[swift-dev] Foundation Data Behavior Different (read: crashes) in Swift 3.1 and Swift 3.2

Michael Gottesman mgottesman at apple.com
Thu Aug 24 13:05:41 CDT 2017

> On Aug 24, 2017, at 10:47 AM, Ryan Lovelett via swift-dev <swift-dev at swift.org> wrote:
> I've found what I believe is a bug. Though I'm unclear if the bug is in
> Swift 3.1 or Swift 3.2/4.0. All I can say for sure is the behavior is
> quite drastically different between the two.
> For the code below (and attached):
>    import Cocoa
>    var data = Data(bytes: [0x50, 0x4B, 0x01, 0x02, 0x41, 0x61])
>    data.removeFirst(4)
>    let base64 = data.base64EncodedString()
>    let str = String(bytes: data[0..<2], encoding: .utf8)!
>    print("Base64: \(base64), String: \(str)")
> If I compile and run that with the Swift included in Xcode 8.3.3 (e.g.,
> swift ./data-bug.swift) it outputs: Base64: QWE=, String: Aa. Which is
> what I expect.
> With the Swift that is included with Xcode 9.0 beta 6 (9M214v) (e.g.,
> swift -swift-version 3 ./data-bug.swift). It performs an illegal
> hardware instruction and crashes. It also does this if I use use the
> version 4 of the compiler.
> Is this a bug? If so where is the bug? Was this always meant to not work
> and Swift 3.1 just happened to work or is there now an issue in the
> Swift 3.2 implementation?

I have not engaged my brain with the particulars of the rest of the email, but high level question: does this happen without optimization? Or does it happen only with optimization?


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