On Tue, Oct 4, 2011 at 8:59 PM, Robert Jacques <sandf...@jhu.edu> wrote: > On Tue, 04 Oct 2011 10:54:58 -0400, Andrew Wiley <wiley.andre...@gmail.com> > wrote: > >> On Tue, Oct 4, 2011 at 3:52 AM, Walter Bright >> <newshou...@digitalmars.com> wrote: >>> >>> On 10/4/2011 1:22 AM, deadalnix wrote: >>>> >>>> Do you mean manage the memory that way : >>>> Shared heap -> TL pool within the shared heap -> allocation in thread >>>> from >>>> TL pool. >>>> >>>> And complete GC collect. >>> >>> Yes. >>> >>> >>>> This is a good solution do reduce contention on allocation. But a very >>>> different >>>> thing than I was initially talking about. >>> >>> Yes. >>> >>> >>>> Back to the point, >>>> >>>> Considering you have pointer to immutable from any dataset, but not the >>>> other >>>> way around, this is also valid to get a flag for it in the allocation >>>> interface. >>>> >>>> What is the issue with the compiler here ? >>> >>> Allocate an object, then cast it to immutable, and pass it to another >>> thread. >>> >> >> Assuming we have to make a call to the GC when an object toggles its >> immutable/shared state, it seems like this whole approach would >> basically murder anyone doing message passing with ownership changes, >> because the workflow tends to be create an object -> cast to shared -> >> send to another thread -> cast away shared -> do work -> cast to >> shared... >> On the other hand, I guess the counterargument is that locking an >> uncontended lock is on the order of two instructions (or so I'm told), >> so casting away shared probably isn't ever necessary. It just seems >> somewhat counterintuitive that casting to and from shared would be >> slower than unnecessarily locking the object. >> > > It's entirely possible to simply allocate the memory for the object from the > shared heap to start with. Then no more calls to the GC are needed. >
When an object is created and later cast to shared, the compiler *can't* know that it should allocate from the shared heap because the cast may not be anywhere near where the object was created. The same problem goes for immutable.
