RFR: 8214363: HeapWord should not be a fake class

[Kim Barrett]

> 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).