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[SR-15259] Should be possible to statically optimise out calls to swift_isUniquelyReferenced_nonNull_native immediately after retain call #57581

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Lukasa opened this issue Sep 29, 2021 · 2 comments

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@Lukasa
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Lukasa commented Sep 29, 2021

Previous ID SR-15259
Radar rdar://problem/83825776
Original Reporter @Lukasa
Type Bug
Status Resolved
Resolution Done
Environment

Swift 5.5 RELEASE

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

md5: cae4a3377826580828afb9b77121337e

Issue Description:

Consider the following function:

func foo(_ x: Int) -> [Int] {
    var results: [Int] = []
    results.reserveCapacity(x)

    for _ in 0..<x {
        results.append(x)
    }

    return results
}

When compiled (see this godbolt page) this includes the following lines of assembly for part of the first two lines:

        mov     r15, qword ptr [rip + _swiftEmptyArrayStorage@GOTPCREL]
        mov     rdi, r15
        call    swift_retain@PLT
        mov     rdi, r15
        call    swift_isUniquelyReferenced_nonNull_native@PLT
        test    al, al
        je      .LBB1_1

In this instance, we load the empty array storage into r15. We then retain r15 and then pass it to swift_isUniquelyReferenced_nonNull_native.

This pattern has a statically-known result: the call to isUniquelyReferenced can only return false. In this instance, we have issued a retain call on a strong reference and, without any intervening release, we have then asked if it is uniquely referenced. Of course, it cannot be: we just issued a +1 and the object was already held strongly (at +1).

In principle we could have safely elided the call to isUniquelyReferenced, the jump, and just hit the resize operation in straight-line code. This would be a bit of a code size win.

@eeckstein
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eeckstein commented Oct 4, 2021

@swift-ci create

@eeckstein
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eeckstein commented Nov 29, 2021

Fixed with #40291

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