Data processing systems

A data processing system determines for a stream of instructions to be executed, whether there are any instructions that can be re-ordered in the instruction stream 41 and assigns each such instruction to an instruction completion tracker and includes in the encoding for the instruction an indication of the instruction completion tracker it has been assigned to 42. For each instruction in the instruction stream, an indication of which instruction completion trackers, if any, the instruction depends on is also provided 43, 44. Then, when an instruction that is indicated as being dependent on an instruction completion tracker is to be executed, the status of the relevant instruction completion tracker is checked before executing the instruction.