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, wherein the subsequent instruction is an instruction having a width greater than a width of each of the plurality of instruction execution slices; 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, wherein the dispatching dispatches a first portion of the subsequent instruction to the selected instruction execution slice and a second portion of the subsequent instruction to another instruction execution slice that executed an instruction previous to the first instruction for the first instruction stream, wherein the event is an SIMD instruction and wherein the dispatching dispatches the subsequent instruction to the selected instruction execution slice and another instruction execution slice that executed an instruction previous to the first instruction for the first instruction stream, wherein the selected instruction execution slice and the another instruction execution slice process different data associated with the SIMD instruction.