[swift-evolution] ABI in Layman's terms?

[David Hart]

I'd also like to understand this more and this answer does not completely satisfy me. I understand backwards compatibility, especially in terms of source breaking changes.

I have more difficulties understanding what breaks or not the ABI and how to make educated guesses about what features will require breaking it.

> On 11 Aug 2016, at 13:24, Anton Zhilin via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 2016-08-11 11:52 GMT+03:00 Jonathan Hull via swift-evolution <swift-evolution at swift.org>:
>> Could someone explain in simple terms what the ABI is and what sorts of things might affect it?
>> 
>> I had thought it was the layout in memory of structs/classes/etc… and how the program knows where to go to find a particular field.  This seems to be incorrect though, as I have seen many features that I would assume have some affect on this layout ruled “out of scope for phase 1”.  For example, I would think that many generics features would have an impact on the ABI, or the idea of COW (via secret boxing) for structs, or even union types.
>> 
>> At the very least, I would think that mix-ins would have a fairly significant effect.
> 
> ABI stability means that changes will have to be backwards compatible after a certain stage. If we can add mixins feature without modifying old code (and its SIL and IR and whatever), then we are fine. 
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