Pure dynamic casts?

[Jeremie Pelletier]

Daniel Keep wrote:
> 
> Jason House wrote:
>> Dynamic casts are pure. They don't use global state, and have the same output for the same reference as input. Interestingly, dynamic cast results are independent of intervening mutable calls... So there's even greater opportunity for optimization.
> 
> What if the GC just happens to re-use that address for a different object?

I don't know about druntime's GC but I used to have this issue with my 
own GC. I would cast an object to an interface type which would make its 
only reference point to a different 16bytes block, remember that bit 
fields in the GC are aligned to 16bytes, and the allocation had 
therefore no more reachable pointers, or so the GC thought.

What fixed it was to adjust the scan phase and align found pointers to 
the block size of its corresponding allocation. That way the GC won't 
reuse the address of reachable objects no matter what interface or 
subclass they are casted to.