The "no gc" crowd

[deadalnix]

On Wednesday, 9 October 2013 at 14:17:44 UTC, Sean Kelly wrote:
> On Oct 9, 2013, at 4:30 AM, Jacob Carlborg <doob at me.com> wrote:
>> 
>>> On 2013-10-09 05:31, Walter Bright wrote:
>>> 
>>> Making this work is fraught with difficulty. It is normal 
>>> behavior in D
>>> to create local data with new(), build a data structure, and 
>>> then cast
>>> it to shared so it can be transferred to another thread. This 
>>> will fail
>>> miserably if the data is allocated on a thread local heap.
>> 
>> I agree with Andrei here. Alternatively perhaps the runtime 
>> can move the data to a global pool if it's casted to shared.
>
> Generally not, since even D's precise GC is partially 
> conservative.  It's also way more expensive than any cast 
> should be. For better or worse, I think being able to cast data 
> to shared means that we can't have thread-local pools. Unless a 
> new attribute were introduced like "local" that couldn't ever 
> be cast to shared, and that sounds like a disaster.

That isn't accurant. Allocator like tcmalloc use thread local 
info to allocate shared chunk of memory. What does matter is that 
the block is tagged as shared as far as the GC is oncerned.

Casting qualifier is a NOOP at machine level, so that won't be 
any slower.