1. A method of processing kernel invocations in a system where a kernel to be executed has defined for it a set of plural index dimensions over which separate invocations of the kernel are to be executed, the method comprising: for a kernel to be executed having a defined set of index dimensions: determining the number of bits needed to represent a maximum value to be indicated for each dimension of the kernel's index; determining a bitfield index representation to be used for indicating the indices for invocations of the kernel based on the determined number of bits needed to represent the maximum value to be indicated for each dimension of the kernel's index; preparing a bit field placement data structure for the kernel, indicating the placement of the bits representing each dimension of the kernel's index within the determined bitfield index representation to be used for indicating the indices for invocations of the kernel; providing the bitfield placement data structure to a processor or processors that are to execute the kernel; and when a kernel invocation is to be executed, associating with the kernel invocation a bitfield index representation configured in accordance with the bitfield placements indicated by the bitfield placement data structure and indicating in the respective bit positions indicated by the bitfield placement data structure, a value for the respective index dimension for the kernel invocation, so as to indicate a kernel invocation index for the kernel invocation.