[lworld] RFR: 8303416: [lworld] Fix JVM crash at Unsafe_FinishPrivateBuffer [v2]

[Tobias Hartmann]

On Fri, 3 Mar 2023 08:37:39 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> When calling Unsafe.finishPrivateBuffer(), JVM crashes with following
>>  assertion failure:
>> 
>> 
>>  Internal Error (/mnt/local/code/valhalla/src/hotspot/share/prims/unsafe.cpp:388), pid=29517, tid=29518
>>   assert(v->mark().is_larval_state()) failed: must be a larval value
>> 
>>  JRE version: OpenJDK Runtime Environment (21.0) (fastdebug build 21-internal-git-05e4d0dba)
>>  Java VM: OpenJDK 64-Bit Server VM (fastdebug 21-internal-git-05e4d0dba, mixed mode, compressed oops, compressed class ptrs, g1 gc, linux-aarch64)
>>  Problematic frame:
>>  V  [[libjvm.so](http://libjvm.so/)+0x1a92a64]  Unsafe_FinishPrivateBuffer+0xc0
>> 
>> The assertion is used to check whether the input value is in larval
>>  state. Usually this method is called after `Unsafe.makePrivateBuffer()`,
>>  which will create a new oop and set it to larval state. If everything
>>  works fine, the above assertion won't fail. But the issue happens if
>>  all the fields of the input value object of `Unsafe.makePrivateBuffer()`
>>  are the relative java default values (i.e. `0`). The root cause is the
>>  C2 compiler will optimize the new buffered oop to the default oop for
>>  such value object, and the larval state is missed during the optimization.
>>  Marking the larval bit of the optimized oop can fix the issue.
>
> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert the larval bit setting change, and discard the default oop replacement for larval state oop

Looks good to me!

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Marked as reviewed by thartmann (Committer).

PR: https://git.openjdk.org/valhalla/pull/828