My Language Feature Requests

[Christopher Wright]

Frits van Bommel wrote:
> Christopher Wright wrote:
>> While there is a fixed reference to the GC object, it is pinned. If 
>> that reference is rebound to another GC object, the original object is 
>> unpinned and the new one is pinned.
>>
>> How to mark these is a difficult problem. On a 64-bit machine, I'd say 
>> you just use the most significant bit as a flag; you're not going to 
>> use petabytes of address space.
> 
> Since "fixedness" as proposed would be a compile-time property, and you 
> already need metadata to find pointers to implement a moving GC, such a 
> flag could be in that metadata instead of in the pointer itself. (The 
> OffsetTypeInfo could say "there's a pointer at offset 8, of type Object, 
> and it's fixed")

Yes, I thought of that. Currently, however, offTi isn't populated. Just 
like the Interface* that's supposed to be the first element of each 
interface's vtbl pointer. It would be useful if it existed, but no cheese.

> If run-time pinning is used instead (where whether the GC cell pointed 
> to by a pointer is pinned is not known at compile time), it could be a 
> simple (synchronized) counter that starts out at 0 for each memory cell, 
> that's incremented when pinned and decremented when unpinned. The GC is 
> then only allowed to move cells whose counter is 0.

You would do both. During a collection, you mark each block to see if 
it's referenced, and mark it again if it's got a fixed reference. Then 
you collect every section that's not referenced and optionally move the 
sections that aren't marked as pinned to a more advantageous layout.

If you are proposing a compile-time garbage collector, one that 
determines when to delete an object using static analysis, I will be 
quite impressed if you come up with an implementation.