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[SR-13114] switch statement over enum instance should release references in associated values on successful pattern match #55560

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tayloraswift opened this issue Jun 29, 2020 · 4 comments
Open

[SR-13114] switch statement over enum instance should release references in associated values on successful pattern match #55560

tayloraswift opened this issue Jun 29, 2020 · 4 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior.

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@tayloraswift
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Previous ID SR-13114
Radar rdar://problem/64953109
Original Reporter @Kelvin13
Type Bug
Environment

$ swift --version
Swift version 5.2-dev (LLVM 4cd92ac, Swift 79716ef)
Target: x86_64-unknown-linux-gnu

Additional Detail from JIRA
Votes 0
Component/s
Labels Bug
Assignee None
Priority Medium

md5: 8181bfb6c40eae032241ad95f2b96f51

is duplicated by:

  • SR-13113 assignment to enum instance should recursively deinitialize associated values

Issue Description:

Right now, it seems that `switch` statements appear to retain a strong reference to the associated values in the `enum` case for the duration of the `case` block's execution. However, the payloads are no longer needed after the pattern matching succeeds, so they should be released at the beginning of the `case` block, with locally bound associated values taking an additional retain as needed.

    // example
    e = .s(.init()) // +1 (total: 1)
    switch e // +1 (total: 2)
    {
    // +1 for var binding (total: +3)
    case .s(var s): 
    // -1 for return from pattern match (total: +2)
        s.foo() 
        e = .empty // -1 (total: +1)
        s.foo() // should be uniquely referenced
   
    default:
    // -1 for return from pattern match (total: +1)
        break
    }

this issue is probably related to: https://bugs.swift.org/browse/SR-13113
@rjmccall
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@gottesmm, this is related to your work.

@CodaFi
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CodaFi commented Jun 30, 2020

@swift-ci create

@swift-ci
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swift-ci commented Jul 3, 2020

Comment by Sighoya (JIRA)

John McCall wrote in SR-13112:

>I'm going to close this one; it's acceptable for `isKnownUniquelyReferenced` to change from `false` to `true` under optimization as a general matter. Whether `switch` should guarantee to not separately keep the switch value alive in unoptimized builds is better tracked as SE-13114.

Afaik, it is desireable that a program after optimization shares the same semantics as the program before.

Flipping true to false or vice versa doesn't belong to this kind of desire.

@rjmccall
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rjmccall commented Jul 5, 2020

That is generally true, but `isKnownUniquelyReferenced` is an exception: it is testing something that is generally an implementation detail that the optimizer is permitted to affect (specifically by changing the result from `false` to `true` by eliminating unnecessary extra references).

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@rjmccall @CodaFi @tayloraswift @swift-ci