8385728: Serial: Check empty MemRegion in maintain_old_to_young_invariant

[albertnetymk]

Add explicit is_empty() checks before calling clear_MemRegion() and dirty_MemRegion() in maintain_old_to_young_invariant to avoid passing empty MemRegions.

The alternative would be adding early returns in those two shared methods, but inspecting all existing callers shows they never pass empty regions, so I chose to fix the Serial GC caller rather than modify shared code.

Test: tier1-3

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Issue

• JDK-8385728: Serial: Check empty MemRegion in maintain_old_to_young_invariant (Enhancement - P4)

Reviewers

• Thomas Schatzl (@tschatzl - Reviewer)
• Joel Sikström (@jsikstro - Reviewer)
• Guoxiong Li (@lgxbslgx - Reviewer)

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8385728: Serial: Check empty MemRegion in maintain_old_to_young_invariant

Reviewed-by: tschatzl, jsikstro, gli

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At the time when this comment was updated there had been 59 new commits pushed to the master branch:

• 5719b67: 8385369: G1: Concurrent Cleanup For Next Mark accesses uncommitted bitmaps after region uncommit
• cc69221: 8381618: VectorExpressionFuzzer.java: enable test for all platforms
• 9229878: 8385355: NullPointerException in jdk.tools.jlink.internal.ImageResourcesTree after JDK-8377070
• ... and 56 more: https://git.openjdk.org/jdk/compare/886eb11c645b52ec0af9024d5d2c94fe8d3ceea7...master

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[tschatzl]

Lgtm, but I would somewhat strongly consider adding at least an assert(!mr.is_empty()) to both CardTable methods - in the worst case it will crash the VM (passing MemRegion() accidentally.

[albertnetymk]

I would somewhat strongly consider adding at least an assert(!mr.is_empty()) to both CardTable methods

Sure; will do so in a followup PR. Wanna avoid adding risky logic in shared code as part of Serial-only PR.

[jsikstro]

Looks good.

I agree with Thomas' review, it would be nice with asserts in the shared CardTable functions.

[lgxbslgx]

LGTM