Predicting upcoming control flow

Abstract

An apparatus has a fetch queue to identify a sequence of instructions to be fetched for execution and prediction circuitry to predict upcoming control flow and to control which instructions are identified in the fetch queue in dependence on the prediction. The prediction circuitry predicts multi-taken sequences which are sequences of instructions in which control flow is diverted by a first control flow changing instruction to a series of instructions terminating in a second control flow changing instruction that diverts control flow to a target address. The apparatus also has prediction confidence calculation circuitry to calculate confidence levels for respective multi-taken sequences. Each confidence level is indicative of a confidence in an accuracy of prediction of its respective multi-taken sequence. When the confidence level for a particular multi-taken sequence satisfies a prediction confidence condition, the prediction confidence tracking circuitry allows the particular multi-taken sequence to be predicted by the prediction circuitry. The prediction circuitry causes the series of instructions and the target instruction for the particular multi-taken sequence to be identified in the fetch queue when the prediction circuitry predicts the particular multi-taken sequence and further predictions to be made starting from the target address for the particular multi-taken sequence.