Switching a predicted branch type following a misprediction of a number of loop iterations

Abstract

When a predicted branch type for a given address is a first branch type corresponding to a predicated-loop instruction for triggering processing circuitry to perform a variable number of iterations of a predicated loop body, branch prediction circuitry generates a first type of branch prediction indicative of a predicted number of iterations for the predicated-loop instruction, and omits speculatively updating history information based on the predicted number of iterations. The processing circuitry is able to tolerate at least one unnecessary iteration of the predicated loop body being processed when the predicted number of iterations is too large. For a second branch type, the branch prediction circuitry generates a second type of branch prediction and, at least when a taken branch is predicted in the second type of branch prediction, speculatively updates the history information. Following detection of a misprediction of the predicted number of iterations indicated by the first type of branch prediction for the given address, it is determined whether a branch type update condition is satisfied, and if so the predicted branch type associated with the given address is switched from the first branch type to the second branch type.