> On Oct 24, 2017, at 8:32 AM, Sam Ding via swift-dev <swift-dev@swift.org>
> wrote:
>
> Hi,
>
> It is found that "for" loop iterate and "enum" does not work on s390x.
> Here is a sample code:
> enum IntKey : Int {
> case a = 3
> case b = 4 // Implicitly 4
> case c = 1
> }
> for ( a1, a2) in [(IntKey.a, 1)] {
> print ("+++++ test \n")
> }
>
> The for loop inifinitly iterates on s390x. Note that it works fine if
> replacing the enum with normal tuple in the above code.
> When debugging this code, after one iteration, the logic gets a "nil" in
> "IndexingIterator.next()" as follows:
> ....
> frame #0: 0x000003fffd5b4088
> libswiftCore.so`IndexingIterator.next(self=0x000003fffffff460) at
> Collection.swift:411
> 408 @inline(__always)
> 409 public mutating func next() -> Elements.Element? {
> 410 if _position == _elements.endIndex {
> -> 411 return nil
> 412 }
> 413 let element = _elements[_position]
> ..
> Then comes "Enum.cpp:211" (swift::swift_storeEnumTagSinglePayload) and no
> difference on both s390x and x86_64.
> However, It continues goes into the loop body on s390x, and terminates on
> x86_64, respectively.
>
> Have anyones know well this implementation and give us a hint for the
> problems?
>
> Thanks,
>
At a binary level, Swift is going to represent the result of the next()
operation as an value of the element type wrapped in an Optional, `(IntKey,
Int)?`, and the compiler is going to recognize that it can represent the `nil`
case by using unused representations of IntKey. Our code for doing enum layout
was originally written in a very little-endian-centric way, and if s390x is
big-endian, there's a possibility that the code in IRGen or the runtime that
encodes the `nil` result is not correct, or that the runtime and IRGen do not
agree on the representation of `nil`. That could lead to the infinite loop
you're seeing.
-Joe
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