8272041: [lworld] CDS heap dump fails with primitive objects

[iklam]

Two simple fixes to handle primitive objects when dumping the CDS heap:

• When dumping the objects, we enter them into a hashtable. Do not use oopDesc::identity_hash() as the hashcode for this table.
• When resetting the header of archived objects, do not call oopDesc::identity_hash() on primitive objects.

---

Progress

• Change must not contain extraneous whitespace

Issue

• JDK-8272041: [lworld] CDS heap dump fails with primitive objects

Reviewers

• Frederic Parain (@fparain - Committer)
• Mandy Chung (@mlchung - Committer)

Reviewing

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8272041: [lworld] CDS heap dump fails with primitive objects

Reviewed-by: fparain, mchung

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[mlbridge]

Webrevs

• 00: Full (3dd0d3e1)

[fparain]

Looks good to me.

Just a question by curiosity, using object addresses instead of the identity hash code is likely to change the distribution of keys for the hashtable (all objects being in the same memory area and being aligned on heap words, some bits in their addresses are the same for all objects). Is it going to have a significant impact of the performance of the hashtable or is it negligeable?

Thank you,

Fred

[mlchung]

Looks good. I verified with the patch that converts some JDK classes to ref-default primitive class.

[iklam]

Looks good to me.

Just a question by curiosity, using object addresses instead of the identity hash code is likely to change the distribution of keys for the hashtable (all objects being in the same memory area and being aligned on heap words, some bits in their addresses are the same for all objects). Is it going to have a significant impact of the performance of the hashtable or is it negligeable?

Hi Fred,

Thanks for the review. I've added code to calculate the stats of the hashtable. The max number of entries per bucket is no more than 3, so I think the distribution is acceptable.

Also, the hash function already shifts down by LogBytesPerWord, so it will get rid of the lower 3 zero bits.

My stats code is in https://github.com/iklam/valhalla/commits/heap-shared-seen-obj-table-stats, but it's not part of this PR. I will clean it up and integrate into jdk/jdk.

[iklam]

Thanks @fparain and @mlchung for the review.
/integrate

[openjdk]

Going to push as commit a8994ba.

[openjdk]

@iklam Pushed as commit a8994ba.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

[fparain]

Ioi,

Thank you for the additional effort to measure statistics of the hashtable.

Fred