The problem with Google Code Search and the one that you mentioned is, you can never be certain the match is relevant/correct. And the reason for this is, you a

The problem with Google Code Search and the one that you mentioned is, you can never be certain the match is relevant/correct. And the reason for this is, you are always searching against a single branch/timeline. Definitions can change from one release to another. Function behavior can change from one release to another and so forth. Searching across millions of repos is not difficult, if you are willing to accept the matches returned may or may not be relevant. What is extremely difficult, is building a search solution that can take into consideration all the changes, on every branch, from the millions of repos. And this is the problem nobody is going to solve anytime soon.