Wednesday, 29 June 2016

Relearning Debugging With rr

As I've mentioned before, once you have a practical reverse-execution debugger like rr, you need to learn new debugging strategies to exploit its power, and that takes time. (Almost all of your old debugging strategies still work --- they're just wasting your time!) A good example presented itself this morning. A new rr user wanted to stop at a location in JIT-generated code, and modified the JIT compiler to emit an int3 breakpoint instruction at the desired location --- because that's what you'd do with a regular debugger. But with rr there's no need: you can just run past the generation of the code, determine the address of your generated instruction after the fact (by inserting a logging statement at the point where you would have triggered generation of int3, if you must), set a hardware execution breakpoint at that address, and reverse-execute until that location is reached.

One of the best reasons I've heard for not using rr was given by Jeff: "I don't want to forget how to debug on other platforms".

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