Appratus and method for using predicted result values

Abstract

An apparatus and method are provided for using predicted result values. The apparatus has processing circuitry for executing a sequence of instructions, and value prediction storage that comprises a plurality of entries, where each entry is used to identify a predicted result value for an instruction allocated to that entry. Dispatch circuitry maintains a record of pending instructions awaiting execution by the processing circuitry, and selects pending instructions from the record for dispatch to the processing circuitry for execution. The dispatch circuitry is arranged to enable at least one pending instruction to be speculatively executed by the processing circuitry using as a source operand a predicted result value provided by the value prediction storage. Allocation circuitry is arranged to apply a default allocation policy to identify a first instruction to be allocated an entry in the value prediction storage. However, the allocation circuitry is further responsive to a trigger condition to identify a dependent instruction whose result value will be dependent on the result value produced by executing the first instruction, and to then allocate an entry in the value prediction storage to store a predicted result value for the identified dependent instruction. Such an approach can enable performance improvements to be achieved through the use of predicted result values even in situations where the prediction accuracy of the predicted result value for the first instruction proves not to be that high, by instead enabling a predicted result value for the dependent instruction to be used to allow speculative execution of further dependent instructions.