[9] RFR(S): 8174164: SafePointNode::_replaced_nodes breaks with irreducible loops

[Roland Westrelin]

>>> Also what happens for the rest of replacing nodes on the list?
>>
>> They stay on the list. I suppose I could prune the list when it's
>> propagated from a callee to a caller.
>
> Can you put old nodes pruned from list on IGVN list so they have chance 
> to be optimized later?

Actually, no, thinking more about this, I don't see how the list can be
pruned more than it is today. The code collects (initial, improved)
pairs of nodes every time replace_in_map is called. Then, at parse time,
on method exit, the list of pairs is passed to the caller and nodes on
the current safepoint state are replaced. ReplacedNodes::transfer_from()
already has some pruning (pairs for which initial is a new node in a
callee are not passed to the caller). I don't see what other nodes can
be removed because we're sure they can't be candidates for replacement
in callers. I don't think wether improved is new in a callee tells us
anything about whether it can be replaced in a caller or not. Also, I
don't think I understand why we would want to push nodes on the igvn
list: assuming all those nodes are control dependent CheckCastPP or
CastPP nodes, why do we think they can be optimized further at this
point?

Roland.