Building a debugger for a high-level language on top of a low-level recording is unexplored territory but it's definitely possible and it would have some nice features. However, you can't get much leverage from any existing debugging support built into a VM.
We could build some JS debugging support on top of rr like this: 1) Add support to extract JS program state from a stopped Firefox process, by reading memory and register contents. Basically, completely passive stack walking and value/heap inspection. There are two sub-approaches that can be mixed and matched here: * Duplicating some functionality of the JS engine into the debugging layer so it can run outside the stopped Firefox process. * Add a way to interpret JS engine machine code *as if* it was running in the context of the stopped process. Side effects (e.g. memory writes) would be buffered temporarily and thrown away once we've got the results we need. Basically, this would let us support running user code for debugging purposes during an rr replay. 2) Add support for JS breakpoints. Map a JS breakpoint to a conditional breakpoint in the interpreter C++ code, or one (or more) locations in compiled code. This mapping may be possible just by inspecting VM state, or we may need to monitor VM execution (e.g. compilation) to maintain this mapping during replay. I think if we can accept some heuristics and approximations here, it could work OK. E.g., just setting breakpoints at the beginning of non-inlined functions and at loop heads would go quite a long way. That's makeshift. Ultimately you want to take a completely different approach to debugging both JS and C++, e.g. by building something like Chronicle + Chronomancer on top of rr. But, baby steps. Rob -- Jtehsauts tshaei dS,o n" Wohfy Mdaon yhoaus eanuttehrotraiitny eovni le atrhtohu gthot sf oirng iyvoeu rs ihnesa.r"t sS?o Whhei csha iids teoa stiheer :p atroa lsyazye,d 'mYaonu,r "sGients uapr,e tfaokreg iyvoeunr, 'm aotr atnod sgaoy ,h o'mGee.t" uTph eann dt hwea lmka'n? gBoutt uIp waanndt wyeonut thoo mken.o w _______________________________________________ dev-platform mailing list [email protected] https://lists.mozilla.org/listinfo/dev-platform
