RFR: JDK-8212603: Need to step over GC barriers in Node::eqv_uncast()

Roman Kennke rkennke at redhat.com
Wed Oct 17 20:33:37 UTC 2018

Thanks Erik for reviewing!


> Hi Roman,
> Looks good.
> Thanks,
> /Erik
> On 2018-10-17 19:54, Roman Kennke wrote:
>> Node::eqv_uncast() checks if two nodes are equal (equivalent) behind
>> casts. It's used in many places concerning lock elimination. The trouble
>> is if the actual nodes are behind GC barriers, and we get the same node
>> behind two different GC barrier nodes, this would return false negative.
>> We have seen a bad case of this with Shenandoah, where lock elimination
>> was subtly thrown off by this, which led to eliminated locks not
>> re-locked properly during deoptimization.
>> I propose to also strip any possible GC barriers like this:
>> http://cr.openjdk.java.net/~rkennke/JDK-8212603/webrev.00/
>> Testing: hotspot/tier1, will push through jdk/submit in a bit
>> Roman

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