Techniques for managing instruction execution for multiple instruction streams using a processor core having multiple parallel instruction execution slices provide flexibility in execution of program instructions by a processor core. An event is detected indicating that either resource requirement or resource availability will not be met by the execution slice currently executing the instruction stream. In response to detecting the event, dispatch of at least a portion of the subsequent instruction is made to another instruction execution slice. The event may be a compiler-inserted directive, may be an event detected by logic in the processor core, or may be determined by a thread sequencer. The instruction execution slices may be dynamically reconfigured as between single-instruction-multiple-data (SIMD) instruction execution, ordinary instruction execution, wide instruction execution. When an instruction execution slice is busy processing a current instruction for one of the streams, another slice can be selected to proceed with execution.