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. -- -Dave _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
