[swift-dev] Resilient dynamic dispatch ABI. Notes and mini-proposal.

John McCall rjmccall at apple.com
Mon Feb 6 16:03:47 CST 2017

> On Feb 4, 2017, at 6:45 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
> on Fri Feb 03 2017, John McCall <swift-dev-AT-swift.org> wrote:
>>> On Feb 3, 2017, at 7:12 PM, Joe Groff via swift-dev <swift-dev at swift.org> wrote:
>>> Given that most open-coded resilient method lookup paths require an
>>> extra load dependency to grab the method offset before loading the
>>> method address itself, we might possibly consider indirecting the
>>> vtables for each class, so that the top-level vtable contains
>>> [address of root class vtable, address of first child class vtable,
>>> etc.]. If a class hierarchy is fixed once exported (in other words,
>>> you can't insert a superclass into an inheritance chain without an
>>> ABI break), then the offset into each superclass's pointer in the
>>> vtable would be statically known, and the offset into each
>>> second-level vtable could be statically known via sorting by
>>> availability. This somewhat matches how we lay out protocol witness
>>> tables, where each parent protocol's witness table is indirectly
>>> referenced instead of being inlined into the leaf witness
>>> table. (OTOH, method offsets can be cached and reused to dispatch
>>> the same method on different objects, whereas we would have to
>>> perform the load chain once per object per method with this
>>> approach.)
>> Great point.
>> I'm still uncomfortable with the idea of assuming that we can't insert
>> a superclass into an inheritance chain.  This isn't an assumption
>> that's otherwise necessary or even useful, unless we decide to start
>> optimizing dynamic casts.
> Does that mean we don't need to make such an assumption for protocols
> either?  That would be a big relief for me, if so.

I think we can probably implement the ability to extract a super-protocol resiliently, yeah.
You'd get some requirements in redundant positions, but that's not so bad.


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