Apparatus and method for performing branch prediction

A method of performing branch prediction comprising:

• employing processing circuitry to execute instructions;
• fetching the instructions for execution by the processing circuitry, and when encountering a branch instruction during the fetching, determining subsequent instructions to be fetched in dependence on an initial branch direction prediction for the branch instruction made by branch prediction circuitry;
• employing value prediction circuitry to maintain a predicted result value for one or more instructions;
• maintaining a record of pending instructions that have been fetched and are awaiting execution by the processing circuitry, and selecting pending instructions from the record for dispatch to the processing circuitry;
• when a given instruction whose predicted result value is maintained by the value prediction circuitry has a dependent instruction whose outcome is dependent on a result value of the given instruction, and the dependent instruction is in the record of pending instructions, enabling speculative execution of the dependent instruction by the processing circuitry using the predicted result value of the given instruction;
• when the dependent instruction is the branch instruction, detecting a mispredict condition when an additional branch direction prediction for the branch instruction determined using the predicted result value for the given instruction is considered more accurate than the initial branch direction prediction, and the additional branch direction prediction differs to the initial branch direction prediction;
• and
• on detection of the mispredict condition, issuing a control signal to indicate that the branch instruction has been mispredicted.

An apparatus comprising:

• processing means for executing instructions;
• fetch means for fetching the instructions for execution by the processing means, the fetch means, when encountering a branch instruction, for determining subsequent instructions to be fetched in dependence on an initial branch direction prediction for the branch instruction made by branch prediction means;
• value prediction means for maintaining a predicted result value for one or more instructions;
• dispatch means for maintaining a record of pending instructions that have been fetched by the fetch means and are awaiting execution by the processing means, and for selecting pending instructions from the record for dispatch to the processing circuitry;
• wherein when a given instruction whose predicted result value is maintained by the value prediction means has a dependent instruction whose outcome is dependent on a result value of the given instruction, and the dependent instruction is in the record of pending instructions, the dispatch means is arranged to enable speculative execution of the dependent instruction by the processing means using the predicted result value of the given instruction;
• and
• analysis means for detecting, when the dependent instruction is the branch instruction, a mispredict condition when an additional branch direction prediction for the branch instruction determined using the predicted result value for the given instruction is considered more accurate than the initial branch direction prediction, and the additional branch direction prediction differs to the initial branch direction prediction, and on detection of the mispredict condition for issuing a control signal to indicate that the branch instruction has been mispredicted.