RFR: 8276901: Implement UseHeavyMonitors consistently [v4]

[Coleen Phillimore]

On Wed, 17 Nov 2021 20:50:15 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> The flag UseHeavyMonitors seems to imply that it makes Hotspot always use inflated monitors, rather than stack locks. However, it is only implemented in the interpreter that way. When it calls into runtime, it would still happily stack-lock. Even worse, C1 uses another flag UseFastLocking to achieve something similar (with the same caveat that runtime would stack-lock anyway). C2 doesn't have any such mechanism at all.
>> I would like to experiment with disabling stack-locking, and thus, having this flag work as expected would seem very useful.
>> 
>> The change removes the C1 flag UseFastLocking, and replaces its uses with equivalent (i.e. inverted) UseHeavyMonitors instead. I think it makes sense to make UseHeavyMonitors develop (I wouldn't want anybody to use this in production, not currently without this change, and not with this change). I also added a flag VerifyHeavyMonitors to be able to verify that stack-locking is really disabled. We can't currently verify this uncondiftionally (e.g. in debug builds) because all non-x86_64 platforms would need work.
>> 
>> Testing:
>>  - [x] tier1
>>  - [x] tier2
>>  - [x] tier3
>>  - [ ] tier4
>
> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix formatting
>  - Keep UseHeavyMonitors as release flag, but deprecate it

src/hotspot/share/runtime/globals.hpp line 1072:

> 1070:                                                                             \
> 1071:   product(bool, UseHeavyMonitors, false,                                    \
> 1072:           "use heavyweight instead of lightweight Java monitors")           \

For deprecated flags, make the description:
"(Deprecated) Use heavyweight instead of lightweight Java monitors"

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PR: https://git.openjdk.java.net/jdk/pull/6320