Source-level debugging of compiled code only works when compilers generate the necessary metadata. Currently, that means it rarely works well, at least in optimising ahead-of-time compilers like LLVM and GCC. I'll give an overview of how compiler-generated metadata enables source-level debugging, the challenges of making it work for optimised code, and our recent work on doing better. Whereas compilers have so far taken a "best-effort" approach with no particular correctness criterion, I'll outline a correctness condition for local variable information that seems to balance the relevant trade-offs. I'll then describe a tool we've built that can use this to mechanically find valid LLVM bugs capturing avoidable losses or corruptions of debug info. A theme will be how the textbook framing of compiler optimisations as "eliminating" code or variables could be more constructively thought of as "residualising" them into debug info; I'll finish with some thoughts on what that could mean for how compilers are built. All this is joint work with J. Ryan Stinnett.

Stephen Kell does practical research on programming systems, with the the goal of making computers work for human beings and not vice-versa, and a focus on infrastructure software including operating systems and language runtimes. Some past and present research topics include: realistic formal and metaprogrammable specifications of operating systems' linking, loading and system call interfaces; reflective run-time services in Unix-like processes; using the latter to provide new kinds of dynamic checking in C and other 'unsafe' languages; and making debugging of optimized code more reliable. Currently he is a Lecturer (Assistant Professor) in Computer Science at King's College London.