8264783: G1 BOT verification should not verify beyond allocation threshold

[tschatzl]

The G1 BOT contains an allocation threshold which basically acts as a "last known valid entry" index for the per-region BOT (which are views on the global BOT table).

Currently G1 BOT verification actually "verifies" the BOT within a region past that BOT index.

This causes issues with young regions; actually there is already code that prevents their BOT verification. This is perfectly fine, allocations in young regions do not update the BOTs.

With JDK-8262068/PR #2760 this existing filtering of young regions (such that their BOT is not verified) does not work because there may be young regions that are not compacted (so with a BOT that have that last known valid entry at the start of the region) are still labelled as old.

This change proposes to not try to verify the BOT beyond the last known valid index (which is arguably not worth doing), which also covers the existing young filtering.

There are alternatives that may work in particular for JDK-8262068:
a) always compact young regions (which recreates the BOT)
b) create a "dummy" BOT that spans the entire part of the region containing live objects

However they were rejected by me because
option a) takes time and directly counters that optimization for no reason
option b) makes finding the start of an object within these regions slow (they are not refined when there is at least some bot)

and finally I do not think there is much point in trying to be clever about areas in the BOT that are known to not contain useful values.

Testing: tier1-3

---

Progress

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Issue

• JDK-8264783: G1 BOT verification should not verify beyond allocation threshold

Reviewers

• Hamlin Li (@Hamlin-Li - Reviewer)
• Albert Mingkun Yang (@albertnetymk - Committer)

Reviewing

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[Hamlin-Li]

The change looks good to me.
Thanks Thomas!

[openjdk]

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

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8264783: G1 BOT verification should not verify beyond allocation threshold

Reviewed-by: mli, ayang

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At the time when this comment was updated there had been 66 new commits pushed to the master branch:

• ecef1fc: 8264972: Unused TypeFunc declared in OptoRuntime
• 440c34a: 8264644: Add PrintClassLoaderDataGraphAtExit to print the detailed CLD graph
• b1ebf82: 8264358: Don't create invalid oop in method handle tracing
• 627ad9f: 8262328: Templatize JVMFlag boilerplate access methods
• c15680e: 8264868: Reduce inclusion of registerMap.hpp and register.hpp
• 5784f6b: 8264948: Check for TLS extensions total length
• 42f4d70: 8264649: runtime/InternalApi/ThreadCpuTimesDeadlock.java crash in fastdebug C2 with -XX:-UseTLAB
• 76bd313: 8264872: Dependencies: Migrate to PerfData counters
• 07c8ff4: 8264871: Dependencies: Miscellaneous cleanups in dependencies.cpp
• 863feab: 8005295: Use mandated information for printing of repeating annotations
• ... and 56 more: https://git.openjdk.java.net/jdk/compare/a611c462f938176d8fad303ec7a53191ce85c5d1...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[mlbridge]

Webrevs

• 00: Full (1ab7e29b)

[tschatzl]

Thanks @Hamlin-Li @albertnetymk for your revies.

[tschatzl]

/integrate

[openjdk]

@tschatzl Since your change was applied there have been 67 commits pushed to the master branch:

• cb2806d: 8265018: [AIX] FileDispatcherImpl.c:31:10: fatal error: 'sys/mount.h' file not found
• ecef1fc: 8264972: Unused TypeFunc declared in OptoRuntime
• 440c34a: 8264644: Add PrintClassLoaderDataGraphAtExit to print the detailed CLD graph
• b1ebf82: 8264358: Don't create invalid oop in method handle tracing
• 627ad9f: 8262328: Templatize JVMFlag boilerplate access methods
• c15680e: 8264868: Reduce inclusion of registerMap.hpp and register.hpp
• 5784f6b: 8264948: Check for TLS extensions total length
• 42f4d70: 8264649: runtime/InternalApi/ThreadCpuTimesDeadlock.java crash in fastdebug C2 with -XX:-UseTLAB
• 76bd313: 8264872: Dependencies: Migrate to PerfData counters
• 07c8ff4: 8264871: Dependencies: Miscellaneous cleanups in dependencies.cpp
• ... and 57 more: https://git.openjdk.java.net/jdk/compare/a611c462f938176d8fad303ec7a53191ce85c5d1...master

Your commit was automatically rebased without conflicts.

Pushed as commit e604320.

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