Loop end prediction using loop counter updated by inflight loop end instructions

Abstract

In a data processing apparatus loop end prediction is carried out to predict whether a branch represented by a loop end instruction will be taken, branching to the start of the loop for a further iteration to be carried out, or will be not taken leading to the further instructions following the loop. A loop iteration counter at the fetch stage of the apparatus maintains a count on the basis of which the prediction is made. The loop iteration counter is decremented both by loop end instructions reaching the end of the pipeline for which no prediction was made and by later loop end instructions for which a prediction is made, once it has been established that a loop is being executed. This dual counting mechanism allows 'shadow' loop end instructions, which were already in the pipeline by the time it was established that a loop is being executed, to be accounted for.