RFR: 8316566: RISC-V: Zero extended narrow oop passed to Atomic::cmpxchg

[Robbin Ehn]

On Tue, 26 Sep 2023 12:19:56 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>> Hi, please consider!
>> 
>> There is bug in gcc < 12 where __synch_synchronize() in some corner-cases don't enforce the compiler barrier.
>> This causes some code to be placed after the __synch_synchronize(), and in this case causing a word to to be not be sign extended as a collateral issue of the bug.
>> You can see the 'bad' assembly in JBS, where a branch is moved over the compiler barrier.
>> 
>> Trying to get information from gcc folks.
>> 
>> It seems like either adding a extra compiler barrier, or use  __atomic_thread_fence(__ATOMIC_SEQ_CST) fixes it.
>> 
>> Tested https://bugs.openjdk.org/browse/JDK-8316186 with this fix.
>> Manually verified assembly, with this fix we generate the same as gcc 12.
>
> Marked as reviewed by luhenry (Committer).

Thank you @luhenry !

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PR Comment: https://git.openjdk.org/jdk/pull/15917#issuecomment-1735511844