RFR: 8229258: Rework markOop and markOopDesc into a simpler mark word value carrier

[coleen.phillimore at oracle.com]

https://cr.openjdk.java.net/~stefank/8229258/webrev.valueMarkWord.03.delta/src/hotspot/share/runtime/synchronizer.cpp.udiff.html

+ lock->set_displaced_header(markWord::from_pointer(NULL));


Should this be markWork::zero ?

+ m->set_header(markWord(markWord::zero));


ew.

I just clicked through the latest incremental and the full oops directory.

The to_pointer and from_pointer casts are awkward, particularly because 
you can do (Edge*)mark.value() too, but it's good to find where we're 
using the markWord for various purposes.

This is a massive improvement.  Dont' change a thing and check it in.  
We can do minor cleanups in followup RFEs.

Thanks!
Coleen


On 8/15/19 7:46 AM, Stefan Karlsson wrote:
> Thanks Kim, Roman, Dan and Coleen for reviews and feedback.
>
> I rebased the patch, fixed more alignments, renamed the bug, and rerun 
> the test through tier1-3.
>
> https://cr.openjdk.java.net/~stefank/8229258/webrev.valueMarkWord.03.delta/ 
>
> https://cr.openjdk.java.net/~stefank/8229258/webrev.valueMarkWord.03/
>
> Could I get reviews for this version? I'd also like to ask others to 
> at least partially look at this:
>
> 1) Platform maintainers probably want to run this patch through their 
> build system.
> 2) SA maintainers (CC:ed serviceability-dev)
> 3) JVMCI maintainers
>
> Thanks,
> StefanK
>
> On 2019-08-14 11:11, Roman Kennke wrote:
>>
>> Am 14.08.19 um 01:26 schrieb Kim Barrett:
>>>> On Aug 12, 2019, at 12:19 PM, Stefan Karlsson 
>>>> <stefan.karlsson at oracle.com> wrote:
>>>>
>>>> Hi Roman,
>>>>
>>>> Kim helped me figuring out how to get past the volatile issues I 
>>>> had with the class markWord { uintptr_t value; ... } version. So, 
>>>> I've created a version with that:
>>>>
>>>> https://cr.openjdk.java.net/~stefank/8229258/webrev.valueMarkWord.01/
>>>>
>>>> I can go with either approach, so let me now what you all think.
>>> I've finally had time to look at the first proposed change.
>>>
>>> Comparing the first approach (an AllStatic MarkWord class and markWord
>>> typedef'd to uintptr_t) vs the second approach (markWord is a thin
>>> class wrapping around uintptr_t), I prefer the second.
>>>
>>> * I think the markWord class provides better type safety. It still
>>> involves too many casts sprinkled over the code base, but I think it
>>> also provides a better basis for further cast reduction and
>>> prevention.
>>>
>>> * I think having one markWord class for the data and behavior is
>>> better / more natural than having a markWord typedef for the data and
>>> a MarkWord AllStatic class for the behaviour.
>>>
>>> * I like that the markWord class eliminates the markWord vs MarkWord
>>> homonyms, which I think will be annoying.
>>>
>>> * The markWord class is a trivially copyable class, allowing it to be
>>> efficiently passed around by value, so no disadvantage there.
>>>
>>> I haven't found anything that I think argues for the first over the
>>> second. Other folks might have different priorities or taste. I think
>>> either is better than the status quo.
>>>
>>> I'm still reviewing webrev.valueMarkWord.02, but so far haven't found
>>> anything that makes me want to suggest backing off from that direction.
>>>
>>> Note that the bug summary doesn't describe the second approach.
>> +1 :-)
>>
>> Roman
>>
>