Previously, we relied on a quirk in the ARC optimizer so that we only
need to visit terminators top down. This simplified the dataflow. Sadly,
try_apply changes this since it is a terminator that provides a call
with the value, causing this assumption to break program correctness.
Now during the bottom up traversal, while performing the dataflow for a
block B, we (after visiting all instructions), visit B's predecessors to
see if any of them have a terminator that is a use or decrement. We then
take the most conservative result among all of the terminators and
advance the sequence accordingly.
I do not think that we can have multiple such predecessors today since all
interesting terminators can not have any critical edges to successors. Thus if
our block is a successor of any such block, it can not have any other
predecessors. This is mainly for future proofing if we decide that this is able
to be done in the future.
rdar://23853221
SR-102
(libraries now)
It has been generally agreed that we need to do this reorg, and now
seems like the perfect time. Some major pass reorganization is in the
works.
This does not have to be the final word on the matter. The consensus
among those working on the code is that it's much better than what we
had and a better starting point for future bike shedding.
Note that the previous organization was designed to allow separate
analysis and optimization libraries. It turns out this is an
artificial distinction and not an important goal.
