RFR: JDK-8213381: Hook to allow GC to inject Node::Ideal() calls

[Roman Kennke]

It now appears to me that for sake of consistency, we shall probably
provide a similar hook for Identity() (and we might actually have a use
for this in Shenandoah too). This is a bit more involved, because
Identity() seems to be called from a few places around the code. I made
the two PhaseGVN::apply_ideal(..) and PhaseGVN::apply_identity(..)
methods public for this reason. Other than that, same concept as
previous patch:

Incremental:
http://cr.openjdk.java.net/~rkennke/JDK-8213381/webrev.01.diff/
Full:
http://cr.openjdk.java.net/~rkennke/JDK-8213381/webrev.01/

Thoughts?

Roman

> Shenandoah injects a couple of reshapings into some implementations of
> Node::Ideal() (namely CallLeafNode::Ideal() and CmpPNode::Ideal()). I
> propose to provide a hook into BarrierSetC2 to better abstract this.
> This might be useful for other GCs too.
> 
> The approach that I've chosen here is to call BSC2::ideal_node(..) from
> the places in phaseX.cpp where we also call Node::Ideal(..). I've
> introduced a helper method to do this: first call BSC2::ideal_node(..),
> if that returns NULL, call into the usual Node::Ideal(..).
> 
> BarrierSetC2::ideal_node(..) must follow the same contract as
> Node::Ideal(..) of course.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8213381
> Webrev:
> http://cr.openjdk.java.net/~rkennke/JDK-8213381/webrev.00/
> 
> Testing: hotspot/jtreg:tier1 passes locally.
> 
> Can I please get reviews?
> 
> Roman
> 

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