RFR: 8292082: Deprecate UseRTM* for removal

[Roman Kennke]

On Tue, 9 Aug 2022 16:34:47 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> HotSpot supports RTM (restricted transactional memory) to be used for locking and deoptimization. RTM has since been disabled in Intel processors due to security vulnerabilities [0] and IBM removed support for it since its Power 10 line. RTM adds unnecessarily to complexity and maintenance burden.
>> 
>> I would like to propose to deprecate the relevant flags for removal, and actually remove the flags and all related code in a later release, unless somebody comes up with a good reason and performance comparison to show that it's worth keeping.
>> 
>> [0] https://en.wikipedia.org/wiki/Transactional_Synchronization_Extensions#History_and_bugs
>> 
>> Testing:
>>  - [x] runtime/CommandLine/VMDeprecatedOptions.java
>>  - [x] tier1
>
> Good.
> 
> @sviswa7 do you agree with deprecating this feature?

> @vnkozlov @rkennke RTM feature is available on Intel ICX as well as upcoming Xeon platforms.

Ok, interesting. I did not know that.
Is our RTM locking implementation still useful, though? Does it have advantages over locking without RTM, and if so, what are they?

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PR: https://git.openjdk.org/jdk/pull/9810