[SR-7354] Swift 4.1 Regression: EXC_BAD_ACCESS for Optimized Builds in Xcode 9.3

[swift-ci]

Previous ID	SR-7354
Radar	rdar://problem/39209102
Original Reporter	fabb (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 9.3 + iPhone X Simulator 11.3, or iPhone 6 Device 11.3

Additional Detail from JIRA

Votes	3
Component/s	Compiler
Labels	Bug, 4.1Regression, OptimizedOnly, RunTimeCrash
Assignee	@atrick
Priority	Medium

md5: 01fbdfe890b85cc4dcce92b1c7fe221d

Issue Description:

Since updating to Xcode 9.3 our project crashes on launch for optimized builds. It happens both on device and simulator, but not necessarily in the same locations. It worked fine with the previous Xcode. It happens for all builds with -O or -Osize optimizations, independent of WMO de/activated, and independent of Swift 3 / Swift 4 mode.

Changing arbitrary things in the method makes it run without crash. Even putting it into a different file might make it not crash anymore.

Example repro code:

// crashes when called on an optimized build (-O or -Osize) both on simulator and device
func crash() {
    let blah = ProblematicEnum.problematicCase.problematicMethod()
    NSLog("\(blah)")
}

enum ProblematicEnum {
    case first, second, problematicCase
    
    func problematicMethod() -> SomeClass {
        let someVariable: SomeClass
        
        switch self {
        case .first:
            someVariable = SomeClass()
        case .second:
            someVariable = SomeClass()
        case .problematicCase:
            someVariable = SomeClass(someParameter: NSObject())
            _ = NSObject().description
            return someVariable // EXC_BAD_ACCESS (simulator: EXC_I386_GPFLT, device: code=1)
        }
        
        let _ = [someVariable]
        return SomeClass(someParameter: NSObject())
    }
    
}

class SomeClass: NSObject {
    override init() {}
    init(someParameter: NSObject) {}
}

Example project for convenience: https://github.com/fabb/swift_4_1_crash

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

it's a very strange crash, e.g. removing the NSObject as a parent class of SomeClass results in not crashing anymore. So does not passing someParameter anymore in either of the 2 locations. Or removing the `second` case, or merging the 2 cases in the `switch`.

[belkadan]

Very strange. Thanks, Fabian!

@swift-ci create

[gottesmm]

Looking at this.

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

@gottesmm any news on this? it would be helpful for us if after analyzing, you could give an exact description under which circumstances the bug happens, so we can analyze the rest of our codebase for this pattern. currently we're afraid to update as it's hard to test all the special cases in our app.

[gottesmm]

We have already triaged the issue and have a fix:

#15840

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

@gottesmm awesome, thanks for the fast fix!

@gottesmm @belkadan Do you consider the bug bad enough to warrant an Xcode bugfix release?
As a non-compiler-engineer I couldn't really deduce which kind of Swift code might be affected from the explanation in the PR. Is it even possible to do that, or are the possibly dangerous patterns endless?

[atrick]

This has to do with the interaction between passes. There's nothing really unusual about the Swift code that triggers it. It should be fixed at the next possible opportunity.

[belkadan]

We can't ever talk about whether there'll be an Xcode bugfix release, or whether particular fixes would make it into any such release. This bug is pretty bad, especially in that you can't tell whether you're going to hit it beforehand, but every bug without a workaround is blocking somebody.

[atrick]

[[master] https://github.com/apple/swift/pull/15840\|#15840]

[4.2] #15876

[[4.1] https://github.com/apple/swift/pull/15882\|#15882]

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

@belkadan thank you. too bad that no fixing xcode release seems in sight. we rolled back to xcode 9.2.

[belkadan]

Since the beta is out now I can say that this should be in Xcode 9.4b2.

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

@belkadan Xcode 9.4 just was released, but this bugfix is not mentioned in the release notes. Is it included?

[belkadan]

Yes, it is!

[swift-ci]

Comment by Fabian Ehrentraud (JIRA)

Awesome, thank you!