addRequired vs addRequiredTransitive

I am having a hard time figuring out the difference between addRequired
and addRequiredTransitive. Could someone please clarify the difference?
  Let's say I have a function pass P1 that relies on a function pass P2.
P1 computes some sort of analysis on a function F and P2 uses that
analysis when it processes F. Does that mean that I should use
addRequiredTransitive?

Thanks,
Ryan

An analysis pass has two interesting stages:

1. When it's runOnFunction (or whatever) method is called. Call this the
    'computation' phase of an analysis.
2. After its computation phase when some other pass is using it.

If you are writing some analysis A that needs analysis B, you use one of these two methods. Use addRequired if A uses B in phase #1. Use addRequiredTransitive if A uses B in phase #1 or phase #2.

If this helps, I'd appreciate it if you would create a doc patch to improve the how-to-write-a-pass document with something that would have helped you to understand the situation better.

-Chris