8366117: Shenandoah: Notify barrier nodes when transitive inputs change during IGVN

[krk]

ShenandoahLoadReferenceBarrierNode::Identity() determines if a barrier can be eliminated by traversing through intermediate nodes (Phi, DecodeN, CastPP, etc.) via needs_barrier_impl(). When these intermediate nodes change during IGVN, the barrier is not re-evaluated because IGVN only notifies direct users. This causes TestVerifyIterativeGVN to fail with Shenandoah, reporting missed optimization opportunities.

This fix adds a BarrierSetC2::enqueue_dependent_gc_barriers() virtual method that Shenandoah overrides to traverse outputs through the same node types and enqueue dependent barriers for re-evaluation.

There are more barrier eliminations enabled with this, e.g. in the arbitrarily chosen scrabble of renaissance benchmarks:

java -XX:+UseShenandoahGC -Xbatch -Xcomp -XX:-TieredCompilation -jar ~/.cache/stress/renaissance-gpl-0.16.1.jar scrabble -r 1 |& tee /dev/stderr | grep "optimize barrier" | sort | uniq -c | sort -rn

# with fix
236 optimize barrier on call
     75 optimize barrier on alloc
     35 optimize barrier on barrier
     23 optimize barrier on parm
     15 optimize barrier on const
      1 optimize barrier on barrier
      1 optimize barrier on null
TOTAL = 386

# without fix
193 optimize barrier on call
     72 optimize barrier on alloc
     32 optimize barrier on barrier
     23 optimize barrier on parm
      1 optimize barrier on barrier
      1 optimize barrier on null
      1 optimize barrier on const
TOTAL = 323

This requires uncommenting prints with optimize barrier in c2/shenandoahSupport.cpp.

Also tested with make test CONF=linux-x86_64-server-fastdebug TEST=compiler/c2/TestVerifyIterativeGVN.java JTREG="VM_OPTIONS=-XX:+UseShenandoahGC -Xcomp -Xbatch -XX:-TieredCompilation" which passes with the fix and fails without.

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Issue

• JDK-8366117: Shenandoah: Notify barrier nodes when transitive inputs change during IGVN (Bug - P4)

Reviewers

• Cesar Soares Lucas (@JohnTortugo - Committer)

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[JohnTortugo]

I'd prefer if you used a Stack instead of recursion here.

[JohnTortugo]

LGTM. However, note that we are working on Shenandoah late-barrier expansion and once that happens we won't have these Shenandoah-specific IR nodes anymore.

[krk]

Hi! Could I get a review on this one?