Data processing apparatus and method for executing a stream of instructions out of order with respect to original program order

A data processing apparatus and method are provided for executing a stream of instructions out-of-order with respect to original program order. At least some of the instructions in the stream identify one or more architectural registers from a set of architectural registers. The apparatus comprises a plurality of out-of-order components configured to manage execution of a first subset of instructions out-of-order, the plurality of out-of-order components being configured to remove false dependencies between instructions in the first subset. The plurality of out-of-order components include a first issue queue into which the instructions in the first subset are buffered prior to execution. A second issue queue is used to buffer a second subset of instructions prior to execution, the second subset of instructions being constrained to execute in order. Issue control circuitry is configured to reference both issue queues in order to determine an order of execution of instructions, and is configured to constrain the order of execution of the first subset of instructions by true dependencies between the instructions in both the first and second issue queues, and to constrain the order of execution of the second subset of instructions by both the true dependencies and the false dependencies between the instructions in both the first and second issue queues. This approach provides improved performance and/or reduced energy consumption.