Monday, August 20, 2007

Real-world trace analysis

Robert O'Callahan has just released a debugger called Chronomancer, which is based on a technology he's been working on for a little while: Chronicle is a database for storing and querying program events, built on top of Valgrind, a dynamic analysis tool that instruments Linux binaries to monitor their runtime behavior. Chronomancer is a C/C++ debugger deployed as an Eclipse plug-in. There's a rough screencast here, though I didn't follow everything he was doing.

One of the cooler aspects of the debugger is the idea of history-based stack reconstruction. There's a tug-of-war that goes on between debuggers and compilers: compilers want to mess with data structures and control that is not supposed to be observable to programs, and debuggers want to observe them! As a result, there's constant disagreement or confusion about the interface between the two, resulting in impartial stack information or corrupted debugging information. Chronicle avoids these issues by relying on the history of events, rather than the stack layout, to determine what calls have happened when, and which ones have returned. This means, for example, that tail calls don't interfere with stack traces, because stack traces are based on program history instead of continuations.

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