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[SR-5267] Optional<Bool> layout is inefficient #47842

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dabrahams opened this issue Jun 20, 2017 · 4 comments
Open

[SR-5267] Optional<Bool> layout is inefficient #47842

dabrahams opened this issue Jun 20, 2017 · 4 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@dabrahams
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Previous ID SR-5267
Radar rdar://problem/19859539
Original Reporter @dabrahams
Type Bug
Status Reopened
Resolution
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: 0aefdbe4f7f07547cb62f2bd02f6a8c0

is duplicated by:

  • SR-3725 Determine if struct data members are addressable or bit-packed

relates to:

  • SR-3727 Determine the layout algorithm for payloaded enums

Issue Description:

I need to store a bunch of true/false/indeterminate values, but Swift doesn't handle it efficiently:

(swift) MemoryLayout<(Bool?,Bool?)>.size
// r0 : Int = 2
(swift) MemoryLayout<(Bool?,Bool?,Bool?)>.size
// r1 : Int = 3
(swift) struct X { var a, b, c: Bool? }
(swift) MemoryLayout<X>.size
// r2 : Int = 3

These should all fit in one byte.
@dabrahams
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@swift-ci create

@belkadan
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There are sort of two parts here:

  • Bool could be bit-packable.

  • Enum discriminators could be bit-packable.

(And making Bool an enum would collapse these two problems into one…)

@jckarter
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Enum discriminators are already bit-packed; Bool? takes two bits. Tuples are constrained by the (T,T,T) abstraction boundary, so we can't bit-pack their fields unless we're willing to reabstract them at generic boundaries. We could conceivably bit-pack concrete structs.

@bob-wilson
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I'm reopening this. I redefined SR-3726 to track the larger task of defining the layout algorithm for tuples, not just whether they are bit-packed. Let's use this report to track the packing issue.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@dabrahams @jckarter @belkadan @bob-wilson