1. Apparatus for processing data under control of a program having program instructions including sequences of individual program instructions corresponding to computational subgraphs within said program, said apparatus comprising: an operand store operable to store operand data; an execution unit coupled to said operand store and responsive to an individual program instruction within said program: (i) to read one or more input operand values from said operand store; (ii) to perform a data processing operation specified by said individual program instruction upon said one or more input operand values to generate one or more output operand values; and (iii) to write said one or more output operand values to said operand store; and an accelerator unit coupled to said operand store and triggered by reaching an execution point within said program corresponding to a sequence of individual program instructions corresponding to a computational subgraph within said program to apply a selected one of a plurality of predetermined sets of configuration data inputs to said accelerator to configure said accelerator to: (v) to read one or more input operands from said operand store; (vi) to perform an accelerated data processing operation specified by said sequence of program instructions upon said one or more input operands to generate one or more output operand values and at least one intermediate operand value being an operand value generated by one of said individual program instructions within said sequence of program instructions and determined not to be referenced outside of said sequence of program instructions; and (vii) to write said one more output operand values to said operand store with said at least one intermediate operand value not being written to said operand store.