draft proposal for ref counting in D

[Michel Fortin]

On 2013-10-15 02:20:49 +0000, "deadalnix" <deadalnix at gmail.com> said:

> It will indeed cause trouble for code that mutate a large amount of 
> shared pointers. I'd say that such code is probably asking for trouble 
> in the first place, but as always, no silver bullet. I still think 
> solution is the one that fit D the best.

I think there's a small mistake in your phrasing, but it makes a difference.

When the collector is running, it needs to know about any mutation for 
pointers to its shared memory pool, including pointers that are 
themselves thread-local but point to shared memory. So COW will be 
trouble for code that mutate a large amount of **pages containing 
pointers to shared memory**. And this which includes **pointers to 
immutable data** because immutable is implicitly shared. And this 
includes **pointers to const data** since those pointers might point to 
immutable (thus shared) memory.

So any memory page susceptible of containing pointers to shared memory 
would need to use COW during collection. Which means all the thread's 
stacks, and also all objects with a pointer to shared, immutable, and 
const data. At this point I think it is fair to approximate this to 
almost all memory that could contain pointers.

-- 
Michel Fortin
michel.fortin at michelf.ca
http://michelf.ca