8383912: Deduplicate klass fetching and decoding in oopDesc

[stefank]

I've been investigating the regression reported in JDK-8382041, which is caused by JDK-8363996: "Obsolete UseCompressedClassPointers". While doing that I've found that calls to oopDesc::klass() bloat the code and that affects inilining, which cause performance regressions in the GC.

A localized fix for JDK-8382041, is to deduplicate multiple calls to klass() into fewer calls. That fixes the imminent performance issue. However, while looking into klass() I see that there's opportunity to somewhat simplify the implementation, and that seems to have almost the same level of performance improvements. I propose that we make that change because it both makes the code smaller, cleaner, and faster.

The performance was measured with the org.openjdk.bench.vm.gc.systemgc.AllLive.gc microbenchmark + UseZGC.

I will run tier1-tier3

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Integration blocker

 ⚠️ Dependency #31053 must be integrated first

Issue

• JDK-8383912: Deduplicate klass fetching and decoding in oopDesc (Enhancement - P4)

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)
• Albert Mingkun Yang (@albertnetymk - Reviewer)

Reviewing

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[mlbridge]

Webrevs

• 01: Full - Incremental (f32d4eb9)
• 00: Full (6b373ce3)

[shipilev]

Looks reasonable, but:

[shipilev]

Right, this makes sense. Kim would probably crash into this PR and say _compressed_klass should probably be Atomic in itself then :)

[stefank]

Right, this makes sense. Kim would probably crash into this PR and say _compressed_klass should probably be Atomic in itself then :)

Probably. :) I used AtomicAccess here because that is what is used inside oopDesc::release_set_klass.