Source-Level Debugging of Compiler-Optimised Code: Ill-Posed, but Not Impossible

Debuggability and optimisation are traditionally regarded as in fundamental tension. This paper disputes that idea, arguing instead that it is possible to compile programs such that they are both fully source-level-debuggable and fully optimised, and that the essential problem to be solved is loss of state. Although these two properties are usually not achievable at the same time, it argues the feasibility of providing the desired one 'on demand', and that metadata-based approaches extended with residual state can do so in a manner that generalises beyond dynamic deoptimisation. Correctness of debugging metadata is introduced as an ill-posed problem, a partial correctness criterion is proposed, and further approaches are discussed.