8376813: [lworld] Add print for dummy field reused as null-marker

[jsikstro]

Hello,

Right now it's not clear that the dummy field that is injected into empty inline klasses can be/is reused for the null-marker when looking at the printed layout from -XX:+PrintInlineLayout. I suggest we enhance this print to indicate if the dummy field has been reused for the null-marker.

I've tested that the added comment in the print is there when nullability is turned on (default), and not there when turned off (-XX:-UseNullableValueFlattening -XX:-UseNullableNonAtomicValueFlattening).

Before:
Instance fields:
 @0 RESERVED 12/-
 @12 REGULAR 1/1 ".empty" B

After:
Instance fields:
 @0 RESERVED 12/-
 @12 REGULAR 1/1 ".empty" B (reused as null-marker)

Testing:

• GHA
• Oracle's tier1

---

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Issue

• JDK-8376813: [lworld] Add print for dummy field reused as null-marker (Enhancement - P4)

Reviewers

• Frederic Parain (@fparain - Committer) ⚠️ Review applies to a94f1bf5
• Paul Hübner (@Arraying - Committer)

Reviewing

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[openjdk]

@jsikstro This change now passes all automated pre-integration checks.

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8376813: [lworld] Add print for dummy field reused as null-marker

Reviewed-by: fparain, phubner

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At the time when this comment was updated there had been 116 new commits pushed to the lworld branch:

• 2b06c93: 8372954: [lworld] Remove old implementation of the substitutability test method
• 750a2f9: 8375547: [lworld] Dependency verification fails with assert "Should have been marked for deoptimization"
• c09d99d: 8376702: [lworld] MemNode::optimize_simple_memory_chain asserts with "invalid node class: Con"
• ... and 113 more: https://git.openjdk.org/valhalla/compare/09b557cceafeedcdf1933ef27bb33a6cb1f0069a...lworld

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[mlbridge]

Webrevs

• 01: Full - Incremental (87849f24)
• 00: Full (a94f1bf5)

[fparain]

Looks good to me.
It's unfortunate that the null marker offset is stored in the FieldLayoutBuilder and not in FieldLayout, this would have prevented adding an argument to the print method.

[Arraying]

Thanks for the changes, this makes debugging and developing a lot easier. One thing that came to mind is that we have a bunch of flat array specific printing logic in e.g. FlatArrayKlass::oop_print_value_on. As a possible extension, would we ever want to convey such information in the array itself, rather than the field? For example when debug-printing the human-version of the array header to indicate the null marker is reused as the dummy field (when applicable)?

[jsikstro]

Thanks for the changes, this makes debugging and developing a lot easier. One thing that came to mind is that we have a bunch of flat array specific printing logic in e.g. FlatArrayKlass::oop_print_value_on. As a possible extension, would we ever want to convey such information in the array itself, rather than the field? For example when debug-printing the human-version of the array header to indicate the null marker is reused as the dummy field (when applicable)?

I'm not entirely sure of how arrays work so I'd need to spend some time on figuring that out before giving a better answer. When trying to understand emtpy InlineKlasses, it wasn't immediately clear to me that the empty dummy-field was reused as the null-marker, so I'd say anything we can do to convey that more clearly is something we'd want to do.

[jsikstro]

Thank you for the reviews everyone! I reran tier1-2 on the tip of lworld, which is green.

/integrate

[openjdk]

@jsikstro
Your change (at version 87849f2) is now ready to be sponsored by a Committer.

[Arraying]

/sponsor

[openjdk]

Going to push as commit ff2e209.
Since your change was applied there have been 116 commits pushed to the lworld branch:

• 2b06c93: 8372954: [lworld] Remove old implementation of the substitutability test method
• 750a2f9: 8375547: [lworld] Dependency verification fails with assert "Should have been marked for deoptimization"
• c09d99d: 8376702: [lworld] MemNode::optimize_simple_memory_chain asserts with "invalid node class: Con"
• ... and 113 more: https://git.openjdk.org/valhalla/compare/09b557cceafeedcdf1933ef27bb33a6cb1f0069a...lworld

Your commit was automatically rebased without conflicts.

[openjdk]

@Arraying @jsikstro Pushed as commit ff2e209.

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