[SR-3727] Determine the layout algorithm for payloaded enums

[ematejska]

Previous ID	SR-3727
Radar	rdar://problem/31411198
Original Reporter	@ematejska
Type	Task
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: fe502935ae3aee66b438641e4a3b8d0a

relates to:

• SR-5267 Optional layout is inefficient

Issue Description:

Ahead of ABI Stability, determine the layout algorithm for multi-payload enums. The algorithm should try to rearrange payloads so as to coalesce cases and save space. This rearrangement can also improve performance and code size. For example, if ARC-ed payload components reside in the same location, operations like copy can be done directly on the values without extensive switching.

[jckarter]

Single-payload enum layout deserves some consideration too. It'd be nice if extra inhabitants were always distributed in such a way that we could figure out the tag of an enum loading only one word; sometimes the tag information is scattered between extra tag bits at the back of the value and extra inhabitants which are biased toward the front of the value.

[bob-wilson]

@swift-ci create

[bob-wilson]

The only change that we're going to make for the stable ABI is to look for extra inhabitants in any field of a fixed-layout struct (#18630 Other enum layout changes might be possible in the future if we add a mechanism to version the layouts.