There are provided a system, a method and a computer program product for selecting an active data stream (a lane) while running Single Program Multiple Data code on a Single Instruction Multiple Data machine. The machine runs an instruction stream over input data streams and machine increments lane depth counters of all active lanes upon the thread-PC reaching a branch operation and updates the lane-PC of each active lane according to targets of the branch operation. An instruction of the instruction stream includes a barrier indicating a convergence point for all lanes to join. In response to a lane reaching a barrier: evaluating whether all lane-PCs are set to a same thread-PC; and if the lane-PCs are not set to the same thread-PC, selecting an active lane from the plurality of lanes; otherwise, incrementing the lane-PCs of all the lanes, and then selecting an active lane from the plurality of lanes.