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.

The song that doesn't end

M Ward's song Chinese Translation sounds like an MIT AI koan from the 70's:

I met an old, old man
beneath a weeping willow tree
He said now if you got some questions
go and lay them at my feet...

And I said
What do you do with the pieces of a broken heart...

...and then the sun went down
and he sang for me this song

See I once was a young fool like you...
I met an old, old man
he sat beneath a sapling tree
He said now if you got some questions
go and lay them at my feet...

And I said
What do you do with the pieces of a broken heart...

The song ends before we reach a base case, but notice the trees get smaller, so there might be an inductive measure after all.

ECMAScript's new home

Thanks to a generous domain-name donation from OpenDomain, the new home of ECMAScript is http://www.ecmascript.org! The previous domains we created for it now redirect to the new domain.