RFR: 8276901: Implement UseHeavyMonitors consistently [v2]

[David Holmes]

On Wed, 10 Nov 2021 19:19:13 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:
> 
>   Verify monitors even in non-debug builds

I know nothing about jcstress but I skimmed through it and it seems synchronized is covered in "CHAPTER 2.c". I don't know how we normally run jcstress, but I don't think there is any file that would allow you set the flag. It would really be up to the person running the tests to add a configuration that includes testing UseHeavyMonitors. We have test configuration files that would do that, but I don't think we can add it to our testing due to resource constraints. So I guess we need at least some minimal tier1 test that does a bit of synchronization with UseHeavyMonitors enabled so that we can avoid breaking it.

-------------

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