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

[Roman Kennke]

On Wed, 13 Apr 2022 14:57:08 GMT, Daniel D. Daugherty <dcubed 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
>
> src/hotspot/share/oops/markWord.hpp line 179:
> 
>> 177:   }
>> 178:   bool has_monitor() const {
>> 179:     return ((value() & lock_mask_in_place) == monitor_value);
> 
> So I'm a bit confused:
> 
> src/hotspot/share/oops/markWord.hpp:
> 
> `static const uintptr_t monitor_value            = 2;`
> 
> In the bug report you say:
>> monitor value is 0b10
> 
> but that not what I'm seeing. What am I missing?

Binary (0b) 10 == decimal 2 ;-)

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