8358339: Handle MethodCounters::_method backlinks after JDK-8355003

[shipilev]

Found this when reading mainline-vs-premain webrev. JDK-8355003 introduced a backlink to Method* in MethodCounters. I believe we need to handle that backlink at least in CodeBuffer::finalize_oop_references(). premain does this, while mainline does not. Also, amusingly, we have MethodCounters::is_methodCounters, but not the super-class Metadata::is_methodCounters.

I pulled in the hunks that use is_methodCounters() and MethodCounters::method() from premain into this PR.

Additional testing:

• Linux x86_64 server fastdebug, runtime/cds
• Linux x86_64 server fastdebug, tier1
• Linux x86_64 server fastdebug, all

---

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Issue

• JDK-8358339: Handle MethodCounters::_method backlinks after JDK-8355003 (Bug - P4)

Reviewers

• Coleen Phillimore (@coleenp - Reviewer)
• Vladimir Kozlov (@vnkozlov - Reviewer)
• Igor Veresov (@veresov - Reviewer)

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8358339: Handle MethodCounters::_method backlinks after JDK-8355003

Reviewed-by: coleenp, kvn, iveresov

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At the time when this comment was updated there had been 77 new commits pushed to the master branch:

• 77c110c: 8357000: Write overview documentation for start of release changes
• 901144e: 8358217: jdk/incubator/vector/PreferredSpeciesTest.java#id0 failures - expected [128] but found [256]
• 8f82117: 8358170: Repurpose testCompat in test/jdk/java/util/TimeZone/Bug8167143.java
• ... and 74 more: https://git.openjdk.org/jdk/compare/a7671e7360ce23df9577110266e66a1c1a790196...master

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[mlbridge]

Webrevs

• 00: Full (a518af9b)

[shipilev]

Actually, I am not sure if it is even a bug, because mainline is using MethodCounters::method() any reasonably only in MethodCounters::metaspace_pointers_do(). But I guess it would be good to make sure we handle this backlink consistently.

[coleenp]

I don't think this is the right thing to do, since the Method* is already handled in finalize_oop_references since it's a backpointer.

[coleenp]

And MethodCounters shouldn't be inhertited from Metadata, they're inherited from MetaspaceObj in mainline. We want to avoid virtual function pointers in this type.

[shipilev]

I don't think this is the right thing to do, since the Method* is already handled in finalize_oop_references since it's a backpointer.

Sorry, I don't understand this comment. I think there is a symmetry between MethodCounters and MethodData. Now that MethodCounters have the backpointer to Method*, like MethodData, it should be handled like MethodData everywhere?

And MethodCounters shouldn't be inhertited from Metadata, they're inherited from MetaspaceObj in mainline. We want to avoid virtual function pointers in this type.

Are you, perhaps, looking at older mainline? Because in current mainline MethodCounters is inherited from Metadata:

jdk/src/hotspot/share/oops/methodCounters.hpp

Line 35 in 78a392a

class MethodCounters : public Metadata {

-- this was also part of JDK-8355003.

[vnkozlov]

Good. This will be needed for AOT caching Level2 C1 compiled nmethods which have profiling: vnkozlov@4659523

[shipilev]

Before I proceed anywhere with this, I need to understand what @coleenp saw in all this :)

[coleenp]

My repo was two weeks old so I didn't see this change to give MethodCounters a vptr, and don't know why. At worst the backpointer to Method* in MethodCounters is redundant with the Method* that you're creating the oop_references for, but it shouldn't create two oops.
ie, md == ((MethodCounter*)m)->method();
But maybe that's not the case here.

[shipilev]

OK, phew. I thought I am not seeing some huge gap here. Thanks!
I think we are ready to integrate this. Just checking if @veresov is also okay with it.

[veresov]

Sure why not, we'll eventually need it.

[shipilev]

Cool, here goes.

/integrate

[openjdk]

Going to push as commit 3cf3e4b.
Since your change was applied there have been 77 commits pushed to the master branch:

• 77c110c: 8357000: Write overview documentation for start of release changes
• 901144e: 8358217: jdk/incubator/vector/PreferredSpeciesTest.java#id0 failures - expected [128] but found [256]
• 8f82117: 8358170: Repurpose testCompat in test/jdk/java/util/TimeZone/Bug8167143.java
• ... and 74 more: https://git.openjdk.org/jdk/compare/a7671e7360ce23df9577110266e66a1c1a790196...master

Your commit was automatically rebased without conflicts.

[openjdk]

@shipilev Pushed as commit 3cf3e4b.

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