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[SR-12427] Pointer stored in C union loses pointee storage when optimised #54866

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swift-ci opened this issue Mar 27, 2020 · 3 comments
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[SR-12427] Pointer stored in C union loses pointee storage when optimised #54866

swift-ci opened this issue Mar 27, 2020 · 3 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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@swift-ci
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Previous ID SR-12427
Radar rdar://problem/60983997
Original Reporter nrbrook (JIRA User)
Type Bug
Status Resolved
Resolution Done

Attachment: Download

Environment

Xcode Version 11.4 (11E146), macOS 10.15.4

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

md5: 8c20184c1e5d46cc15863bed3d2ef704

Issue Description:

The attached project demonstrates the issue.

With optimisation enabled, obtaining a pointer using withUnsafePointer and storing the pointer in a union inside a struct which is passed as a pointer to a C function causes the original struct instance to be deallocated before it is used in the C function.

When run without optimisation (debug scheme) the program runs correctly (prints 1, 2, 3). When run with optimisation (release scheme), `str` is released early and the memory read inside the C function is invalid (prints 0, 0, 0). Uncommenting the print statement causes the struct instance to be retained even when optimised.
@CodaFi
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CodaFi commented Mar 27, 2020

Thanks for the reproducer project, it's just fantastic. It seems like the optimizer is ignoring that taking the address of the temporary counts as a non-removable use and is just erasing everything. In fact, the IR in release just shows everything got inlined into `main` and we're just passing a freshly alloca'd value to your function in C.

Attaching debug and release IR.

@swift-ci create

@swift-ci
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Comment by Nick Brook (JIRA)

Good to know it is a bug and I'm not just misusing swift! For anyone suffering from this until a fix is released, it can be worked around using withExtendedLifetime on the s variable

@eeckstein
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fixed in #30770

@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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@CodaFi @eeckstein @swift-ci