Does the spec preclude objects being movable by the GC?

[Vladimir Panteleev]

On Monday, 25 February 2013 at 23:17:16 UTC, Ben Davis wrote:
> On 25/02/2013 23:15, Ben Davis wrote:
>> Unless I'm mistaken, this page:
>>
>> http://dlang.org/garbage.html
>>
>> states that it's safe to use a union to share storage with a 
>> pointer, e.g.
>>
>> union U { void* ptr; int value }
>>
>> But that wouldn't be safe if the GC ever moved objects. Should 
>> the page
>> be tweaked? Especially as further down on that page, it says 
>> that a
>> moving garbage collector can be implemented if one follows the 
>> rules.
>
> Actually I suppose it's possible for any object affected by 
> that union to be pinned. Is that the intention?

As I understand it, there will always be uncertain references, 
such as those found on the stack (esp. stack of C functions) and 
data segment. Such uncertain references will pin objects they 
point at, however ideally this would only apply to a small 
percentage of heap objects. A moving GC in D would only move 
those objects for which it can be certain that it knows how to 
safely update all existing references to the object.