[swift-dev] Shrinking the heap object header

[Joe Groff]

> On Jan 15, 2016, at 11:51 AM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Jan 15, 2016, at 10:58 AM, Joe Groff via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> Swift heap object headers are fairly large—16 bytes on 64-bit, and 12 bytes on 32-bit. Into this space we pack:
>> 
>> - the 'isa' pointer for the object, pointing to its heap metadata/class object,
>> - the strong and unowned reference counts,
>> - 'pinned' and 'deallocating' flags.
>> 
>> We've also discussed taking a flag bit for 'not refcounted' objects, such as statically-allocated globals and/or stack promotions that need to be ABI compatible with heap objects, and potentially one for thread-local objects, to avoid barriers when refcounting objects we dynamically know are not referenced from multiple threads. We should consider whether we can reduce the header size. Two ideas come to mind:
>> 
>> Dropping the unowned reference count
>> 
>> If we adopt a sufficiently fast implementation for normal weak references, such as the activity count implementation suggested by Kevin and Mike, the unowned reference count might not be worth the expense. If we dropped it, that would be enough to bring the 32-bit object header down to 8 bytes. The tradeoff would be that unowned references become fatter, like weak references would, which might complicate our plans to eventually allow unowned to transparently become unowned(unsafe) in unchecked builds.
> 
> Unless I’m misunderstanding something, the activity count implementation doesn’t actually change anything about the need to register the existence of the weak/unowned reference with the object.
> 
> We could compact the two reference counts into a single 32-bit header; that would help a lot on 32-bit targets.  However, doing so would shrink both reference counts to the point that leaking on overflow would become unacceptable; we would definitely need some ability to spill over into a side table.

I probably misunderstood too. If that's the case, we could still probably allocate more bits to the strong refcount than to the unowned one, since it's probably more acceptable to spill into a side table for weak references more often than for strong ones.

-Joe

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