RFR: 8284816: Make markWord::has_monitor() more robust

[Daniel D.Daugherty]

On Wed, 13 Apr 2022 09:59:49 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Currently, markWord::has_monitor() is implemented like this:
> 
> `    return ((value() & monitor_value) != 0);`
> 
> monitor value is `0b10`. This means that it also reports marked or forwarded objects (`0b11`) as having a monitor, which is wrong. As far as I can tell, it does not cause any problems because relevant code is either not affected by marked/forwarded objects, or by testing bits in an order that hides the problem.
> 
> I suggest to test the bits properly to make it more robust and avoid potenial future bugs.
> 
> Testing:
>  - [x] tier1
>  - [ ] tier2
>  - [ ] tier3

I agree that the new code is more precise.

Thumbs up.

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Marked as reviewed by dcubed (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/8219