2An Execution-Driven Approach to Code Optimisation

Whether code is created by a programmer or generated by a compiler, its construction is traditionally based on a strategy developed before code execution begins. The strategy determines the conditions most likely to occur during execution and optimises code generation accordingly. Execution is non-optimal when the conditions expected by the code generation strategy differ from those actually experienced. An optimisation technique is described here which allows an executing system to be incrementally improved as information about the execution conditions become available. Optimisations are performed by recompiling source code using new code generation strategies and then linking the new code segments into the executing system. The body of data collected or constructed during execution is unaffected, making the technique particularly suited to long-lived, data-intensive systems. This paper describes the technique in general, implementation requirements and a particular instantiation of the technique in which the implementation of polymorphism is optimised. Further examples of the technique are also outlined.