Apparatus and method for controlling branch prediction

A method of controlling branch prediction in an apparatus having processing circuitry to execute instructions, the method comprising:

• providing branch prediction circuitry comprising a plurality of branch prediction mechanisms used to predict target addresses for branch instructions to be executed by the processing circuitry, the branch instructions comprising a plurality of branch types, where one branch type is a return instruction;
• using by default a return prediction mechanism amongst the plurality of branch prediction mechanisms to predict a target address when a return instruction is detected by the branch prediction circuitry, wherein the return prediction mechanism comprises a return stack, each target address pushed onto the return stack is a return address calculated for a function call and forms an entry on the return stack, and each time the return stack is used to predict a target address for a return instruction detected by the branch prediction circuitry the return stack is arranged to determine the predicted target address for that return instruction by popping an entry from the return stack;
• and
• responsive to a trigger condition indicative of misprediction of the target address when using the entry popped from the return stack to predict the target address for one or more preceding instances of a given return instruction, switching to using an alternative branch prediction mechanism amongst the plurality of branch prediction mechanisms for predicting the target address for at least one subsequent instance of the given return instruction.

An apparatus comprising:

• processing means for executing instructions;
• and
• branch prediction means comprising a plurality of branch prediction mechanisms for use when predicting target addresses for branch instructions to be executed by the processing means, the branch instructions comprising a plurality of branch types, where one branch type is a return instruction;
• wherein the branch prediction mechanisms include a return prediction mechanism comprising a return stack used by default to predict a target address when a return instruction is detected by the branch prediction means, wherein each target address pushed onto the return stack is a return address calculated for a function call and forms an entry on the return stack, and each time the return stack is used to predict a target address for a return instruction detected by the branch prediction means the return stack is arranged to determine the predicted target address for that return instruction by popping an entry from the return stack;
• the branch prediction means, responsive to a trigger condition indicative of misprediction of the target address when using the entry popped from the return stack to predict the target address for one or more preceding instances of a given return instruction, for switching to using an alternative branch prediction mechanism for predicting the target address for at least one subsequent instance of the given return instruction.