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

[Jonathan Hull]

Right, I understand that part.  ABI stability means that we can run past binaries without recompiling.

What I want to know is what sorts of things affect this stability and which things don’t.  I guess I want heuristics for knowing whether an idea will affect the ABI, and thus needs to be talked about now.  I don’t want to distract from the process by proposing features which can be tackled later, but at the same time I REALLY don’t want to wait until phase 2 and then be told the feature can never be added because it would break the ABI (and I should have proposed it during phase 1).

Thanks,
Jon

> On Aug 11, 2016, at 4:24 AM, Anton Zhilin <antonyzhilin at gmail.com> wrote:
> 
> 2016-08-11 11:52 GMT+03:00 Jonathan Hull via swift-evolution <swift-evolution at swift.org <mailto: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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