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[SR-3727] Determine the layout algorithm for payloaded enums #46312

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ematejska mannequin opened this issue Jan 25, 2017 · 3 comments
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[SR-3727] Determine the layout algorithm for payloaded enums #46312

ematejska mannequin opened this issue Jan 25, 2017 · 3 comments
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affects ABI Flag: Affects ABI task

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@ematejska
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Mannequin

ematejska mannequin commented Jan 25, 2017

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
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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
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@bob-wilson
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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.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@jckarter @bob-wilson