RFR: 8291555: Implement alternative fast-locking scheme [v27]

[Thomas Stuefe]

On Thu, 16 Mar 2023 08:36:45 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - More RISCV changes (by Fei Yang)
>>  - Use -w instructions in fast_unlock()
>>  - Increase stub size of C2HandleAnonOwnerStub to 18
>
> I like -XX:+UseNewLocks, too. I wouldn't overcomplicate things: this flag is meant to be transitional, it is not meant to be used by end-users (except the bravest nerds) at all. When it lands, the Lilliput flag (e.g. +UseCompactObjectHeaders) will also control the locking flag. Eventually (e.g. release+1) both flags would become on by default and afterwards (e.g. release+2) would go away entirely, at which point the whole original stack-locking would disappear.

@rkennke I must be missing something. In aarch64, why do we handle the non-symmetric-unlock case in interpreter, but not in C1/C2? There, we just seem to pop whatever is on top.

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PR: https://git.openjdk.org/jdk/pull/10907