> On Feb 4, 2017, at 6:45 PM, Dave Abrahams via swift-dev <swift-dev@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@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. John. _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
