The sequence of instructions for a shader program 60 to be executed by a shader core of a graphics processor is divided into an initial set of instructions 61 that perform “global” common expressions of the shader program, a set of instructions 62 in the shader program that perform expressions that are common to a given work group within a set of work items that the shader program is to process, and a main instruction sequence 63 that needs to be executed independently for each work item.Execution threads are then able to start executing the shader program either at the beginning of the global common expressions 64, or at the beginning of the work group common expressions 65, or at the beginning of the main instruction sequence 66.