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[SR-12715] Optimiser fails to elide bounds check that is guaranteed to succeed on constant-size array. #55160

Lukasa opened this issue May 1, 2020 · 1 comment


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Lukasa commented May 1, 2020

Previous ID SR-12715
Radar rdar://problem/62894039
Original Reporter @Lukasa
Type Bug


Additional Detail from JIRA
Votes 1
Labels Bug, Optimizer
Assignee None
Priority Medium

md5: a42e10c38447fad0977a4ea580b205ea

Issue Description:

The following code:

func test(index: Int) -> Int {
    var array = Array(0..<1024)
    return array[index & 1023]

Generates this assembly in Swift 5.2 when compiled with -O:

output.test(index: Swift.Int) -> Swift.Int:
        push    rbp
        mov     rbp, rsp
        push    rbx
        push    rax
        mov     rbx, rdi
        mov     esi, 1024
        xor     edi, edi
        call    (generic specialization <Swift.Range<Swift.Int>> of Swift._copyCollectionToContiguousArray<A where A: Swift.Collection>(A) -> Swift.ContiguousArray<A.Element>)
        and     ebx, 1023
        cmp     rbx, qword ptr [rax + 16]
        jae     .LBB1_2
        mov     rbx, qword ptr [rax + 8*rbx + 32]
        mov     rdi, rax
        call    swift_release@PLT
        mov     rax, rbx
        add     rsp, 8
        pop     rbx
        pop     rbp

Due to the outlined call to generic specialization <Swift.Range<Swift.Int>> of Swift._copyCollectionToContiguousArray<A where A: Swift.Collection>(A) -> Swift.ContiguousArray<A.Element> the compiler has failed to observe that [rax + 16] will always be 1024, and so the jae branch will never be taken. It would be good if the compiler could elide the bounds check.

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beccadax commented May 5, 2020

@swift-ci create

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