Transactional recovery storage for branch history and return addresses to partially or fully restore the return stack and branch history register on transaction aborts

An apparatus comprising:

• processing circuitry to perform data processing operations in response to instructions, the processing circuitry comprising transactional memory support circuitry to support execution of a transaction, the transaction comprising a sequence of instructions executed speculatively and for which the processing circuitry is configured to prevent commitment of results of the speculatively executed instructions until the transaction has reached a transaction end point;
• prediction circuitry to generate predictions in relation to instruction flow changing instructions, with the generated predictions being used to determine the instructions fetched for execution by the processing circuitry;
• prediction storage to store a plurality of items of prediction information that are referenced by the prediction circuitry when generating the predictions, wherein the items of prediction information maintained by the prediction storage change based on the instructions being executed by the processing circuitry;
• and
• a recovery storage activated by the transactional memory support circuitry at a transaction start point to store a restore pointer identifying a chosen location in the prediction storage, and between the transaction start point and the transaction end point to receive any item of prediction information removed from the prediction storage that was present in the prediction storage at the transaction start point;
• wherein:
• the recover storage and the prediction storage are separate elements;
• the transactional memory support circuitry is responsive to the transaction being aborted to reference the restore pointer in order to discard from the prediction storage any items of prediction information added to the prediction storage after the transaction start point, and to store back into the prediction storage any items of prediction information stored in the recovery storage;
• the prediction storage is arranged to store, as the items of prediction information, predicted target addresses for return instructions;
• the prediction circuitry is target address prediction circuitry used to generate, as the predictions, predicted target addresses for return instructions;
• each return instruction is provided in association with an earlier call instruction;
• the prediction storage is arranged as a return stack, and in association with each encountered call instruction a return address for the call instruction is determined and pushed onto the return stack to form the predicted target address for the associated return instruction;
• the processing circuitry is responsive to completing execution of a return instruction to pop a return address from the return stack;
• the recovery storage is arranged to store as the restore pointer a pointer identifying a location in the return stack storing the most recently added return address, and is arranged to mark every location storing a valid return address at the transaction start point as being a recoverable location;
• each time during execution of the transaction a return address is popped from a recoverable location in the return stack, the recovery storage is arranged to receive that return address;
• the recovery storage has capacity to store M return addresses;
• when during execution of the transaction more than M return addresses are popped from recoverable locations in the return stack, the recovery storage is arranged to retain the least recently popped M return addresses;
• the recovery storage is arranged to maintain an indication of the number of items of prediction information discarded by the recovery storage;
• and
• the prediction circuitry is arranged, following recovery of the prediction storage using the recovery storage in response to the transaction being aborted, and when execution of the transaction is retried, to use said indication to determine a point at which to disable prediction of target addresses for return instructions using the return addresses held in the return stack.

A method of handling prediction information, comprising:

• employing processing circuitry to perform data processing operations in response to instructions, the processing circuitry comprising transactional memory support circuitry to support execution of a transaction, the transaction comprising a sequence of instructions executed speculatively and for which the processing circuitry is configured to prevent commitment of results of the speculatively executed instructions until the transaction has reached a transaction end point;
• generating predictions in relation to instruction flow changing instructions, with the generated predictions being used to determine the instructions fetched for execution by the processing circuitry;
• storing in prediction storage a plurality of items of prediction information that are referenced when generating the predictions, wherein the items of prediction information maintained by the prediction storage change based on the instructions being executed by the processing circuitry;
• activating a recovery storage at a transaction start point to store a restore pointer identifying a chosen location in the prediction storage, and between the transaction start point and the transaction end point to receive any item of prediction information removed from the prediction storage that was present in the prediction storage at the transaction start point, wherein the recovery storage and the prediction storage are separate elements;
• and
• in response to the transaction being aborted, causing the transactional memory support circuitry to reference the restore pointer in order to discard from the prediction storage any items of prediction information added to the prediction storage after the transaction start point, and to store back into the prediction storage any items of prediction information stored in the recovery storage;
• wherein
• the prediction storage is arranged to store, as the items of prediction information, history information indicative of instruction flow for a number of previously encountered instruction flow changing instructions;
• and
• the generating of predictions comprises generating direction predictions for the instruction flow changing instructions during which an entry in each of a plurality of direction predicting resources are identified using an associated group of items of prediction information maintained within the prediction storage.