On 2014-02-05 15:01:04 +0000, Manu <turkeyman@gmail.com> said:

Aligned data is a challenge. I have often wondered if it would be feasible
to access the RC via a pointer hash or something, and keep it in a side
table... sounds tricky, but I wonder if it's possible.

That's what Apple is doing (as seen in the CoreFoundation source code). They actually have eight such tables on OS X, each protected by a spin lock. The right table is chosen according to a few bits of the pointer.

The obvious advantage is you can put immutable data in read-only memory without making the reference count immutable. The downside is that it's more complicated to access the counter.

Fat pointers might be necessary. That's a bit annoying. Hmmm...

Since we're talking about adding reference counts to GC-allocated memory, you could use the GC to find the base address of the memory block. What is the cost of that?

I'm sure there's a clever solution out there which would allow the ARC to
detect if it's a raw C pointer or not...

Ask the GC for the base address of the memory block. If it does not come from a GC block, there's no reference counter to update.


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