There is provided a method for executing a program comprising a function call and one or more subsequent instructions. The method comprises processing, on a first thread, a function defined by the function call, the function having one or more programmer predefined typical return values. For each predefined return value, the one or more subsequent instructions are pre-processed on an additional thread assuming that the function returned that pre-defined return value. In this way the processor, on completion of processing said function is able to make use of the pre-processing completed by the additional thread which used the actual return value.