Why is Xcode's debugger jumping around this way with Swift?

Question

I debugged the following code with F6 in Xcode 6, and the sequence of execution is very interesting.

Here is the code - 7 lines, a breakpoint is set on line 1:

    let request = AWSDynamoDBPutItemInput()
    request.tableName = "blah"

    let card = AWSDynamoDBAttributeValue()
    card.S = "1234"
    let email = AWSDynamoDBAttributeValue()
    email.S = "notset"

    request.item = ["card_number" : card, "email" : email]

When I F6'd through the code, it showed the following sequence (the numbers are line numbers):

1,2,4,2,3,4,6,4,5,6,7,6,7

Why is this? Is this something with Xcode or the language? Those classes are defined in Amazon's AWS SDK, not sure whether that matters, they are accessed through swift-objective-c bridging, could this be related to the bridging.

By the way, the net result of the execution is correct.

Answer 1

I think what you're observing is the effect of the compiler optimizations. It rewrites your code at compile time. For this reason it's normal to disable optimization (-Onone) on debug builds, but enable it (-Ofast or -Os) on release builds.

Answer 2

Finally I got a reply to my bug report from Apple, and sounds like it was a bug and got fixed in a beta version of XCode. If you are seeking for a fix, please try the beta:

We believe this issue has been addressed in the latest Xcode 6.3 beta, including iOS 8.3 SDK with Swift 1.2.

Please test with this release. If you still have issues, please include any relevant logs or information that could help us investigate.

This is a pre-release version of the complete Xcode developer toolset for Mac, iPhone, iPad, and Apple Watch. This release requires OS X Yosemite.

Xcode 6.3 - Build 6D520o https://developer.apple.com/xcode/downloads/