How to detect whether we're inside of a loop??

["Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>]

I was interested in experimenting with heuristics which might expand a
builtin depending on whether at the point of expansion we're in a loop
or not.  E.g. It might make sense to expand something which might
create slightly larger code only if we think it's likely that it'll be
executed a number of times.  I'm guessing that being inside a loop is
one way to increase that probability.  Thus I was curious if there's a
way to detect this.  So my questions are:

1.  Can we detect this?  (C'mon there's gotta be a way.)

2.  Is it cheap to detect or a heavy calculation?  Ie. I don't want to
    have to do an expensive optimization pass just to figure this out.
    I'm hoping that I can just access a bit somewhere that tells me
    yes or no.

3.  Is it practical to detect this at the point I'm interested in,
    ie. inside an expand_builtin_foo function?

4.  Is there another way to conceptually do what I want, ie. detect
    that the code in question is likely to be executed more than once?

5.  Can you explain it all simply enough for me to understand? :-)

		Thanks,
		--Kaveh
--
Kaveh R. Ghazi			ghazi at caip dot rutgers dot edu