RFR: 8214363: HeapWord should not be a fake class

[Mikael Vidstedt]

> On Feb 15, 2019, at 12:44 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Feb 15, 2019, at 3:27 PM, coleen.phillimore at oracle.com wrote:
>> 
>> 
>> moving to hotspot-dev.
>> Coleen
>> 
>> On 2/15/19 8:07 AM, coleen.phillimore at oracle.com wrote:
>>> 
>>> This is nice but what does this mean?
>>> 
>>> http://cr.openjdk.java.net/~kbarrett/8214363/open.02/src/hotspot/share/gc/shared/spaceDecorator.cpp.udiff.html 
>>> 
>>> + return badHeapWord == (max_juint & reinterpret_cast<uintptr_t>(*q));
>>> 
>>> 
>>> I thought since it has no implementation class, you can't use *q ?
> 
> The type of q is HeapWord*, which is (with this change) HeapWordImpl**.
> Dereferencing q => HeapWordImpl*, which is then cast to a uintptr_t.
> 
> A pretty similar effect could have been achieved by changing HeapWord to
> struct HeapWord { HeapWordImpl* value; };
> e.g. making the member public (to eliminate the new gcc warning) and changing
> the pointee type of the member to an incomplete type (to prevent dereference).

I was also confused by that. Not enough to have a strong opinion though.

Looks good to me. For completeness I too tried your change with gcc 8.2 and the warnings are indeed gone.

Cheers,
Mikael