1. A method of processing instructions of multiple instruction streams in a processor core having a plurality of instruction execution slices, the method comprising: analyzing the multiple instruction streams and detecting, in a first one of the multiple instruction streams, an event indicating that a first instruction requires processing that changes resource availability for execution of a subsequent instruction of the first instruction stream or that the subsequent instruction has execution requirements such that a change in mapping between the instruction execution slices and corresponding ones of the multiple instruction streams is indicated; and responsive to detecting the event, modifying the first one of the multiple instruction streams by inserting a directive in the first one of the multiple instruction streams, wherein the directive, when executed by the another processor core will cause the change in mapping so that at least a portion of the subsequent instruction is dispatched to a selected one of the instruction execution slices that was assigned to a second one of the multiple instruction streams during a previous execution cycle.