1. A data processing apparatus comprising: a register data store operable to store data elements; an instruction decoder operable to decode an instruction with generated constant, said instruction having a data value associated therewith; a data processor operable to perform data processing operations within parallel processing lanes on at least one source operand in response to a data processing instruction decoded by said instruction decoder; and said data processor being operable in response to said decoded instruction with generated constant and associated data value to expand at least a data portion of said associated data value, said expansion being performed in response to said instruction with generated constant and depending on a selected function, to generate a constant, said generated constant forming one of said at least one source operands.