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

Changes requested by dcubed (Reviewer).

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?

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

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