropriate instructions in the group. Thus older code which performs partial-register writes can benefit from superscalar processing by dispatching the instructions together as a group and then merging the writes together at the end of the pipelines. The dispatch and decode stage, which is often a critical path on the processor, is reduced in complexity by not checking for destination-register dependencies. Performance increases because more kinds of instructions can be dispatched together in a group, increasing the use of the superscalar features.