1. A compiler system for converting an application source program into an executable program according to a predetermined instruction set architecture (ISA) that is executable by a general purpose processor implemented according to the predetermined ISA, in which the processor includes a programmable execution unit (PEU) that is programmable to execute a specified user-defined instruction (UDI) received by the processor for execution, said compiler system comprising: a PEU programming tool that converts a functional description of a processing operation to be performed by the PEU of the processor into programming information for programming the PEU to perform the processing operation in response to the specified UDI; and a compiler that converts the application source program into the executable program, wherein said compiler comprises at least one optimization routine that represents a portion of the application source program with the specified UDI and that inserts the UDI into the executable program, and that further inserts into the executable program a UDI load instruction that specifies the UDI and a location of said programming information in the executable program.