> On May 2, 2017, at 4:38 PM, Jordan Rose <jordan_r...@apple.com> wrote:
>> On Apr 28, 2017, at 16:28, John McCall via swift-users
>> <swift-us...@swift.org <mailto:swift-us...@swift.org>> wrote:
>>
>>> On Apr 28, 2017, at 7:03 AM, Johannes Weiss via swift-dev
>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote:
>>> Hi swift-users,
>>>
>>> (sorry for the cross post to swift-dev, but wasn't sure where it belongs)
>>>
>>> I tried to find guarantees about the memory layout Swift tuples but
>>> couldn't find anything. The reason I ask is because I'd like to use them as
>>> fixed sized (stack allocated) arrays. I'm pretty sure they're actually not
>>> guaranteed to be stack allocated but highly likely I assume :).
>>
>> Tuples are guaranteed to use a standard C-style layout wherever that layout
>> is ABI-observable, e.g. when you construct an UnsafePointer to one.
>
> Ah, is this true for non-homogeneous tuples? I thought we only guaranteed it
> for homogeneous ones.
It's true of all tuples; we have to have some agreement on tuple layout that
works generically, and the simple C left-to-right layout is the obvious one.
Of course, like I said, we totally reserve the right to not actually store
tuples that way in specific classes, structs, enums, locals, argument, results,
or whatever else; but in places where a universal ABI is necessary, including
pointers, we do make that guarantee.
This notion of universal ABI generally also affects things like generic data
structures, which can mean that e.g. a [(MyClass, Bool)] might be stored less
efficiently than a [MyPair] where
struct MyPair { var object: MyClass; var flag: Bool }
But that's just the price of supporting dynamic generics.
John.
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