1. An information processor for executing a program comprising a plurality of separate program instructions: processing logic operable to individually execute said separate program instructions of said program; an operand store operable to store operand values; and an accelerator having an array comprising a plurality of functional units, said accelerator being operable to execute a combined operation corresponding to a computational subgraph of said separate program instructions by configuring individual ones of said plurality of functional units to perform particular processing operations associated with one or more processing stages of said combined operation; wherein said accelerator executes said combined operation in dependence upon operand mapping data providing a mapping between operands of said combined operation and storage locations within said operand store and in dependence upon separately specified configuration data providing a mapping between said plurality of functional units and said particular processing operations such that said configuration data can be re-used for different operand mappings.