Execution Paths

6/17/2007 6:19:01 AM

Execution Paths

There were a few items that I didn't show in previous posts on my attempts at visually illustrating execution traces leading to an error or breakpoint.

One is the superimposing of arrows of all possible paths together. For all errors or breakpoints of the same type in the same location with the same call stack, all possible paths leading to the error are shown simultaneously. This has interesting consequences inside various control structures such as finally clauses, loops.

For one thing, this does means NStatic's reported error count will actually be an underestimate of the number of errors it found, as each reported error count represents multiple bugs. On the other hand, it cuts down on false positive rates in which not every possible path will likely be explored in actual use.

There are some reasons for this. For example, there may be an exponential number of paths leading to the error, so it would not be possible for me enumerate all of them. I do plan on providing a later option in a minor update where additional assumptions may applied after the fact to selectively choose which arrows are most important.

In the bug above, there is a warning given for the cases where we may encounter an error. Inactive code in the function is blanked out.

The execution path is general. From within a function, all types of control structures are supported and accurately depicted from arbitrary gotos, throws, loops, finally clauses. Probably, the main limitations are due to asynchronous exception handling (thread aborts, stack overflows, out of memory), which I am still thinking about. The execution path is shown for all functions in the current call stack, but it would not be difficult in the future to add support for examining the traces of previously completed functions that have vanished from the call chain.

The implementation is simple but completely general--a real functional pearl. The execution path is internally represented as an expression with a sequence operator, which is only simplified with current set of assumptions at the time the user views the error. The expression becomes arbitrarily complex with lambda functions and conditionals.






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