1. Apparatus comprising: processor circuitry configured to execute data processing instructions, wherein the processor circuitry is configured to maintain a call stack data structure in dependence on the data processing instructions it executes, and wherein the processor circuitry is configured to operate in a transactional execution mode when the data processing instructions executed access stored data items shared with further processor circuitry; and pre-transactional stack pointer storage circuitry configured to store a stack depth indication for the call stack data structure, wherein the processor circuitry is configured to store the stack depth indication prior to entering the transactional execution mode, wherein the processor circuitry is configured, when operating in the transactional execution mode, to determine a relative stacking position for a modification to the call stack data structure with respect to the stack depth indication, and if the relative stacking position is in a positive stack growth direction with respect to a position indicated by the stack depth indication, to store in association with the modification an indication that the modification is non-speculative, and if the relative stacking position not in a positive stack growth direction with respect to the position indicated by the stack depth indication, to store in association with the modification an indication that the modification is speculative.