[SR-3728] Explore using spare bits in references for ARC optimizations

[ematejska]

Previous ID	SR-3728
Radar	rdar://problem/31411206
Original Reporter	@ematejska
Type	Task
Status	Resolved
Resolution	Won't Do

Additional Detail from JIRA

Votes	0
Component/s
Labels	Task, AffectsABI
Assignee	None
Priority	Medium

md5: c2f9f85d68a6a079d894fbb5c1c7e156

Issue Description:

Ahead of ABI Stability, explore using spare bits in references to store local reference counts in order to perform some ARC operations more efficiently . These would need to be flushed to the object whenever a reference may escape or the local reference count reaches zero. If these local reference counts can cross ABI boundaries, then such a change will have to be implemented in an ABI-additive way with deployment target checking.

[bob-wilson]

@swift-ci create

[bob-wilson]

This could potentially give some big performance improvements, but it would be a lot of work to implement and it's not feasible to fit it into Swift 5. It is worth noting that this optimization is not needed for calls within a module, because we can get the same benefit there using function signature optimization.