How can I verify that refactoring preserves code flow, not just behavior?

Question

Sometimes, I see if-statements that could be written in a better way. Usually these are cases where we have several layers of nested if-statements and I've identified a simpler way of rewriting the block of if-statements. Of course the biggest concern is that the resulting code will have a different code flow in certain cases.

How can I compare the two code-blocks and determine if the code flow is the same or different? Is there a way to support this analysis with static analysis tools? Are there any other techniques that might help?

Answer 1

Find some way to exercise all possible paths through the code that you want to refactor. You could

• write unit tests by hand
• use Daikon http://plse.cs.washington.edu/daikon/, which exercises code automatically and systematically to infer invariants (I haven't used it myself, but I have tried a commercial descendant targeted at Java)

Either way, use a code coverage tool to verify that you have complete statement and decision coverage. Use a coverage tool that reports the number of times each statement is executed during the coverage run. You might even be able to get trucov, which actually generates diagrams of code paths, to work.

Do your refactoring.

Run the coverage tool again and compare statement execution counts before and after the refactoring. If any statement execution count changed, the flow must have changed. The opposite isn't guaranteed to be true, but it's probably close enough to true for practical applications. Alternatively, if you got trucov to work, compare execution graphs before and after; that would be definitive.