On 9/19/17 2:06 PM, Craig Black wrote:
Thank you for the clarification. I understand mow that @nogc is only for functions and not for data types. Thinking out loud, it would seem beneficial if there was a way to mark a pointer or data structure as not pointing to the GC heap. A guarantee to the compiler and run-time to ignore it during GC sweeps. Now I know that pointer arithmetic breaks every kind of guarantee that would have with pointers, but aside from that it would seem to me that the compiler could help to enforce data marked as non-GC to not be assigned GC heap allocations. This wouldn't be allowed for classes or class references, since they are always pointing to GC data, but perhaps for pointers and structs. It seems like it would be a helpful feature, but maybe I'm way off base.
In order for that to work, we would first need a precise scanner, as the current scanner scans ALL values in a block to see if any point at any heap memory, regardless of whether it is truly a pointer or not.
Also, if a pointer points to something outside the heap, there is no scanning of that data, since the GC doesn't know about it. The cost of having a pointer that points to a non-GC memory location is small (not entirely trivial, but it's just a few comparisons). The only true benefit would be to avoid scanning the entire block. This means that along with precise scanning, you couldn't have any GC pointers in the block.
As with most things, we need to prove the benefit with profiling before spending any time thinking about how to implement, and certainly before adding any language/library features.
-Steve
