Apart from that, I'm not surprised you find your code as shown to be slow... it has an infinite loop! You should recast it as something like: ...

Um. No? I've played around with my implementation a couple of different ways. I've not yet encountered it failing to terminate with the desired result.

update: I didn't realise at first that you only cared about the depth, not what's at each level. In which case, the following snippet will be pretty efficient (since it is blockless):

Ah, yeah that's a little faster, about 15% faster than the version with the block.

By the way, in benchmarking these two methods I found that getting the stack depth (either way) is really quite fast, I had to do a million repetitions of my test code (each run had a stack depth of 3). That's much less overhead than my naive expectations would have predicted.

Update: I played around with the two versions a little more and found that really the difference between the block and the inline versions is only about 4%. My block implementation had some extra overhead unrelated to the 'blockiness' of it.