RFR: 8276901: Implement UseHeavyMonitors consistently [v10]

[Roman Kennke]

On Wed, 1 Dec 2021 12:36:02 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 one additional commit since the last revision:
> 
>   Use heavy monitors in runtime only on supported architectures

Thank you, Martin!

> I don't like hacking the regular assembler implementations. Better would be to change C2 such that it doesn't generate FastLockNodes. But that may be a bit cumbersome.

That is a good suggestion, and would help ease the work in the backend.

I believe you still have to change something in sharedRuntime_ppc.cpp, similar to what I did in, e.g., sharedRuntime_aarch64.cpp.

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